LT8644S [ADI]
8V, 16A Synchronous Step-Down Silent Switcher 2;
| 型号： | LT8644S |
| 厂家： | 
ADI |
| 描述： | 8V, 16A Synchronous Step-Down Silent Switcher 2 |
| 文件： | 总28页 (文件大小：2073K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT8638S
42V, 10A/12A Peak Synchronous
Step-Down Silent Switcher 2
FEATURES
DESCRIPTION
Silent Switcher®2 Architecture
The LT®8638S synchronous step-down regulator
features second generation Silent Switcher architecture
designed to minimize EMI emissions while delivering high
n
n
Ultralow EMI Emissions on Any PCB
n
Eliminates PCB Layout Sensitivity
n
efficiency at high switching frequencies. This includes
the integration of input capacitors to optimize all the fast
current loops inside and make it easy to achieve advertised
EMI performance by reducing layout sensitivity. This
performance makes the LT8638S ideal for noise sensitive
applications and environments.
Internal Bypass Capacitors Reduce Radiated EMI
n
Optional Spread Spectrum Modulation
High Efficiency at High Frequency
n
n
Up to 96% Efficiency at 1MHz, 12V to 5V
Up to 94% Efficiency at 2MHz, 12V to 5V
IN
IN
OUT
OUT
n
n
n
n
n
Wide Input Voltage Range: 2.8V to 42V
10A Maximum Continuous, 12A Peak Transient Output
The fast, clean, low overshoot switching edges enable high
efficiency operation even at high switching frequencies,
leading to a small overall solution size. Peak current mode
control with a 25ns minimum on-time allows high step
down ratios even at high switching frequencies. External
compensation via the VC pin allows for fast transient
response. PolyPhase operation allows multiple LT8638S
regulators to run with interleaving phase shift to provide
more output current.
Fast Transient Response with External Compensation
Low Quiescent Current Burst Mode® Operation
n
90µA I Regulating 12V to 5V
Q
IN
P-P
OUT
n
Output Ripple < 10mV
n
n
n
n
n
n
n
n
n
Reference Accuracy: 1ꢀ Over Temperature
Fast Minimum Switch On-Time: 25ns
PolyPhase® Operation: Up to 12 Phases
Low Dropout Under All Conditions: 45mV at 1A
Adjustable and Synchronizable: 200kHz to 3MHz
Output Soft-Start and Power Good
Burst Mode operation enables low standby current
consumption, forced continuous mode can control
frequency harmonics across the entire output load
range, or spread spectrum operation can further reduce
EMI emissions. Soft-start and tracking functionality is
accessed via the SS pin, and an accurate input voltage
UVLO threshold can be set using the EN/UV pin.
Safely Tolerates High Reverse Current
28-Lead 5mm × 4mm LQFN Package
AEC-Q100 Qualified for Automotive Applications
APPLICATIONS
n
All registered trademarks and trademarks are the property of their respective owners. Protected
by U.S. patents, including 8823345.
Automotive and Industrial Supplies
General Purpose Step-Down
n
12VIN to 5VOUT Efficiency
TYPICAL APPLICATION
ꢀ00
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.0
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
0.ꢀ
0
5V 10A Step-Down Converter
ꢋ
ꢌꢍ
ꢀꢁꢁꢂꢃꢂꢀꢄꢃꢅ
ꢋ
ꢊꢅꢁ
ꢌꢍ
ꢕ.ꢗꢋ ꢁꢛ ꢗꢜꢋ
0.ꢇꢙꢒ
ꢇꢙꢝ
ꢗ.ꢘꢙꢒ
ꢎꢍꢏꢐꢋ
ꢋ
ꢛꢐꢁ
ꢕꢋ
ꢅꢉ
ꢇ0ꢆ
ꢌꢍꢁꢋ
ꢊꢌꢆꢅ
ꢑꢑ
ꢇꢙꢒ
ꢇꢜ.ꢇꢚ
ꢀꢁꢂꢃꢄꢂꢅ
ꢀꢁꢂꢃR ꢄꢁꢅꢅ
ꢇ00ꢚ
ꢇꢕꢖꢒ
ꢋ
ꢑ
Rꢁ
ꢒꢊ
ꢗꢘꢙꢒ
ꢀ00ꢁꢂꢃꢄ ꢅ ꢆ ꢇ.ꢇꢈꢂ
ꢀꢁꢂꢃꢄ ꢅ ꢆ ꢀꢇꢂ
ꢀꢁꢂꢃꢄ ꢅ ꢆ 0.ꢇꢈꢉꢂ
ꢄꢄ0ꢖꢒ
ꢄꢂ.ꢄꢚ
ꢇꢄ.ꢘꢚ
ꢓꢍꢔ
ꢞ
ꢟ ꢇꢠꢝꢡ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ0
ꢅꢉ
ꢂꢃꢄꢂꢅ ꢁꢆ0ꢇꢈ
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
ꢀꢁꢂꢀꢃ ꢄꢅ0ꢆꢇ
Rev. 0
1
Document Feedback
For more information www.analog.com
LT8638S
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
ꢉꢊꢋ ꢌꢍꢎꢏ
V , EN/UV, PG..........................................................42V
IN
BIAS..........................................................................25V
FB, SS, PHMODE . ......................................................4V
SYNC/MODE Voltage . ................................................6V
Operating Junction Temperature Range (Note 2)
LT8638SE .......................................... –40°C to 125°C
LT8638SJ .......................................... –40°C to 150°C
Storage Temperature Range .................. –65°C to 150°C
Maximum Reflow (Package Body) Temperature.....260°C
ꢀꢅ ꢀꢄ ꢀꢃ ꢀꢂ ꢀꢁ ꢀꢆ
ꢋꢩꢦꢊꢙꢎ
ꢞꢍꢔꢨ
ꢈ
ꢀ
ꢆ
ꢁ
ꢂ
ꢃ
ꢄ
ꢅ
ꢀꢀ Rꢉ
ꢀꢈ ꢎꢓꢢꢪꢌ
ꢀ0 ꢗꢓꢙ
ꢈꢇ ꢓꢕ
ꢀꢇ
ꢗꢓꢙ
ꢆ0
ꢗꢓꢙ
ꢍꢓꢉꢌ
ꢕꢕ
ꢞꢨꢉ
ꢨꢏ
ꢨꢏ
ꢨꢏ
ꢨꢏ
ꢈꢅ
ꢈꢄ
ꢈꢃ
ꢈꢂ
ꢌ
ꢌ
ꢌ
ꢌ
ꢍꢓ
ꢍꢓ
ꢍꢓ
ꢍꢓ
ꢆꢈ
ꢗꢓꢙ
ꢆꢀ
ꢗꢓꢙ
ꢇ
ꢈ0 ꢈꢈ ꢈꢀ ꢈꢆ ꢈꢁ
ꢐꢑꢒꢓ ꢋꢔꢕꢖꢔꢗꢎ
ꢀꢅꢘꢐꢎꢔꢙ ꢚꢂꢛꢛ × ꢁꢛꢛ × 0.ꢇꢁꢛꢛꢜ
ꢝꢎꢙꢎꢕ ꢞꢊꢔRꢙꢟ θ ꢠ ꢆ0ꢡꢕꢢꢏꢣ θ
ꢠ ꢈꢁ.ꢄꢡꢕꢢꢏꢣ θ
ꢠ ꢀ.ꢄꢡꢕꢢꢏ ꢚꢓꢤꢥe ꢆꢜ
ꢝꢔ
ꢝꢕꢚꢉꢊꢋꢜ
ꢝꢕꢚꢋꢔꢙꢜ
ꢙꢎꢦꢊ ꢞꢊꢔRꢙꢟ θ ꢠ ꢈꢇꢡꢕꢢꢏꢣ Ψ ꢠ 0.ꢈꢡꢕꢢꢏ
ꢝꢔ
ꢝꢉ
ꢎꢧꢋꢊꢨꢎꢙ ꢋꢔꢙ ꢚꢋꢍꢓꢨ ꢀꢇꢘꢆꢀꢜ ꢔRꢎ ꢗꢓꢙꢣ ꢨꢩꢊꢪꢐꢙ ꢞꢎ ꢨꢊꢐꢙꢎRꢎꢙ ꢉꢊ ꢋꢕꢞ
ORDER INFORMATION
PART MARKING*
PAD OR BALL
PACKAGE
TYPE**
MSL
RATING
TEMPERATURE RANGE
(SEE NOTE 2)
PART NUMBER
LT8638SEV#PBF
LT8638SJV#PBF
FINISH
DEVICE
FINISH CODE
–40°C to 125°C
–40°C to 150°C
LQFN (Laminate Package
with QFN Footprint)
Au (RoHS)
8638S
e4
3
AUTOMOTIVE PRODUCTS***
LT8638SEV#WPBF
–40°C to 125°C
–40°C to 150°C
LQFN (Laminate Package
with QFN Footprint)
Au (RoHS)
8638S
e4
3
LT8638SJV#WPBF
• Contact the factory for parts specified with wider operating temperature
ranges. *Pad or ball finish code is per IPC/JEDEC J-STD-609.
• Recommended LGA and BGA PCB Assembly and Manufacturing
Procedures
• Device temperature grade is identified by a label on the shipping container.
• LGA and BGA Package and Tray Drawings
Parts ending with PBF are RoHS and WEEE compliant. **The LT8638S package has the same dimensions as a standard 5mm × 4mm QFN package.
***Versions of this part are available with controlled manufacturing to support the quality and reliability requirements of automotive applications. These
models are designated with a #W suffix. Only the automotive grade products shown are available for use in automotive applications. Contact your
local Analog Devices account representative for specific product ordering information and to obtain the specific Automotive Reliability reports for
these models.
Rev. 0
2
For more information www.analog.com
LT8638S
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETER
CONDITIONS
f = 2MHz
SW
MIN
TYP
2.6
6
MAX
2.8
9
UNITS
V
l
Minimum Input Voltage
V
V
Quiescent Current in Shutdown
Quiescent Current in Sleep
V
V
= 0V, V = 12V
µA
IN
IN
EN/UV
EN/UV
IN
= 2V, V > 0.6V, V
= 0V, V
= 0V
125
125
195
245
µA
µA
FB
SYNC
BIAS
l
V
V
= 2V, V > 0.6V, V
= 0V, V
= 0V, V
= 5V
= 5V
20
29
µA
µA
EN/UV
FB
SYNC
BIAS
BIAS
BIAS Quiescent Current in Sleep
Feedback Reference Voltage
= 2V, V > 0.6V, V
100
145
EN/UV
FB
SYNC
V
IN
V
IN
= 12V
= 12V
0.598
0.594
0.6
0.6
0.602
0.604
V
V
l
l
Feedback Voltage Line Regulation
Feedback Pin Input Current
Error Amp Transconductance
Error Amp Gain
V
V
= 4.0V to 40V, V = 1.25V
0.004
0.03
20
ꢀ/V
nA
IN
CC
= 0.6V
–20
FB
V = 1.25V
C
1.05
1.4
700
320
320
12
1.75
mS
V Source Current
C
V
V
= 0.4V, V = 1.25V
µA
µA
A/V
V
FB
C
V Sink Current
C
= 0.8V, V = 1.25V
C
FB
V Pin to Switch Current Gain
C
V Clamp Voltage
C
2.3
45
BIAS Pin Current Consumption
Minimum On-Time
V
BIAS
= 3.3V, f = 2MHz, V = 12V
mA
ns
SW
IN
l
I
= 3A, FCM
25
40
LOAD
Minimum Off-Time
80
100
ns
l
l
l
Oscillator Frequency
R = 226k
170
0.96
1.85
200
1
2
230
1.04
2.15
kHz
MHz
MHz
T
R = 38.3k
T
R = 16.9k
T
Top Power NMOS On-Resistance
Top Power NMOS Current Limit
Bottom Power NMOS On-Resistance
Bottom Power NMOS Current Limit
SW Leakage Current
I
= 1A
20
20
mΩ
A
SW
l
17
23
V
V
V
= 3.4V, I = 1A
8
mΩ
A
INTVCC
SW
= 3.4V
12
15.5
19
1.5
INTVCC
= 42V, V = 0V, 42V
–1.5
0.93
µA
V
IN
SW
l
EN/UV Pin Threshold
EN/UV Rising
0.98
40
1.03
EN/UV Pin Hysteresis
mV
nA
ꢀ
EN/UV Pin Current
V
V
V
= 2V
–20
6
20
9.5
–6
EN/UV
l
l
PG Upper Threshold Offset from V
Rising
7.75
–7.75
0.4
FB
FB
FB
PG Lower Threshold Offset from V
PG Hysteresis
Falling
–9.5
ꢀ
FB
ꢀ
PG Leakage
V
V
= 3.3V
= 0.1V
–80
80
nA
Ω
PG
l
PG Pull-Down Resistance
SYNC/MODE Threshold
600
2000
PG
l
l
l
SYNC/MODE DC and Clock Low Level Voltage
SYNC/MODE Clock High Level Voltage
SYNC/MODE DC High Level Voltage
0.7
2.2
V
V
V
1.5
2.9
Spread Spectrum Modulation Frequency Range R = 38.3k
24
ꢀ
T
Rev. 0
3
For more information www.analog.com
LT8638S
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETER
CONDITIONS
MIN
1.3
35
TYP
3
MAX
UNITS
kHz
µA
Spread Spectrum Modulation Frequency
SS Source Current
l
2.0
200
37
2.7
SS Pull-Down Resistance
Fault Condition, SS = 0.1V
Ω
V
IN
to Disable Forced Continuous Mode
V
Rising
IN
39
V
PHMODE Thresholds
Between 180° and 120°
Between 120° and 90°
0.7
2.2
1.5
2.9
V
V
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 3: values determined per JEDEC 51-7, 51-12. See the Applications
Information section for information on improving the thermal resistance
and for actual temperature measurements of a demo board in typical
operating conditions.
Note 2: The LT8638SE is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization, and correlation with statistical process controls. The
LT8638SJ are guaranteed over the full –40°C to 150°C operating junction
temperature range. High junction temperatures degrade operating
lifetimes. Operating lifetime is derated at junction temperatures greater
Note 4: This IC includes overtemperature protection that is intended to
protect the device during overload conditions. Junction temperature will
exceed 150°C when overtemperature protection is active. Continuous
operation above the specified maximum operating junction temperature
will reduce lifetime.
than 125˚C. The junction temperature (T , in °C) is calculated from the
J
ambient temperature (T in °C) and power dissipation (PD, in Watts)
A
according to the formula:
T = T + (PD • )
JA
J
A
where (in °C/W) is the package thermal impedance.
JA
Rev. 0
4
For more information www.analog.com
LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
12VIN to 5VOUT Efficiency
vs Frequency
12VIN to 3.3VOUT Efficiency
vs Frequency
Efficiency at 5VOUT
ꢀ00
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
0.ꢀ
0
ꢀ00
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.0
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
0.ꢀ
0
ꢀ00
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ.ꢁ
ꢀꢁ ꢂꢃꢄꢅ
ꢀꢁ ꢂꢃꢄꢅ
ꢀꢁ ꢂꢃꢄꢅ
ꢀ.ꢁ
ꢀ.0
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀꢁꢁꢂꢃꢂꢀꢄꢃꢅ
ꢀꢁꢁꢂꢃꢂꢀꢄꢃꢅ
ꢀꢁꢁꢂꢃꢂꢀꢄꢃꢅ
ꢀꢁꢂꢃR ꢄꢁꢅꢅ
ꢀꢁꢂꢃR ꢄꢁꢅꢅ
ꢀꢁꢂꢃR ꢄꢁꢅꢅ
ꢀ
ꢀ
ꢀ
ꢀ ꢁꢂꢃ
ꢀ ꢁꢂꢃ
ꢀ ꢁꢂꢃ
ꢀ ꢁ ꢂꢃꢀꢄ0ꢄ0
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀ ꢁ ꢂꢃꢀꢄ0ꢄ0
ꢀ00ꢁꢂꢃꢄ ꢅ ꢆ ꢇ.ꢇꢈꢂ
ꢀꢁꢂꢃꢄ ꢅ ꢆ ꢀꢇꢂ
ꢀꢁꢂꢃꢄ ꢅ ꢆ 0.ꢇꢈꢉꢂ
ꢀ00ꢁꢂꢃꢄ ꢅ ꢆ ꢇ.ꢇꢈꢂ
ꢀꢁꢂꢃꢄ ꢅ ꢆ ꢀꢇꢂ
ꢀꢁꢂꢃꢄ ꢅ ꢆ 0.ꢇꢈꢉꢂ
0.ꢀ
ꢀ
ꢀ ꢁ00ꢂꢃꢄꢅ ꢆꢀ ꢇꢈꢆꢉ0ꢉ0ꢅ ꢊ.ꢊꢋꢃ
ꢀꢁ
0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ0
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
ꢀꢁꢂꢀꢃ ꢄ0ꢅ
ꢀꢁꢂꢀꢃ ꢄ0ꢅ
ꢀꢁꢂꢀꢃ ꢄ0ꢂ
Efficiency at 3.3VOUT
Light Load Efficiency at 5VOUT
Light Load Efficiency at 3.3VOUT
ꢀ00
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ.ꢁ
ꢀ00
ꢀ00
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
0
ꢀꢁ ꢂꢃꢄꢅ
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ.ꢁ
ꢀ.0
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
0.ꢀ
0
ꢀꢁꢁꢂꢃꢂꢀꢄꢃꢅ
ꢀꢁꢂꢃR ꢄꢁꢅꢅ
ꢀ
ꢀ
ꢀ
ꢀ ꢁꢂꢃ
ꢀ ꢁꢂꢃ
ꢀ ꢁꢂꢃ
ꢀ
ꢀ
ꢀ
ꢀ ꢁꢂꢃ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀ ꢁꢂꢃ
ꢀ ꢁꢂꢃ
ꢀ
ꢀ
ꢀ
ꢀ ꢁꢂꢃ
ꢀ ꢁꢂꢃ
ꢀ ꢁꢂꢃ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀ
ꢀ ꢁ00ꢂꢃꢄ
ꢀ
ꢀ ꢁ00ꢂꢃꢄ
ꢀꢁ
ꢀꢁ
ꢀ ꢁ ꢂꢃꢀꢄ0ꢄ0ꢅ ꢆ.ꢆꢇꢈ
ꢀ ꢁ ꢂꢃꢀꢄ0ꢄ0ꢅ ꢆ.ꢆꢇꢈ
ꢀ
ꢀ ꢁ00ꢂꢃꢄꢅ ꢆ ꢀ ꢇꢈꢆꢉ0ꢉ0ꢅ ꢊ.ꢊꢋꢃ
ꢀꢁꢂꢃꢄ ꢅꢆꢇe ꢈꢉꢊRꢋꢌꢍꢈꢎ
ꢀꢁ
ꢀꢁꢂꢃꢄ ꢅꢆꢇe ꢈꢉꢊRꢋꢌꢍꢈꢎ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ0
0.ꢀ
ꢀ
ꢀ0
ꢀ00
ꢀ000
ꢀ0000
0.ꢀ
ꢀ
ꢀ0
ꢀ00
ꢀ000
ꢀ0000
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢊꢂꢋ
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢊꢂꢋ
ꢀꢁꢂꢀꢃ ꢄ0ꢅ
ꢀꢁꢂꢀꢃ ꢄ0ꢅ
ꢀꢁꢂꢀꢃ ꢄ0ꢁ
Burst Mode Operation Efficiency
vs Inductor Value
Reference Voltage
Efficiency vs Frequency
ꢀ00
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀ0
ꢀꢀ
ꢀꢁ
ꢀꢁ
ꢀ00
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0ꢁ
ꢀ0ꢁ
ꢀ0ꢁ
ꢀ0ꢁ
ꢀ0ꢁ
ꢀꢁꢁ
ꢀꢁꢂ
ꢀꢁꢀ
ꢀꢁꢂ
ꢀꢁꢂ
ꢀ
ꢀ
ꢀ
ꢀ ꢁꢂꢃ
ꢀꢁ
ꢀꢁꢂ
ꢀ
ꢀ ꢁꢂ
ꢀ ꢁ.ꢁꢂ
ꢀꢁꢂ
ꢀ
ꢀ
ꢀꢁꢂꢃ
ꢀꢁꢂꢃ
ꢀꢁ
ꢀꢁ
ꢀ
ꢀ ꢁ0ꢂꢃ
ꢀꢁꢂꢃ
ꢀ ꢁꢂ
ꢀꢁꢂꢃ
ꢀ ꢁ ꢂꢃꢀꢄ0ꢄ0ꢅ ꢆ.ꢆꢇꢈ
ꢀ ꢁ ꢂꢃꢀꢄ0ꢄ0
0.ꢀ
0.ꢀ
ꢀ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢀ
0.ꢀ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ
ꢀꢁ0 ꢀꢁꢂ
0
ꢀꢁ ꢀ0 ꢀꢁ ꢀ00 ꢀꢁꢂ ꢀꢁ0
ꢀꢁꢂꢃꢄꢅꢂꢆꢇ ꢈRꢉꢊꢋꢉꢆꢄꢌ ꢍꢎꢅꢏꢐ
ꢀꢁꢂꢃꢄꢅꢆR ꢇꢈꢉꢃꢊ ꢋꢌꢍꢎ
ꢀꢁꢂꢃꢁRꢄꢀꢅRꢁ ꢆꢇꢈꢉ
ꢀꢁꢂꢀꢃ ꢄ0ꢅ
ꢀꢁꢂꢀꢃ ꢄ0ꢀ
ꢀꢁꢂꢀꢃ ꢄ0ꢅ
Rev. 0
5
For more information www.analog.com
LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
EN Pin Thresholds
Load Regulation
Line Regulation
ꢀ.00
0.ꢀꢀ
0.ꢀꢁ
0.ꢀꢁ
0.ꢀꢁ
0.ꢀꢁ
0.ꢀꢁ
0.ꢀꢁ
0.ꢀꢁ
0.ꢀ0
0.ꢀꢁ
ꢀ.00
0.ꢀꢁ
0.ꢀ0
0.ꢀꢁ
0
ꢀ
ꢀ
ꢀ ꢁꢂꢃ
ꢀꢁꢂ
ꢀꢁ
ꢀ ꢁꢂ
0.ꢀ0
ꢀꢁ Rꢂꢃꢂꢁꢄ
0.0ꢀ
0.00
ꢀ0.0ꢁ
ꢀ0.ꢁ0
ꢀ0.ꢁꢂ
ꢀ0.ꢁ0
ꢀ0.ꢁꢂ
ꢀ0.ꢁ0
ꢀ0.ꢁꢂ
ꢀꢁ.00
ꢀ
ꢀꢁꢂꢃ
ꢀ ꢁꢂ
ꢀꢁ ꢂꢃꢄꢄꢅꢁꢆ
ꢀꢁꢂ
ꢀ
ꢀ ꢁꢂ
ꢀꢁRꢂꢃꢄ ꢂꢁꢅꢆꢇꢅꢈꢁꢈꢉ ꢊꢁꢄꢃ
ꢀꢁꢂꢃꢄ ꢅꢆꢇe ꢈꢉꢊRꢋꢌꢍꢈꢎ
ꢀꢁ ꢂꢃꢄꢅ
ꢀ0 ꢀꢁ
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
ꢀꢁ0 ꢀꢁꢂ
0
ꢀꢁ ꢀ0 ꢀꢁ ꢀ00 ꢀꢁꢂ ꢀꢁ0
ꢀ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ0
ꢀꢁꢂꢃꢁRꢄꢀꢅRꢁ ꢆꢇꢈꢉ
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
ꢀꢁꢂꢀꢃ ꢄꢅ0
ꢀꢁꢂꢀꢃ ꢄꢅꢆ
ꢀꢁꢂꢀꢃ ꢄꢅꢅ
Top FET Current Limit vs
Temperature
No-Load Supply Current
Top FET Current Limit vs Duty Cycle
ꢀ0
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢀ
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀ00
ꢀꢁꢂ
ꢀꢁ0
ꢀꢁꢂ
ꢀ00
ꢀꢁ
ꢀ
ꢀ ꢁꢂ
ꢀꢁꢂ
ꢀ ꢁ ꢂꢃꢄ
ꢀꢁꢂꢃꢄ ꢅꢆꢇe ꢈꢉꢊRꢋꢌꢍꢈꢎ
ꢀꢁ Rꢂꢃꢄꢅꢆꢇꢀꢈꢁ
ꢀꢁ ꢂꢃ
ꢀ0
ꢀꢁ
0.ꢀ
0.ꢀ
0.ꢀ
0.ꢀ
0.ꢀ
ꢀꢁ0 ꢀꢁꢂ
0
ꢀꢁ ꢀ0 ꢀꢁ ꢀ00 ꢀꢁꢂ ꢀꢁ0
ꢀ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁꢂꢃ ꢄꢃꢄꢅꢆ
ꢀꢁꢂꢃꢁRꢄꢀꢅRꢁ ꢆꢇꢈꢉ
ꢀꢁꢂꢃꢄ ꢅꢆꢇꢄꢈꢉꢊ ꢋꢅꢌ
ꢀꢁꢂꢀꢃ ꢄꢅꢆ
ꢀꢁꢂꢀꢃ ꢄꢅꢆ
ꢀꢁꢂꢀꢃ ꢄꢅꢂ
Minimum On-Time
Switch RDS(ON) vs Temperature
Dropout Voltage
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ00
ꢀꢁ0
ꢀ00
ꢀꢁ0
ꢀ00
ꢀꢁ0
ꢀ00
ꢀ0
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀ
ꢀ
ꢀ ꢁꢂ
ꢀꢁꢂ
ꢀꢁ
ꢀꢁRꢂꢃꢄ ꢂꢁꢅꢆꢇꢅꢁꢈꢉ ꢊꢁꢄꢃ
ꢀꢁꢂꢃꢄ ꢅꢆꢇe ꢈꢉꢊRꢋꢌꢍꢈꢎ
ꢀ
ꢀꢁꢂ ꢂꢃ Rꢁꢄꢅꢆꢇꢂꢁ ꢇꢂ ꢈꢉ
ꢀ ꢁ ꢂꢃꢀꢄ0ꢄ0ꢅ ꢆꢇꢈ
ꢀꢁꢂ ꢃꢄꢅꢀꢆꢇ
ꢀꢁꢂꢂꢁꢃ ꢄꢅꢆꢂꢇꢈ
ꢀ
ꢀ 0.ꢁꢂ
ꢀꢁꢂꢃ
ꢀ ꢁꢂꢃꢄ
ꢀꢁꢂ
ꢀ
ꢀ ꢁꢂ
ꢀ
ꢀꢁ
0
0
ꢀꢁ0 ꢀꢁꢂ
0
ꢀꢁ ꢀ0 ꢀꢁ ꢀ00 ꢀꢁꢂ ꢀꢁ0
0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ0
ꢀꢁ0 ꢀꢁꢂ
0
ꢀꢁ ꢀ0 ꢀꢁ ꢀ00 ꢀꢁꢂ ꢀꢁ0
ꢀꢁꢂꢃꢁRꢄꢀꢅRꢁ ꢆꢇꢈꢉ
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
ꢀꢁꢂꢃꢁRꢄꢀꢅRꢁ ꢆꢇꢈꢉ
ꢀꢁꢂꢀꢃ ꢄꢅꢀ
ꢀꢁꢂꢀꢃ ꢄꢅꢆ
ꢀꢁꢂꢀꢃ ꢄꢅꢁ
Rev. 0
6
For more information www.analog.com
LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
Soft-Start Tracking
Switching Frequency
Burst Frequency
ꢀ.0ꢁ
ꢀ.0ꢁ
ꢀ.0ꢁ
ꢀ.0ꢀ
ꢀ.00
0.ꢀꢀ
0.ꢀꢁ
0.ꢀꢁ
0.ꢀꢁ
0.ꢀ
0.ꢀ
0.ꢀ
0.ꢀ
0
ꢀꢁ00
ꢀ000
ꢀ00
ꢀ00
ꢀ00
ꢀ00
0
R
ꢀ
ꢀ ꢁꢂ.ꢁꢃ
ꢀRꢁꢂꢃ ꢄꢅꢆꢇ ꢅꢄꢄꢈꢉꢊꢅꢃꢉꢁꢂ
ꢀ
ꢀ
ꢀ ꢁꢂꢃ
ꢀꢁ
ꢀꢁꢂ
ꢀ ꢁꢂ
ꢀꢁ0 ꢀꢁꢂ
0
ꢀꢁ ꢀ0 ꢀꢁ ꢀ00 ꢀꢁꢂ ꢀꢁ0
0
0.ꢀ 0.ꢀ 0.ꢀ 0.ꢀ ꢀ.0 ꢀ.ꢁ ꢀ.ꢁ
0
0.ꢀ
0.ꢀ
0.ꢀ
0.ꢀ
ꢀ
ꢀꢁꢂꢃꢁRꢄꢀꢅRꢁ ꢆꢇꢈꢉ
ꢀꢀ ꢁꢂꢃꢄꢅꢆꢇ ꢈꢁꢉ
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
ꢀꢁꢂꢀꢃ ꢄꢅꢆ
ꢀꢁꢂꢀꢃ ꢄꢅꢆ
ꢀꢁꢂꢀꢃ ꢄꢅ0
Soft-Start Current
Error Amp Output Current
PG Thresholds Above VREF
ꢀ.ꢁ
ꢀ.0
ꢀ.ꢁ
ꢀ.0
ꢀ.ꢁ
ꢀ.0
ꢀ.ꢁ
ꢀ.0
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢀ
ꢀ.ꢁ
ꢀ.0
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ00
ꢀ00
ꢀ
ꢀ 0.ꢁꢂ
ꢀꢀ
ꢀ00
ꢀ00
ꢀꢁ Rꢂꢃꢂꢄꢅ
0
ꢀꢁ00
ꢀꢁ00
ꢀꢁ00
ꢀꢁ00
ꢀꢁ ꢀꢂꢃꢃꢄꢅꢆ
ꢀ
ꢀ ꢁ.ꢂꢃꢄ
ꢀ
ꢀꢁ0 ꢀꢁꢂ
0
ꢀꢁ ꢀ0 ꢀꢁ ꢀ00 ꢀꢁꢂ ꢀꢁ0
ꢀꢁ0 ꢀꢁꢂ
0
ꢀꢁ ꢀ0 ꢀꢁ ꢀ00 ꢀꢁꢂ ꢀꢁ0
ꢀꢁ00
ꢀꢁ00
0
ꢀ00
ꢀ00
ꢀꢁꢂꢃꢁRꢄꢀꢅRꢁ ꢆꢇꢈꢉ
ꢀꢁꢂꢃꢁRꢄꢀꢅRꢁ ꢆꢇꢈꢉ
ꢀꢁ ꢂꢃꢄ ꢅRRꢆR ꢇꢆꢈꢉꢊꢋꢅ ꢌꢍꢇꢎ
ꢀꢁꢂꢀꢃ ꢄꢅꢆ
ꢀꢁꢂꢀꢃ ꢄꢅꢅ
ꢀꢁꢂꢀꢃ ꢄꢅꢂ
RT Programmed
Switching Frequency
PG Thresholds Below VREF
Minimum Input Voltage
ꢀ.0
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢀ
ꢀ.0
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢁ
ꢀ.ꢀ
ꢀꢁ.0
ꢀꢁ.ꢂ
ꢀꢁ.0
ꢀꢁ.ꢂ
ꢀꢁ.0
ꢀꢁ.ꢂ
ꢀꢁ.0
ꢀꢁ.ꢂ
ꢀꢁ Rꢂꢃꢂꢄꢅ
ꢀꢁ ꢀꢂꢃꢃꢄꢅꢆ
ꢀ
ꢀ ꢁꢂꢃꢄ
ꢀ
ꢀ ꢁꢂꢃꢄ
ꢀꢁ
ꢀꢁ
ꢀꢁ0 ꢀꢁꢂ
0
ꢀꢁ ꢀ0 ꢀꢁ ꢀ00 ꢀꢁꢂ ꢀꢁ0
ꢀꢁ0 ꢀꢁꢂ
0
ꢀꢁ ꢀ0 ꢀꢁ ꢀ00 ꢀꢁꢂ ꢀꢁ0
ꢀꢁ0 ꢀꢁꢂ
0
ꢀꢁ ꢀ0 ꢀꢁ ꢀ00 ꢀꢁꢂ ꢀꢁ0
ꢀꢁꢂꢃꢁRꢄꢀꢅRꢁ ꢆꢇꢈꢉ
ꢀꢁꢂꢃꢁRꢄꢀꢅRꢁ ꢆꢇꢈꢉ
ꢀꢁꢂꢃꢁRꢄꢀꢅRꢁ ꢆꢇꢈꢉ
ꢀꢁꢂꢀꢃ ꢄꢅꢆ
ꢀꢁꢂꢀꢃ ꢄꢅꢆ
ꢀꢁꢂꢀꢃ ꢄꢅꢁ
Rev. 0
7
For more information www.analog.com
LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
Bias Pin Current vs Switching
Frequency
Bias Pin Current vs Input Voltage
ꢀꢁ
ꢀꢁ
ꢀꢀ
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ ꢁꢂꢃ
ꢀ
ꢀ
ꢀ
ꢀ ꢁꢂ
ꢀꢁ
ꢀꢁꢂꢃ
ꢀꢁꢂ
ꢀꢁ
ꢀ ꢁꢂ
ꢀ ꢁꢂ
ꢀꢁꢂ
ꢀ ꢁꢂ
ꢀ ꢁꢂꢃꢄ
ꢀꢁ ꢂꢃꢄꢅ
ꢀꢁꢂꢃ
ꢀꢁ ꢂꢃꢄꢅ
ꢀ
ꢀ 0ꢁ
ꢀ ꢁꢂ
ꢀ
ꢀ
ꢀ 0ꢁ
ꢀ ꢁꢂ
ꢀꢁꢂ
ꢀꢁꢂ
ꢀꢁꢂ
ꢀ
ꢀꢁꢂ
0
ꢀ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
0
0.ꢀ
ꢀ
ꢀ.ꢁ
ꢀ
ꢀ.ꢁ
ꢀ
ꢀꢁꢂꢃꢄ ꢅꢆꢇꢄꢈꢉꢊ ꢋꢅꢌ
ꢀꢁꢂꢃꢄꢅꢂꢆꢇ ꢈRꢉꢊꢋꢉꢆꢄꢌ ꢍꢎꢅꢏꢐ
ꢀꢁꢂꢀꢃ ꢄꢅꢀ
ꢀꢁꢂꢀꢃ ꢄꢅꢆ
Case Temperature Rise
vs 12A Pulsed Load
Case Temperature Rise
ꢀꢁ0
ꢀꢁ0
ꢀꢀ0
ꢀ00
ꢀ00
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ ꢁꢂ ꢄ ꢅ ꢀ ꢁ00ꢂꢃꢄꢅ ꢆ ꢀ ꢇ.ꢇꢈꢃ
ꢀꢁꢂꢃꢂꢃꢄ ꢀꢅꢆꢇ ꢈꢇꢄRꢀ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀ ꢁꢂ ꢄ ꢅ ꢀ ꢁ00ꢂꢃꢄꢅ ꢆ ꢀ ꢇ.ꢇꢈꢃ
ꢀ
ꢀ
ꢀ
ꢀ ꢁꢂꢃ
ꢀ ꢁ.ꢁꢂ
ꢀ ꢁꢂꢃꢄ
ꢀꢁ
ꢀꢁ
ꢀꢁꢂ
ꢀꢁ
ꢀ ꢁꢂ ꢄ ꢅ ꢀ ꢁꢂꢃꢄꢅ ꢆ ꢀ 0.ꢇꢈꢉꢃ
ꢀꢁ
ꢀ ꢁꢂ ꢄ ꢅ ꢀ ꢁꢂꢃꢄꢅ ꢆ ꢀ 0.ꢇꢈꢉꢃ
ꢀꢁ
ꢀꢁꢂꢃꢄꢅꢆ ꢇꢈꢂꢄ ꢉ 0.ꢊꢋꢂ
ꢀ0 ꢀꢁꢂꢃ ꢄꢃꢅRꢀ ꢃꢆ ꢇꢈꢉꢊꢊ ꢅꢉR
ꢀꢁꢂꢃ ꢄꢅꢆꢇꢈꢉ ꢆꢊꢋꢉ ꢌ ꢀꢍꢋ
ꢀ ꢁ ꢂꢃꢀꢄ0ꢄ0ꢅ ꢂꢃꢀꢄ0ꢆ0
ꢀ0
ꢀ
ꢀ ꢁ.ꢁꢂ
ꢀꢁꢂ
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
0
0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ0
0
0.ꢀ
0.ꢀ
0.ꢀ
0.ꢀ
ꢀ
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
ꢀꢁꢂꢃ ꢄꢃꢄꢅꢆ ꢇꢈ ꢉꢊꢋ ꢅꢇꢋꢀ
ꢀꢁꢂꢀꢃ ꢄꢂ0
ꢀꢁꢂꢀꢃ ꢄꢂꢅ
Switch Rising Edge
CLKOUT Waveforms
ꢀ
ꢀꢁꢂꢃꢄꢅ
ꢀꢁꢂꢃꢄꢁ
ꢀ
ꢀ
ꢀꢁꢂꢃ
ꢀꢁ
ꢀꢁꢂꢃꢄꢁ
ꢀꢁꢂꢃꢄꢁ
Rꢀꢁꢂ ꢃꢀꢄꢂ ꢅ ꢆ.ꢇꢈꢉ
ꢀ
ꢀꢁ
ꢀ0ꢁꢂꢃꢄꢁ
ꢀꢁꢂꢀꢃ ꢄꢂꢅ
ꢀꢁꢂꢀꢃ ꢄꢂꢂ
ꢀꢁꢂꢃꢄꢅꢆ
ꢀ00ꢁꢂꢃꢄꢅꢆ
ꢀ
ꢀꢁꢂꢃ
ꢀ ꢁꢂꢃ
ꢀꢁ
ꢀ
ꢀ ꢁ00ꢂꢃꢄ
ꢀꢁ
ꢀ
ꢀ ꢁꢂ
ꢀꢁꢂꢃꢄꢅ ꢆ 0ꢇ
ꢀꢁꢂꢃꢄRꢅꢂꢆꢇꢈꢉꢆꢅꢂ ꢊꢅꢋꢌ
Rev. 0
8
For more information www.analog.com
LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Waveforms, Burst Mode
Operation
Switching Waveforms, Full
Frequency Continuous Operation
ꢀ
ꢀ
ꢀꢁꢂꢃꢄꢅ
ꢀ
ꢀ
ꢀꢁꢂꢃꢄꢅ
ꢀ
ꢀꢁ
ꢀ
ꢀꢁ
ꢀꢁꢂꢃꢄꢁ
ꢀꢁꢂꢃꢄꢁ
ꢀꢁꢂꢀꢃ ꢄꢂꢅ
ꢀꢁꢂꢀꢃ ꢄꢂꢅ
ꢀ00ꢁꢂꢃꢄꢅꢆ
ꢀ0ꢁꢂꢃꢄꢅꢆ
ꢀRꢁꢂꢃ ꢄꢅꢆꢇ ꢅꢄꢄꢈꢉꢊꢅꢃꢉꢁꢂ
ꢀRꢁꢂꢃ ꢄꢅꢆꢇ ꢅꢄꢄꢈꢉꢊꢅꢃꢉꢁꢂ
ꢀꢁꢂ ꢀꢁ ꢂꢃ
ꢀꢁ ꢂ0ꢃꢀ
ꢀꢁꢂ ꢀꢁ ꢂꢃ
ꢀꢁ ꢂꢀ
ꢀꢁ
ꢀꢁꢂ
ꢀꢁ
ꢀꢁꢂ
ꢀꢁꢂꢃꢄ ꢅꢆꢇe ꢈꢉꢊRꢋꢌꢍꢈꢎ
ꢀꢁ ꢂꢃꢄꢅ
Transient Response; 2.5A to 7.5A
Load Step
Transient Response; 100mA to
5.1A Load Step
ꢀ
ꢀꢁꢂꢃ
ꢀꢁꢂꢃꢄꢅ
ꢀ
ꢀꢁꢂꢃ
ꢀꢁꢂ
ꢀꢁꢂꢃꢄꢅ
ꢀ
ꢀꢁꢂ
ꢀ00ꢁꢂꢃꢄꢅꢂ
ꢀ
ꢀꢁꢂ
ꢀ00ꢁꢂꢃꢄꢅꢂ
ꢀꢁꢂꢃꢄ ꢅꢆꢇe ꢈꢉꢊRꢋꢌꢍꢈꢎ
ꢀꢁꢂꢀꢃ ꢄꢂꢅ
ꢀꢁꢂꢀꢃ ꢄꢂꢁ
ꢀ0ꢁꢂꢃꢄꢅꢆ
ꢀ0ꢁꢂꢃꢄꢅꢆ
ꢀRꢁꢂꢃ ꢄꢅꢆꢇ ꢅꢄꢄꢈꢉꢊꢅꢃꢉꢁꢂ
ꢀ00ꢁꢂ ꢃꢄ ꢅ.ꢀꢂ ꢆRꢂꢇꢈꢉꢊꢇꢆ
ꢀꢁꢂ ꢀ ꢁꢂ ꢀ ꢁ ꢀ ꢁꢂꢃꢄ
ꢀRꢁꢂꢃ ꢄꢅꢆꢇ ꢅꢄꢄꢈꢉꢊꢅꢃꢉꢁꢂ
ꢀ.ꢁꢂ ꢃꢄ ꢅ.ꢁꢂ ꢆRꢂꢇꢈꢉꢊꢇꢆ
ꢀꢁꢂ ꢀ ꢁꢂ ꢀ ꢁ ꢀ ꢁꢂꢃꢄ
ꢀꢁ
ꢀꢁꢂ ꢀꢁ
ꢀꢁ
ꢀꢁꢂ ꢀꢁ
ꢀ ꢀ ꢁꢁ0ꢂ ꢄ R ꢀ ꢁꢂ.ꢁꢃ
ꢀ ꢀ ꢁꢁ0ꢂ ꢄ R ꢀ ꢁꢂ.ꢁꢃ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ ꢁꢂꢃ ꢅ ꢆ
ꢀ ꢁꢂꢃꢄ
ꢀ
ꢀ ꢁꢂꢃ ꢅ ꢆ
ꢀ ꢁꢂꢃꢄ
ꢀꢁꢂ
ꢀꢁꢂꢃ
ꢀꢁꢂ
ꢀꢁꢂꢃ
Rev. 0
9
For more information www.analog.com
LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
Conducted EMI Performance
(CISPR25 Conducted Emission Test with Class 5 Peak Limits)
ꢀ0
ꢀ0
ꢀꢁꢂꢃꢄꢅꢆ ꢇꢆꢃꢈꢁꢉ
ꢀ0
ꢀ0
ꢀ0
ꢀ0
0
ꢀꢁꢂꢃꢃ ꢄ ꢅꢆꢂꢇ ꢁꢈꢉꢈꢊ
ꢀꢁ0
ꢀꢁ0
ꢀꢁRꢂꢃꢄ ꢀꢁꢂꢅꢆRꢇꢈ ꢈꢉꢄꢂ
ꢀꢁꢂꢃꢄ ꢀRꢃꢅꢆꢃꢇꢈꢉ ꢊꢋꢄꢃ
0
ꢀ
ꢀ
ꢀ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀ0
ꢀRꢁꢂꢃꢁꢄꢅꢆ ꢇꢈꢉꢊꢋ
ꢀꢁꢂꢀꢃ ꢄꢂꢀ
ꢀꢁꢂꢃꢂꢃꢄ ꢀꢅꢆꢇ ꢈꢇꢄRꢀ
ꢀꢁꢂꢃꢄ ꢅꢆꢂ ꢇꢂꢈꢃꢅR ꢂꢉꢊꢃꢋꢈꢈꢅꢌꢍ
ꢀꢁꢂ ꢃꢄꢅꢆꢇ ꢇꢈ ꢉ.ꢉꢂ ꢈꢆꢇꢅꢆꢇ ꢊꢇ ꢀ0ꢊꢋ ꢌ ꢀ ꢁꢂꢃꢄ
ꢀꢁ
Radiated EMI Performance
(CISPR25 Radiated Emission Test with Class 5 Peak Limits)
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀ
ꢀꢁRꢂꢃꢄꢅꢆ ꢇꢈꢆꢅRꢃꢉꢅꢂꢃꢈꢊ
ꢀꢁꢂꢃ ꢄꢁꢅꢁꢆꢅꢇR
ꢀꢁꢂꢃꢃ ꢄ ꢅꢆꢂꢇ ꢁꢈꢉꢈꢊ
ꢀꢁRꢂꢃꢄ ꢀꢁꢂꢅꢆRꢇꢈ ꢈꢉꢄꢂ
ꢀꢁꢂꢃꢄ ꢀRꢃꢅꢆꢃꢇꢈꢉ ꢊꢋꢄꢃ
0
0
ꢀ00
ꢀ00
ꢀ00
ꢀ00
ꢀ00
ꢀ00
ꢀ00
ꢀ00
ꢀ00 ꢀ000
ꢀRꢁꢂꢃꢁꢄꢅꢆ ꢇꢈꢉꢊꢋ
ꢀꢁꢂꢀꢃ ꢄꢂꢅ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀꢁ
ꢀ0
ꢀ
ꢀꢁRꢂꢃꢁꢄꢅꢆꢇ ꢈꢁꢇꢆRꢂꢃꢆꢅꢂꢁꢄ
ꢀꢁꢂꢃ ꢄꢁꢅꢁꢆꢅꢇR
ꢀꢁꢂꢃꢃ ꢄ ꢅꢆꢂꢇ ꢁꢈꢉꢈꢊ
ꢀꢁRꢂꢃꢄ ꢀꢁꢂꢅꢆRꢇꢈ ꢈꢉꢄꢂ
ꢀꢁꢂꢃꢄ ꢀRꢃꢅꢆꢃꢇꢈꢉ ꢊꢋꢄꢃ
0
0
ꢀ00
ꢀ00
ꢀ00
ꢀ00
ꢀ00
ꢀ00
ꢀ00
ꢀ00
ꢀ00 ꢀ000
ꢀRꢁꢂꢃꢁꢄꢅꢆ ꢇꢈꢉꢊꢋ
ꢀꢁꢂꢀꢃ ꢄꢅ0
ꢀꢁꢂꢃꢂꢃꢄ ꢀꢅꢆꢇ ꢈꢇꢄRꢀ
ꢀꢁꢂꢃꢄ ꢅꢆꢂ ꢇꢂꢈꢃꢅR ꢂꢉꢊꢃꢋꢈꢈꢅꢌꢍ
ꢀꢁꢂ ꢃꢄꢅꢆꢇ ꢇꢈ ꢉ.ꢉꢂ ꢈꢆꢇꢅꢆꢇ ꢊꢇ ꢀ0ꢊꢋ ꢌ ꢀ ꢁꢂꢃꢄ
ꢀꢁ
Rev. 0
10
For more information www.analog.com
LT8638S
PIN FUNCTIONS
PHMODE (Pin 1): Pin determines the phase relationship
between the LT8638S’s internal clock and CLKOUT. Tie it to
GND for 2-phase operation, float the pin for 3-phase oper-
VIN (Pins 15–18): The VIN pins supply current to the
LT8638S internal circuitry and to the internal topside
power switch. These pins must be tied together and be
locally bypassed with a capacitor of 4.7µF or more. Be
sure to place the positive terminal of the input capacitor
as close as possible to the VIN pins, and the negative
capacitor terminal as close as possible to the GND pins.
ation, or tie it to INTV for 4-phase operation. See Block
CC
Diagram for internal pull-up and pull-down resistance.
BIAS (Pin 2): The internal regulator will draw current from
BIAS instead of V when BIAS is tied to a voltage higher
IN
than 3.1V. For output voltages of 3.3V to 25V this pin
NC (Pins 19): No Connect. This pin is not connected to
internal circuitry and can be tied anywhere on the PCB,
typically ground.
should be tied to V . If this pin is tied to a supply other
OUT
than V
use a 1µF local bypass capacitor on this pin.
OUT
If no supply is available, tie to GND. However, especially
for high input or high frequency applications, BIAS should
be tied to output or an external supply of 3.3V or above.
EN/UV (Pin 21): The LT8638S is shut down when this
pin is low and active when this pin is high. The hyster-
etic threshold voltage is 0.98V going up and 0.94V going
INTV (Pin 3): Internal 3.4V Regulator Bypass Pin. The
down. Tie to V if the shutdown feature is not used. An
CC
IN
internal power drivers and control circuits are powered
external resistor divider from V can be used to program
IN
from this voltage. Do not load the INTV pin with exter-
a VIN threshold below which the LT8638S will shut down.
CC
nal circuitry. INTV current will be supplied from BIAS
CC
RT (Pin 22): A resistor is tied between RT and ground to
set the switching frequency.
if BIAS > 3.1V, otherwise current will be drawn from V .
IN
Voltage on INTV will vary between 2.8V and 3.4V when
CC
CLKOUT (Pin 23): Output Clock Signal for PolyPhase
Operation. In forced continuous mode, spread spectrum,
and synchronization modes, the CLKOUT pin provides a
50% duty cycle square wave of the switching frequency.
The phase of CLKOUT with respect to the LT8638S’s
internal clock is determined by the state of the PHMODE
BIAS is between 3.0V and 3.6V. Place a low ESR ceramic
capacitor of at least 1µF from this pin to ground close to
the IC.
BST (Pin 4): This pin is used to provide a drive volt-
age, higher than the input voltage, to the topside power
switch. Place a 0.1µF boost capacitor as close as possible
to the IC.
pin. CLKOUT’s peak-to-peak amplitude is INTV to GND.
CC
In Burst Mode operation, the CLKOUT pin will be low. Float
this pin if the CLKOUT function is not used.
SW (Pins 5–8): The SW pins are the outputs of the inter-
nal power switches. Tie these pins together and connect
them to the inductor. This node should be kept small on
the PCB for good performance and low EMI.
SYNC/MODE (Pin 24): For the LT8638S, this pin programs
four different operating modes: 1) Burst Mode operation.
Tie this pin to ground for Burst Mode operation at low
output loads—this will result in low quiescent current.
2) Forced Continuous mode (FCM). This mode offers
fast transient response and full frequency operation
over a wide load range. Float this pin for FCM. When
floating, pin leakage currents should be <1µA. 3) Spread
spectrum mode. Tie this pin high to INTVCC (or >3V)
for forced continuous mode with spread spectrum
modulation. 4) Synchronization mode. Drive this pin with
a clock source to synchronize to an external frequency.
During synchronization the part will operate in forced
continuous mode.
GND (Pins 9–14, 20, Exposed Pad Pins 29–32): Ground.
Place the negative terminal of the input capacitor as close
to the GND pins as possible. The exposed pads should
be soldered to the PCB for good thermal performance. If
necessary due to manufacturing limitations Pins 29 to 32
may be left disconnected, however thermal performance
will be degraded.
Rev. 0
11
For more information www.analog.com
LT8638S
PIN FUNCTIONS
PG (Pin 25): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within 7.75% of the final regulation voltage, and there
are no fault conditions. PG is also pulled low when EN/
and the internal reference resumes control of the error
amplifier. An internal 2µA pull-up current from INTV
CC
on this pin allows a capacitor to program output voltage
slew rate. This pin is pulled to ground with an internal
200Ω MOSFET during shutdown and fault conditions; use
a series resistor if driving from a low impedance output.
This pin may be left floating if the soft-start feature is not
being used.
UV is below 1V, INTV has fallen too low, V is too low,
CC
IN
or thermal shutdown. PG is valid when V is above 2.8V.
IN
V (Pin 26): The V pin is the output of the internal error
C
C
amplifier. The voltage on this pin controls the peak switch
current. Tie an RC network from this pin to ground to
compensate the control loop.
FB (Pin 28): The LT8638S regulates the FB pin to 0.6V.
Connect the feedback resistor divider tap to this pin. Also,
connect a phase lead capacitor between FB and V
Typically, this capacitor is 4.7pF to 47pF.
.
OUT
SS (Pin 27): Output Tracking and Soft-Start Pin. This pin
allows user control of output voltage ramp rate during
start-up. A SS voltage below 1V forces the LT8638S to
regulate the FB pin to a function of the SS pin voltage. See
plot in the Typical Performance Characteristics section.
When SS is above 1V, the tracking function is disabled
Corner Pins: These pins are for mechanical support only
and can be tied anywhere on the PCB, typically ground.
Rev. 0
12
For more information www.analog.com
LT8638S
BLOCK DIAGRAM
V
IN
1±–18
C
IN2
10nF
V
IN
15–16
V
IN
C
IN1
C
IN3
–
+
10nF
INTERNAL 0.6V REF
SHDN
BIAS
3.4V
REG
2
3
R3
0.98V
EN/UV
+
–
OPT
INTV
CC
21
SLOPE COMP
R4
OPT
0.1μF
C
VCC
V
C
26
25
OSCILLATOR
200kHz TO 3MHz
C
F
R
C
ERROR
AMP
PG
C
BST
C
±±.±5ꢀ
4
+
+
–
BURST
DETECT
C
BST
V
OUT
M1
SW
5–8
SWITCH
LOGIC
AND
ANTI-SHOOT
THROUGH
L
SHDN
C
R1
PL
THERMAL SHDN
V
OUT
INTV UVLO
CC
C
OUT
V
IN
UVLO
R2
FB
28
29
M2
SHDN
THERMAL SHDN
UVLO
C
SS
OPT
2µA
GND
9–14, 20,
29–32
V
IN
SS
INTV
CC
R
T
CLKOUT
PHMODE
RT
22
24
PLL
23
INTV
INTV
CC
CC
60k
60k
SYNC/MODE
1
600k
600k
8638S BD
Rev. 0
13
For more information www.analog.com
LT8638S
OPERATION
The LT8638S is a monolithic, constant frequency, cur-
rent mode step-down DC/DC converter. An oscillator, with
frequency set using a resistor on the RT pin, turns on
the internal top power switch at the beginning of each
clock cycle. Current in the inductor then increases until
the top switch current comparator trips and turns off the
top power switch. The peak inductor current at which the
top switch turns off is controlled by the voltage on the
internal VC node. The error amplifier servos the VC node
The LT8638S can operate in forced continuous mode
(FCM) for fast transient response and full frequency oper-
ation over a wide load range. When in FCM the oscillator
operates continuously and positive SW transitions are
aligned to the clock. Negative inductor current is allowed.
The LT8638S can sink current from the output and return
this charge to the input in this mode, improving load step
transient response.
To improve EMI, the LT8638S can operate in spread spec-
trum mode. This feature varies the clock with a triangu-
lar frequency modulation of +24%. For example, if the
LT8638S’s frequency is programmed to switch at 2MHz,
spread spectrum mode will modulate the oscillator between
2MHz and approximately 2.5MHz. The SYNC/MODE pin
by comparing the voltage on the V pin with an internal
FB
0.6V reference. When the load current increases it causes
a reduction in the feedback voltage relative to the reference
leading the error amplifier to raise the VC voltage until the
average inductor current matches the new load current.
When the top power switch turns off, the synchronous
power switch turns on until the next clock cycle begins
or in Burst Mode operation, inductor current falls to zero.
If overload conditions result in more than 15.5A flowing
through the bottom switch, the next clock cycle will be
delayed until switch current returns to a safe level.
should be tied high to INTV (or >3V) to enable spread
CC
spectrum modulation with forced continuous mode.
To improve efficiency across all loads, supply current to
internal circuitry can be sourced from the BIAS pin when
biased at 3.3V or above. Else, the internal circuitry will draw
current from V . The BIAS pin should be connected to
IN
The “S” in LT8638S refers to the second generation silent
switcher technology. This technology allows fast switch-
ing edges for high efficiency at high switching frequen-
cies, while simultaneously achieving good EMI perfor-
mance. This includes the integration of ceramic capacitors
V
OUT
if the LT8638S output is programmed at 3.3V to 25V.
The V pin allows the loop compensation of the switch-
C
ing regulator to be optimized based on the programmed
switching frequency, allowing for a fast transient response.
The V and CLKOUT pins enable multiple LT8638S reg-
into the package for V (see Block Diagram). These caps
C
IN
ulators to run with interleaving phase shift, reducing the
amount of required input and output capacitors. The
PHMODE pin selects the phasing of CLKOUT for different
multiphase applications.
keep all the fast AC current loops small, which improves
EMI performance.
If the EN/UV pin is low, the LT8638S is shut down and
draws approximately 6µA from the input. When the
EN/UV pin is above 0.98V, the switching regulator will
become active.
Comparators monitoring the FB pin voltage will pull
the PG pin low if the output voltage varies more than
7.75% (typical) from the set point, or if a fault condition
is present.
To optimize efficiency at light loads, the LT8638S operates
in Burst Mode operation in light load situations. Between
bursts, all circuitry associated with controlling the out-
put switch is shut down, reducing the input supply cur-
rent to 125µA (BIAS = 0). In a typical application, 90µA
(VIN = 12V, BIAS = 5VOUT) will be consumed from the
input supply when regulating with no load. The SYNC/
MODE pin is tied low to use Burst Mode operation and can
be floated to use forced continuous mode (FCM). If a clock
is applied to the SYNC/MODE pin, the part will synchronize
to an external clock frequency and operate in FCM.
The oscillator reduces the LT8638S device’s operating
frequency when the voltage at the FB pin is low. This
frequency foldback helps to control the inductor current
when the output voltage is lower than the programmed
value which occurs during start-up or overcurrent condi-
tions. When a clock is applied to the SYNC/MODE pin, the
SYNC/MODE pin is floated, or held DC high, the frequency
foldback is disabled and the switching frequency will slow
down only during overcurrent conditions.
Rev. 0
14
For more information www.analog.com
LT8638S
APPLICATIONS INFORMATION
Low EMI PCB Layout
placing the capacitors adjacent to the V and GND pins.
IN
Capacitors with small case size such as 0603 are optimal
The LT8638S is specifically designed to minimize EMI
emissions and also to maximize efficiency when switch-
ing at high frequencies. For optimal performance the
due to lowest parasitic inductance.
The input capacitors, along with the inductor and out-
put capacitors, should be placed on the same side of the
circuit board, and their connections should be made on
that layer. Place a local, unbroken ground plane under the
application circuit on the layer closest to the surface layer.
The SW and BOOST nodes should be as small as possi-
ble. Finally, keep the FB and RT nodes small so that the
ground traces will shield them from the SW and BOOST
nodes. The exposed pads on the bottom of the package
should be soldered to the PCB to reduce thermal resis-
tance to ambient. To keep thermal resistance low, extend
the ground plane from GND as much as possible, and add
thermal vias to additional ground planes within the circuit
board and on the bottom side.
LT8638S should use multiple V bypass capacitors.
IN
Two small <1µF capacitors can be placed as close as pos-
sible to the LT8638S (C
, C
) and a third capaci-
OPT1 OPT2
tor with a larger value, 4.7µF or higher, should be placed
nearby.
See Figure 1 for a recommended PCB layout.
For more detail and PCB design files refer to the Demo
Board guide for the LT8638S.
Note that large, switched currents flow in the LT8638S VIN
and GND pins and the input capacitors. The loops formed
by the input capacitors should be as small as possible by
R
R
ꢑ
R R
ꢎ
ꢐ
ꢐ
ꢐ
ꢈ
ꢉꢉ
ꢏ
ꢋꢌꢍꢋ ꢎ0ꢏ
ꢀRꢁꢂꢃꢄ ꢅꢆꢇ
ꢅ
ꢆꢃ
ꢅꢆꢇ
ꢅ
ꢅꢆꢇ
ꢁꢂꢈ
ꢉꢆꢀꢃꢇꢊ ꢅꢆꢇꢉ
Figure 1. Recommended PCB Layout
Rev. 0
15
For more information www.analog.com
LT8638S
APPLICATIONS INFORMATION
Burst Mode Operation
of switching frequency when choosing an inductor. For
example, while a lower inductor value would typically be
used for a high switching frequency application, if high
light load efficiency is desired, a higher inductor value
should be chosen. See curve in Typical Performance
Characteristics.
To enhance efficiency at light loads, the LT8638S oper-
ates in low ripple Burst Mode operation, which keeps the
output capacitor charged to the desired output voltage
while minimizing the input quiescent current and min-
imizing output voltage ripple. In Burst Mode operation
the LT8638S delivers single small pulses of current to
the output capacitor followed by sleep periods where the
output power is supplied by the output capacitor. While
in sleep mode the LT8638S consumes 125µA.
While in Burst Mode operation the current limit of the
top switch is approximately 2A (as shown in Figure 3),
resulting in low output voltage ripple. Increasing the out-
put capacitance will decrease output ripple proportionally.
As load ramps upward from zero the switching frequency
will increase but only up to the switching frequency pro-
grammed by the resistor at the RT pin as shown in Figure 2.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 2) and the percentage
of time the LT8638S is in sleep mode increases, resulting
in much higher light load efficiency than for typical con-
verters. By maximizing the time between pulses, the qui-
escent current approaches 90µA for a typical application
when there is no output load. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider must be minimized as it
appears to the output as load current.
ꢀ
ꢀ
ꢀꢁꢂꢃꢄꢅ
ꢀ
ꢀꢁ
ꢀꢁꢂꢃꢄꢁ
ꢀꢁ00
ꢀꢁꢂꢀꢃ ꢄ0ꢂ
ꢀ0ꢁꢂꢃꢄꢅꢆ
ꢀRꢁꢂꢃ ꢄꢅꢆꢇ ꢅꢄꢄꢈꢉꢊꢅꢃꢉꢁꢂ
ꢀ000
ꢀ00
ꢀ00
ꢀ00
ꢀꢁꢂ ꢀꢁ ꢂꢃ
ꢀꢁ ꢂ0ꢃꢀ
ꢀꢁ
ꢀꢁꢂ
ꢀꢁꢂꢃꢄ ꢅꢆꢇe ꢈꢉꢊRꢋꢌꢍꢈꢎ
Figure 3. Burst Mode Operation
The output load at which the LT8638S reaches the pro-
grammed frequency varies based on input voltage, output
voltage and inductor choice. To select low ripple Burst
Mode operation, tie the SYNC/MODE pin below 0.7V (this
can be ground or a logic low output).
ꢀRꢁꢂꢃ ꢄꢅꢆꢇ ꢅꢄꢄꢈꢉꢊꢅꢃꢉꢁꢂ
ꢀ00
ꢀ
ꢀ ꢁꢂꢃ
ꢀꢁ
ꢀꢁꢂ
ꢀ
ꢀ ꢁꢂ
0
0
0.ꢀ
0.ꢀ
0.ꢀ
0.ꢀ
ꢀ
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
Forced Continuous Mode
ꢀꢁꢂꢀꢃ ꢄ0ꢅ
The LT8638S can operate in forced continuous mode
(FCM) for fast transient response and full frequency oper-
ation over a wide load range. When in FCM, the oscillator
operates continuously and positive SW transitions are
aligned to the clock. Negative inductor current is allowed
at light loads or under large transient conditions. The
LT8638S can sink current from the output and return
this charge to the input in this mode, improving load step
Figure 2. SW Frequency vs Load Information in
Burst Mode Operation
In order to achieve higher light load efficiency, more
energy must be delivered to the output during the sin-
gle small pulses in Burst Mode operation such that the
LT8638S can stay in sleep mode longer between each
pulse. This can be achieved by using a larger value induc-
tor (i.e., 4.7µH), and should be considered independent
Rev. 0
16
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LT8638S
APPLICATIONS INFORMATION
transient response (see Figure 4). At light loads, FCM
operation is less efficient than Burst Mode operation, but
may be desirable in applications where it is necessary to
keep switching harmonics out of the signal band. FCM
must be used if the output is required to sink current. To
enable FCM, float the SYNC/MODE pin. Leakage current
on this pin should be <1µA. See Block Diagram for internal
pull-up and pull-down resistance.
Synchronization
To synchronize the LT8638S oscillator to an external fre-
quency, connect a square wave to the SYNC/MODE pin.
The square wave amplitude should have valleys that are
below 0.7V and peaks above 1.5V (up to 6V) with a min-
imum on-time and off-time of 50ns.
The LT8638S will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will run forced continuous mode to maintain reg-
ulation. The LT8638S may be synchronized over a 200kHz
to 3MHz range. The RT resistor should be chosen to set
the LT8638S switching frequency equal to or below the
lowest synchronization input. For example, if the synchro-
nization signal will be 500kHz and higher, the RT should
be selected for 500kHz. The slope compensation is set
by the RT value, while the minimum slope compensation
required to avoid subharmonic oscillations is established
by the inductor size, input voltage and output voltage.
Since the synchronization frequency will not change the
slopes of the inductor current waveform, if the inductor
is large enough to avoid subharmonic oscillations at the
frequency set by RT, then the slope compensation will be
sufficient for all synchronization frequencies.
ꢀ
ꢀꢁꢂꢃ
ꢀꢁꢂꢃꢄꢅ
ꢀꢁꢂ
ꢀꢁꢂꢃꢄ ꢅꢆꢇe ꢈꢉꢊRꢋꢌꢍꢈꢎ
ꢀꢁꢂꢀꢃ ꢄ0ꢅ
ꢀ
ꢀꢁꢂ
ꢀ00ꢁꢂꢃꢄꢅꢂ
ꢀ0ꢁꢂꢃꢄꢅꢆ
ꢀRꢁꢂꢃ ꢄꢅꢆꢇ ꢅꢄꢄꢈꢉꢊꢅꢃꢉꢁꢂ
ꢀ00ꢁꢂ ꢃꢄ ꢅ.ꢀꢂ ꢆRꢂꢇꢈꢉꢊꢇꢆ
ꢀꢁꢂ ꢀ ꢁꢂ ꢀ ꢁ ꢀ ꢁꢂꢃꢄ
ꢀꢁ
ꢀꢁꢂ ꢀꢁ
ꢀ
ꢀ ꢀ ꢁꢁ0ꢂ ꢄ R ꢀ ꢁꢂ.ꢁꢃ
ꢀ
ꢀ
ꢀ ꢁꢂꢃ ꢅ ꢆ
ꢀ ꢁꢂꢃꢄ
ꢀꢁꢂ
ꢀꢁꢂꢃ
Figure 4. LT8638S Load Step Transient Response with
and without Forced Continuous Mode
FCM is disabled if the V pin is held above 37V or if the FB
IN
pin is held greater than 7.75% above the feedback reference
voltage. FCM is also disabled during soft-start until the
soft-start capacitor is fully charged. When FCM is disabled
in these ways, negative inductor current is not allowed and
the LT8638S operates in pulse-skipping mode.
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to E1.
Spread Spectrum Mode
V
0.6V
⎛
⎜
⎝
⎞
⎠
OUT
R1=R2
–1
(1)
⎟
The LT8638S features spread spectrum operation to
further reduce EMI emissions. To enable spread spec-
trum operation, the SYNC/MODE pin should be tied high
to INTVCC (or >3V). In this mode, triangular frequency
modulation is used to vary the switching frequency
between the value programmed by RT to approximately
24% higher than that value. The modulation frequency is
approximately 3kHz. For example, when the LT8638S is
programmed to 2MHz, the frequency will vary from 2MHz
to approximately 2.5MHz at a 3kHz rate. When spread
spectrum operation is selected, Burst Mode operation is
disabled, and the part will run in forced continuous mode.
Reference designators refer to the Block Diagram.
1% resistors are recommended to maintain output
voltage accuracy.
When using large FB resistors, a 4.7pF to 47pF phase-lead
capacitor should be connected from V
to FB.
OUT
Setting the Switching Frequency
The LT8638S uses a constant frequency PWM architec-
ture that can be programmed to switch from 200kHz to
3MHz by using a resistor tied from the RT pin to ground. A
Rev. 0
17
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LT8638S
APPLICATIONS INFORMATION
table showing the necessary R value for a desired switch-
drops (~0.2V, ~0.08V, respectively at maximum load)
and t is the minimum top switch on-time (see the
T
ing frequency is in Table 1.
ON(MIN)
Electrical Characteristics). Equation 3 shows that a slower
switching frequency is necessary to accommodate a high
IN OUT
The R resistor required for a desired switching frequency
T
can be calculated using Equation 2.
V /V
ratio.
44.8
(2)
RT =
–5.9
For transient operation, V may go as high as the abso-
IN
fSW
lute maximum rating of 42V regardless of the R value,
T
however the LT8638S will reduce switching frequency
as necessary to maintain control of inductor current to
assure safe operation.
where R is in kΩ and f is the desired switching fre-
T
SW
quency in MHz.
Table 1. SW Frequency vs RT Value
The LT8638S is capable of a maximum duty cycle of
f
(MHz)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
3.0
R (kΩ)
SW
T
approximately 99%, and the V -to-V
dropout is lim-
OUT
ited by the RDS(ON) of the topINswitch. In this mode the
LT8638S skips switch cycles, resulting in a lower switch-
ing frequency than programmed by RT.
226
143
105
82.5
66.5
56.2
48.7
38.3
31.6
26.1
22.1
19.1
16.9
15.4
10.5
For applications that cannot allow deviation from the pro-
grammed switching frequency at low V /V
ratios use
IN OUT
Equation 4 to set switching frequency.
VOUT +VSW(BOT)
V
=
– VSW(BOT) +VSW(TOP) (4)
IN(MIN)
1– fSW •tOFF(MIN)
where VIN(MIN) is the minimum input voltage without
skipped cycles, V
SW(BOT)
is the output voltage, V
and
SW(TOP)
V
are theOinUtTernal switch drops (~0.2V, ~0.08V,
respectively at maximum load), fSW is the switching
frequency (set by RT), and tOFF(MIN) is the minimum
switch off-time. Note that higher switching frequency will
increase the minimum input voltage below which cycles
will be dropped to achieve higher duty cycle.
Operating Frequency Selection and Trade-Offs
Selection of the operating frequency is a trade-off
between efficiency, component size, and input voltage
range. The advantage of high frequency operation is that
smaller inductor and capacitor values may be used. The
disadvantages are lower efficiency and a smaller input
voltage range.
Inductor Selection and Maximum Output Current
The LT8638S is designed to minimize solution size by
allowing the inductor to be chosen based on the output
load requirements of the application. During overload
or short-circuit conditions the LT8638S safely tolerates
operation with a saturated inductor through the use of a
high speed peak-current mode architecture.
The highest switching frequency (f
application can be calculated as follows:
) for a given
SW(MAX)
VOUT +VSW(BOT)
A good first choice for the inductor value is given by
Equation 5.
fSW(MAX)
=
(3)
tON(MIN) V – VSW(TOP) +VSW(BOT)
IN
V
OUT +VSW(BOT)
⎛
⎞
where V is the typical input voltage, V
is the output
L=
•0.2
(5)
voltage,INV
and V
are the internal switch
OUT
⎜
⎟
fSW
⎝
⎠
SW(TOP)
SW(BOT)
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LT8638S
APPLICATIONS INFORMATION
where fSW is the switching frequency in MHz, VOUT is
the switch current limit, the inductor value, and the input
and output voltages. The inductor value may have to be
increased if the inductor ripple current does not allow
the output voltage, V
is the bottom switch drop
SW(BOT)
(~0.08V) and L is the inductor value in µH.
sufficient maximum output current (I
) given the
OUT(MAX)
To avoid overheating and poor efficiency, an inductor must
be chosen with an RMS current rating that is greater than
the maximum expected output load of the application.
In addition, the saturation current (typically labeled I
rating of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current (Equation 6)
switching frequency, and maximum input voltage used in
the desired application.
)
In order to achieve higher light load efficiency, more
energy must be delivered to the output during the sin-
gle small pulses in Burst Mode operation such that the
LT8638S can stay in sleep mode longer between each
pulse. This can be achieved by using a larger value induc-
tor (i.e., 4.7µH), and should be considered independent
of switching frequency when choosing an inductor. For
example, while a lower inductor value would typically be
used for a high switching frequency application, if high
light load efficiency is desired, a higher inductor value
should be chosen. See curve in Typical Performance
Characteristics.
SAT
1
IL(PEAK) =ILOAD(MAX) + ΔIL
(6)
2
where ∆I is the inductor ripple current as calculated in
L
Equation 8 and I
for a given application.
is the maximum output load
LOAD(MAX)
As a quick example, an application requiring 3A output
should use an inductor with an RMS rating of greater than
3A and an I
of greater than 4A. During long duration
overload orSsAhTort-circuit conditions, the inductor RMS
rating requirement is greater to avoid overheating of the
inductor. To keep the efficiency high, the series resistance
(DCR) should be less than 8mΩ, and the core material
should be intended for high frequency applications.
The optimum inductor for a given application may differ
from the one indicated by this design guide. A larger value
inductor provides a higher maximum load current and
reduces the output voltage ripple. For applications requir-
ing smaller load currents, the value of the inductor may
be lower and the LT8638S may operate with higher ripple
current. This allows use of a physically smaller inductor,
or one with a lower DCR resulting in higher efficiency. Be
aware that low inductance may result in discontinuous
mode operation, which further reduces maximum load
current.
The LT8638S limits the peak switch current in order
to protect the switches and the system from overload
faults. The top switch current limit (I ) is 20A at low
duty cycles and decreases linearly toLI1M5A at DC = 0.8.
The inductor value must then be sufficient to supply the
desired maximum output current (I
), which is a
OUT(MAX)
For more information about maximum output current and
discontinuous operation, see Analog Devices’ Application
Note 44.
function of the switch current limit (I ) and the ripple
LIM
current (Equation 7).
ΔIL
2
IOUT(MAX) =ILIM
–
(7)
For duty cycles greater than 50% (VOUT/VIN > 0.5), a
minimum inductance is required to avoid subharmonic
oscillation (Equation 9). See Application Note 19 for
more details.
The peak-to-peak ripple current in the inductor can be
calculated using Equation 8.
V (2•DC−1)
⎛
⎞
VOUT
L•fSW
VOUT
V
IN(MAX)
IN
LMIN
=
(9)
ΔIL =
• 1–
(8)
⎜
⎟
5•fSW
⎝
⎠
where DC is the duty cycle ratio (V /V ) and f is the
OUT IN
SW
where fSW is the switching frequency of the LT8638S, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8638S will deliver depends on
switching frequency.
Rev. 0
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LT8638S
APPLICATIONS INFORMATION
Input Capacitors
the addition of a feedforward capacitor placed between
and FB. Increasing the output capacitance will also
decrease the output voltage ripple. A lower value of output
capacitor can be used to save space and cost but transient
performance will suffer and may cause loop instability.
See the Typical Applications in this data sheet for sug-
gested capacitor values.
V
OUT
The V of the LT8638S should be bypassed with at least
IN
three ceramic capacitors for best performance. Two small
ceramic capacitors of <1µF can be placed close to the part
(C
, C
). These capacitors should be 0402 or 0603
in OsPizTe1. FOoPrTa2utomotive applications requiring 2 series
input capacitors, two small 0402 or 0603 may be placed
at each side of the LT8638S near the V and GND pins.
When choosing a capacitor, special attention should be
given to the data sheet to calculate the effective capaci-
tance under the relevant operating conditions of voltage
bias and temperature. A physically larger capacitor or one
with a higher voltage rating may be required.
IN
A third, larger ceramic capacitor of 4.7µF or larger should
be placed close to COPT1 or COPT2. See Low EMI PCB
Layout section for more detail. X7R or X5R capacitors are
recommended for best performance across temperature
and input voltage variations.
Ceramic Capacitors
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8638S due to their piezoelectric
nature. When in Burst Mode operation, the LT8638S’s
switching frequency depends on the load current, and
at very light loads the LT8638S can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT8638S operates at a lower current limit during
Burst Mode operation, the noise is typically very quiet to a
casual ear. If this is unacceptable, use a high performance
tantalum or electrolytic capacitor at the output. Low noise
ceramic capacitors are also available.
A ceramic input capacitor combined with trace or
cable inductance forms a high quality (under damped)
tank circuit. If the LT8638S circuit is plugged into a live
supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT8638S device’s voltage
rating. This situation is easily avoided (see Analog Devices
Application Note 88).
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8638S. As
previously mentioned, a ceramic input capacitor com-
bined with trace or cable inductance forms a high qual-
ity (underdamped) tank circuit. If the LT8638S circuit is
plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8638S
device’s rating. This situation is easily avoided (see Analog
Devices Application Note 88).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT8638S to produce the DC output. In this role it
determines the output ripple, thus low impedance at the
switching frequency is important. The second function is
to store energy in order to satisfy transient loads and sta-
bilize the LT8638S device’s control loop. Ceramic capaci-
tors have very low equivalent series resistance (ESR) and
provide the best ripple performance. For good starting
values, see the Typical Applications section.
Enable Pin
The LT8638S is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 0.98V, with 40mV of hysteresis. The EN pin
Use X5R or X7R types. This choice will provide low output
ripple and good transient response. Transient performance
can be improved with a higher value output capacitor and
can be tied to V if the shutdown feature is not used, or
IN
tied to a logic level if shutdown control is required.
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APPLICATIONS INFORMATION
Adding a resistor divider from V to EN programs the
the BIAS pin is below 3.0V, the internal LDO will consume
IN
LT8638S to regulate the output only when V is above
current from V . Applications with high input voltage and
IN
IN
a desired voltage (see the Block Diagram). Typically, this
high switching frequency where the internal LDO pulls
current from VIN will increase die temperature because
of the higher power dissipation across the LDO. Do not
threshold, V , is used in situations where the input
IN(EN)
supply is current limited, or has a relatively high source
resistance. A switching regulator draws constant power
from the source, so source current increases as source
voltage drops. This looks like a negative resistance load
to the source and can cause the source to current limit or
connect an external load to the INTV pin.
CC
Frequency Compensation
Loop compensation determines the stability and transient
performance, and is provided by the components tied to
latch low under low source voltage conditions. The V
IN(EN)
threshold prevents the regulator from operating at source
voltages where the problems might occur. This threshold
can be adjusted by setting the values R3 and R4 such that
they satisfy Equation 10.
the V pin. Generally, a capacitor (C ) and a resistor (R )
C
C
C
in series to ground are used. Designing the compensation
network is a bit complicated and the best values depend
on the application. A practical approach is to start with
one of the circuits in this data sheet that is similar to
your application and tune the compensation network to
optimize the performance. LTspice® or LTpowerCAD® sim-
ulations can help in this process. Stability should then be
checked across all operating conditions, including load
current, input voltage and temperature. The LT1375 data
sheet contains a more thorough discussion of loop com-
pensation and describes how to test the stability using a
transient load.
⎛
⎞
R3
R4
V
=
+1 •0.98V
(10)
IN(EN) ⎜
⎟
⎝
⎠
where the LT8638S will remain off until VIN is above
VIN(EN). Due to the comparator’s hysteresis, switching
will not stop until the input falls slightly below V
.
IN(EN)
When operating in Burst Mode operation for light load
currents, the current through the VIN(EN) resistor network
can easily be greater than the supply current consumed
Figure 5 shows an equivalent circuit for the LT8638S
control loop. The error amplifier is a transconductance
amplifier with finite output impedance. The power section,
consisting of the modulator, power switches, and inductor,
is modeled as a transconductance amplifier generating an
by the LT8638S. Therefore, the V
resistors should
IN(EN)
be large to minimize their effect on efficiency at low loads.
INTV Regulator
CC
An internal low dropout (LDO) regulator produces the
3.4V supply from VIN that powers the drivers and the
internal bias circuitry and must be bypassed to ground
with a minimum of 1μF ceramic capacitor. The INTV can
supply enough current for the LT8638S device’s cirCcCuitry.
To improve efficiency the internal LDO can also draw cur-
rent from the BIAS pin when the BIAS pin is at 3.1V or
higher. Typically the BIAS pin can be tied to the output of
the LT8638S, or can be tied to an external supply of 3.3V
or above. If BIAS is connected to a supply other than
output current proportional to the voltage at the V pin.
C
Note that the output capacitor integrates this current, and
that the capacitor on the V pin (C ) integrates the error
C
C
amplifier output current, resulting in two poles in the loop.
A zero is required and comes from a resistor R in series
C
with C . This simple model works well as long as the value
of theCinductor is not too high and the loop crossover
frequency is much lower than the switching frequency. A
phase lead capacitor (C ) across the feedback divider can
be used to improve thePtrLansient response and is required
to cancel the parasitic pole caused by the feedback node
to ground capacitance.
V
, be sure to bypass with a local ceramic capacitor. If
OUT
Rev. 0
21
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LT8638S
APPLICATIONS INFORMATION
will regulate to the internal reference voltage. The SS pin
may be left floating if the function is not needed.
ꢔꢌꢕꢖꢗꢕꢇ
ꢁꢉRRꢊꢋꢌ ꢍꢎꢏꢊ
ꢐꢎꢑꢊR ꢇꢌꢒꢓꢊ
An active pull-down circuit is connected to the SS pin
which will discharge the external soft-start capacitor in
the case of fault conditions and restart the ramp when the
faults are cleared. Fault conditions that clear the soft-start
ꢎꢉꢌꢐꢉꢌ
capacitor are the EN/UV pin transitioning low, V voltage
IN
ꢁ
ꢐꢔ
Rꢅ
Rꢆ
ꢂ
ꢃ
ꢄ ꢅꢆꢇ
falling too low, or thermal shutdown.
ꢂ
ꢃ
ꢄ ꢅ.ꢈꢃꢇ
ꢚꢛ
ꢁ
ꢎꢉꢌ
ꢀ
ꢁ
ꢝ
Multiphase Operation
ꢜ
0.ꢖꢀ
R
ꢁ
ꢘ00ꢙ
For output loads that demand more current, multiple
LT8638S devices can be connected in parallel to the
same output. To do this, the VC and FB pins are con-
nected together, and each LT8638S’s SW node is con-
nected to the common output through its own inductor.
The CLKOUT signal can be connected to the SYNC/MODE
pin of the following LT8638S to line up both the frequency
and the phase of the entire system. Tying the PHMODE
ꢁ
ꢚ
ꢁ
ꢁ
ꢕꢖꢗꢕꢇ ꢚ0ꢘ
Figure 5. Model for Loop Response
Table 2 provides a guidance for the compensation val-
ues of several typical applications. Slight tweaks to these
values may be required depending on the specific appli-
cation. All applications were using R1 = 100k
pin to GND, INTV , or floating the pin generates a phase
CC
difference between the LT8638S device’s internal clock
and CLKOUT of 180 degrees, 90 degrees, or 120 degrees
respectively, which corresponds to 2-phase, 4-phase, or
3-phase operation. A total of 12 phases can be paralleled
to run simultaneously with interleaving phase shift with
respect to each other by programming the PHMODE pin
of each LT8638S to different voltage levels. During FCM,
Spread Spectrum, and Synchronization modes, all devices
will operate at the same frequency. Figure 6 shows a
2-phase application where two LT8638Ss are paralleled
to get one output capable of up to 20A.
Table 2. Compensation Values
V
f
C
R
C
C
C
PL
OUT
SW
C
OUT
3.3V
3.3V
5V
400k
2M
820pF
220pF
820pF
220pF
8.87k
12.1k
9.31k
13.7k
47μF ×3
47μF ×2
47μF ×3
47μF
33pF
15pF
33pF
10pF
400k
2M
5V
Output Voltage Tracking and Soft-Start
he LT8638S allows the user to program its output
T
voltage ramp rate by means of the SS pin. An internal
2µA pulls up the SS pin to INTVCC. Putting an external
capacitor on SS enables soft starting the output to prevent
current surge on the input supply. During the soft-start
ramp the output voltage will proportionally track the SS
pin voltage. For output tracking applications, SS can be
externally driven by another voltage source. From 0V to
1V, the SS voltage will override the internal 0.6V refer-
ence input to the error amplifier, thus regulating the FB
pin voltage to a function of the SS pin. See plot in the
Typical Performance Characteristics section. When SS is
above 1V, tracking is disabled and the feedback voltage
ꢉꢒꢓꢊꢔꢕ
ꢂꢁ
ꢍ
ꢊꢋꢌ
ꢍ
ꢀ
ꢇꢏ
ꢃ0ꢎ
ꢂꢌꢄꢅꢆꢄꢇ
ꢀ
ꢉꢂ
ꢀ
ꢊꢋꢌ
Rꢁ
Rꢃ
ꢀꢂꢑꢊꢋꢌ
ꢈꢐ
ꢇꢇ
ꢇꢇ
R
ꢀ
ꢀ
ꢀ
ꢂꢌꢄꢅꢆꢄꢇ
ꢇꢖꢗꢀꢘꢓꢊꢔꢕ
ꢈꢐ
ꢂꢃ
ꢍ
ꢀ
ꢇꢏ
ꢄꢅꢆꢄꢇ ꢈ0ꢅ
Figure 6. Paralleling Two LT8638S devices
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APPLICATIONS INFORMATION
Output Power Good
µA in this state. If the EN pin is grounded the SW pin
current will drop to near 6µA. However, if the V pin is
IN
When the LT8638S device’s output voltage is within the
7.75% window of the regulation point, the output voltage
is considered good and the open-drain PG pin goes high
impedance and is typically pulled high with an external
resistor. Otherwise, the internal pull-down device will pull
the PG pin low. To prevent glitching both the upper and
lower thresholds include 0.4% of hysteresis. PG is valid
grounded while the output is held high, regardless of
EN, parasitic body diodes inside the LT8638S can pull
current from the output through the SW pin and the
VIN pin. Figure 7 shows a connection of the VIN and
EN/UV pins that will allow the LT8638S to run only when
the input voltage is present and that protects against a
shorted or reversed input.
when V is above 2.8V.
IN
ꢃꢄ
The PG pin is also actively pulled low during several fault
ꢀ
ꢁꢂ
ꢀ
ꢁꢂ
conditions: EN/UV pin is below 0.98V, INTV has fallen
CC
ꢅꢆꢇꢈꢉꢇꢊ
ꢋꢂꢌꢍꢀ
ꢐꢂꢃ
too low, V is too low, or thermal shutdown.
IN
Shorted and Reversed Input Protection
ꢇꢈꢉꢇꢊ ꢎ0ꢏ
The LT8638S will tolerate a shorted output. Several fea-
tures are used for protection during output short-cir-
cuit and brownout conditions. The first is the switching
frequency will be folded back while the output is lower
than the set point to maintain inductor current control.
Second, the bottom switch current is monitored such that
if inductor current is beyond safe levels switching of the
top switch will be delayed until such time as the inductor
current falls to safe levels.
Figure 7. Reverse VIN Protection
Thermal Considerations and Peak Output Current
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT8638S. The ground pins on the bottom of the package
should be soldered to a ground plane. This ground should
be tied to large copper layers below with thermal vias;
these layers will spread heat dissipated by the LT8638S.
Placing additional vias can reduce thermal resistance
further. The maximum load current should be derated
as the ambient temperature approaches the maximum
junction rating. Power dissipation within the LT8638S
can be estimated by calculating the total power loss from
an efficiency measurement and subtracting the inductor
loss. The die temperature is calculated by multiplying the
LT8638S power dissipation by the thermal resistance
from junction to ambient.
Frequency foldback behavior depends on the state of the
SYNC pin: If the SYNC pin is low the switching frequency
will slow while the output voltage is lower than the pro-
grammed level. If the SYNC pin is connected to a clock
source, floated or tied high, the LT8638S will stay at the
programmed frequency without foldback and only slow
switching if the inductor current exceeds safe levels.
There is another situation to consider in systems where
the output will be held high when the input to the LT8638S
is absent. This may occur in battery charging applica-
tions or in battery-backup systems where a battery or
some other supply is diode ORed with the LT8638S
The internal overtemperature protection monitors the
junction temperature of the LT8638S. If the junction
temperature reaches approximately 175°C, the LT8638S
will stop switching and indicate a fault condition until the
temperature drops about 10°C cooler.
device’s output. If the V pin is allowed to float and the
IN
EN pin is held high (either by a logic signal or because
it is tied to VIN), then the LT8638S device’s internal
circuitry will pull its quiescent current through its SW
pin. This is acceptable if the system can tolerate several
Rev. 0
23
For more information www.analog.com
LT8638S
APPLICATIONS INFORMATION
Temperature rise of the LT8638S is worst when operating
loads for short periods of time. This time is determined by
how quickly the case temperature approaches the maxi-
mum junction rating. Figure 9 shows an example of how
case temperature rise changes with the duty cycle of a
1kHz pulsed 12A load.
at high load, high V , and high switching frequency. If
IN
the case temperature is too high for a given application,
then either V , switching frequency, or load current can
IN
be decreased to reduce the temperature to an acceptable
level. Figure 8 shows examples of how case tempera-
ꢀ00
ꢀꢁꢂꢃꢂꢃꢄ ꢀꢅꢆꢇ ꢈꢇꢄRꢀ
ture rise can be managed by reducing V , switching fre-
IN
ꢀ0
ꢀ
ꢀ ꢁꢂꢃ
ꢀꢁ
ꢀꢁꢂ
ꢀꢁ
quency, or load.
ꢀ
ꢀ
ꢀ ꢁ.ꢁꢂ
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
0
ꢀ ꢁꢂꢃꢄ
ꢀꢁ0
ꢀꢁꢂꢃꢄꢅꢆ ꢇꢈꢂꢄ ꢉ 0.ꢊꢋꢂ
ꢀꢁꢂꢃ ꢄꢅꢆꢇꢈꢉ ꢆꢊꢋꢉ ꢌ ꢀꢍꢋ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ ꢁꢂ ꢄ ꢅ ꢀ ꢁ00ꢂꢃꢄꢅ ꢆ ꢀ ꢇ.ꢇꢈꢃ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ
ꢀꢁ0
ꢀꢀ0
ꢀ00
ꢀ ꢁꢂ ꢄ ꢅ ꢀ ꢁ00ꢂꢃꢄꢅ ꢆ ꢀ ꢇ.ꢇꢈꢃ
ꢀꢁ
ꢀ ꢁꢂ ꢄ ꢅ ꢀ ꢁꢂꢃꢄꢅ ꢆ ꢀ 0.ꢇꢈꢉꢃ
ꢀꢁ
ꢀ ꢁꢂ ꢄ ꢅ ꢀ ꢁꢂꢃꢄꢅ ꢆ ꢀ 0.ꢇꢈꢉꢃ
ꢀꢁ
ꢀ0 ꢀꢁꢂꢃ ꢄꢃꢅRꢀ ꢃꢆ ꢇꢈꢉꢊꢊ ꢅꢉR
ꢀ ꢁ ꢂꢃꢀꢄ0ꢄ0ꢅ ꢂꢃꢀꢄ0ꢆ0
ꢀꢁꢂ
ꢀ0
ꢀ
ꢀ ꢁ.ꢁꢂ
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
ꢀ0
0
0
0.ꢀ
0.ꢀ
0.ꢀ
0.ꢀ
ꢀ
ꢀꢁꢂꢃ ꢄꢃꢄꢅꢆ ꢇꢈ ꢉꢊꢋ ꢅꢇꢋꢀ
ꢀꢁꢂꢀꢃ ꢄ0ꢅ
Figure 9. Case Temperature Rise vs 12A Pulsed Load
0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ0
ꢀꢁꢂꢃ ꢄꢅRRꢆꢇꢈ ꢉꢂꢊ
The LT8638S device’s top switch current limit decreases
with higher duty cycle operation for slope compensation.
This also limits the peak output current the LT8638S
can deliver for a given application. See curve in Typical
Performance Characteristics.
ꢀꢁꢂꢀꢃ ꢄ0ꢀ
Figure 8. Case Temperature Rise
The LT8638S’s internal power switches are capable of
safely delivering up to 12A of peak output current. However,
due to thermal limits, the package can only handle 12A
Rev. 0
24
For more information www.analog.com
LT8638S
TYPICAL APPLICATIONS
ꢀ
ꢀꢁ
ꢀ
ꢀꢁꢂ
ꢀꢁ
ꢀ.ꢁꢂ ꢃꢄ ꢁꢅꢂ
0.ꢀꢁꢂ
ꢀ.ꢀꢁꢂ
ꢀꢁꢂꢃꢄ
ꢀ.ꢁꢂꢃ
ꢀ
ꢀꢁꢂ
ꢀꢁꢂꢃꢄꢂꢅ
ꢀꢁ
ꢀꢁ
ꢀ0ꢁ
ꢀꢁꢂꢃ
ꢄꢄ
ꢀꢁꢂꢃꢄꢅ
ꢀꢁꢂꢃꢄꢅ
ꢀ00ꢁ
ꢀꢁ
1μF
ꢀꢁꢂꢃꢄꢅꢆꢇꢈ
ꢀ.ꢁꢂꢃ
ꢀꢁꢂꢃ
ꢄꢅ
ꢆꢇꢆ0
ꢈꢉRꢊꢈꢁR
ꢀꢁꢂꢃ
ꢀ
ꢁ
ꢀ00ꢁ
ꢀꢀꢁꢂ
ꢀꢀ
ꢀꢁ0ꢂꢃ
ꢀꢁ
Rꢀ
ꢀ0ꢁꢂ
ꢀꢁꢂ
ꢀꢁ.ꢂꢃ
ꢀ0ꢁꢂ
0
ꢀ
ꢀ ꢁ00ꢂꢃꢄ
ꢀꢁ
ꢀꢁ ꢂꢃꢀꢄ0ꢄ0
Figure 10. 400kHz 5V 10A Step-Down Converter with Soft-Start and Power Good
ꢀ
ꢀꢁ
ꢀ
ꢀꢁꢂ
ꢀꢁ
ꢀ.ꢁꢂ ꢃꢄ ꢅꢆꢂ
0.ꢀꢁꢂ
ꢀꢁꢂꢃꢄ
ꢀ.ꢁꢂꢃ
ꢀꢁꢂꢃꢄꢂꢅ
ꢀ.ꢀꢁꢂ
ꢀ
ꢀꢁꢂ
ꢀꢁ
ꢀ.ꢀꢁ
ꢀꢁꢂꢃ
ꢄꢄ
ꢀꢁꢂꢃꢄꢅ
ꢀꢁꢂꢃꢄꢅ
ꢀ0ꢁ
ꢀ00ꢁ
1μF
ꢀꢁ
ꢀꢁꢂꢃꢄꢅꢆꢇꢈ
ꢀ.ꢀꢁꢂ
ꢀꢁꢂꢃ
ꢀꢁꢂꢃ
ꢀ
ꢁ
ꢀ00ꢁ
ꢀꢀꢁꢂ
ꢄꢅ
ꢀꢀ
ꢆꢇꢆ0
ꢈꢉRꢊꢈꢁR
ꢀꢁ0ꢂꢃ
ꢀꢁ
Rꢀ
ꢀꢁꢂ
ꢀ0ꢁꢂ
ꢀꢀ.ꢁꢂ
ꢀ0ꢁꢂ
ꢀ
ꢀ ꢁ00ꢂꢃꢄ
ꢀꢁ
ꢀꢁ ꢂꢃꢀꢄ0ꢄ0
Figure 11. 400kHz 3.3V, 10A Step-Down Converter with Soft-Start and Power Good
ꢀꢁꢂ
ꢀꢁꢂꢃ
ꢀ
ꢀꢁ
ꢀ.ꢁꢂ ꢃꢄ ꢅꢆꢂ
ꢀ0ꢁꢂ
ꢀꢁꢀ0
ꢀ0ꢁꢂ
ꢀꢁꢀ0
ꢀ0ꢁꢂ
ꢃꢄ
ꢀꢁꢀ0
ꢀꢁꢂꢃꢄ
ꢀ
ꢀ
ꢀꢁ
ꢀꢁ
ꢀꢁꢂ
0ꢀ0ꢁ
ꢀꢁꢂ
0ꢀ0ꢁ
ꢀꢁꢂꢃ ꢂꢄꢅ ꢆꢃꢇꢈ ꢁꢂ
ꢅꢉꢁꢃ ꢊꢁRꢊꢆꢁꢅꢋ
ꢀꢁꢂ
ꢀꢁꢂ
ꢀꢁꢂꢃꢄ ꢆ ꢇꢈꢆ ꢉꢉꢆ
ꢇꢊꢋꢃꢌꢍ
ꢀꢁꢂ
ꢀꢁꢂꢃ
ꢄꢄ
0.ꢀꢁꢂ
ꢀꢁꢂꢃꢄꢂꢅ
0.ꢀꢁꢂꢃ
ꢀ
ꢀꢁꢂ
1μF
ꢀꢁꢂꢃ
ꢀꢁ
ꢀ.ꢀꢁ
ꢀꢀ
ꢀ0ꢁ
ꢀꢁꢂꢃꢄꢅꢆꢇꢈ
ꢀꢁꢂꢃ
ꢀꢁ.ꢀꢂ
ꢀꢁꢂꢃ
ꢄꢅ
ꢆꢅꢆ0
ꢇꢈRꢉꢇꢁR
ꢀ00ꢁ
ꢀꢁꢂꢃ
ꢀ
ꢁ
Rꢀ
ꢀꢁ
ꢀꢀ0ꢁꢂ
ꢀꢁꢂ
ꢀꢁ.ꢂꢃ
ꢀꢀ.ꢁꢂ
ꢀ
ꢀ ꢁꢂꢃꢄ
ꢀꢁ
L: XEL6030
FB1 BEAD: WE-MPSB 10Ω 10.5A 1206
Figure 12. Ultralow EMI 3.3V, 10A Step-Down Converter with Spread Spectrum
Rev. 0
25
For more information www.analog.com
LT8638S
TYPICAL APPLICATIONS
ꢀ
ꢀꢁ
ꢀ
ꢀꢁꢂ
ꢀꢁ
ꢀ.ꢁꢂ ꢃꢄ ꢁꢅꢂ
0.1μF
0.ꢀꢁꢂꢃ
ꢀ.ꢁꢂꢃ
ꢀꢁꢂꢃꢄ
ꢀꢁꢂꢃꢄꢂꢅ
ꢀ
ꢀꢁꢂ
ꢀꢁ
ꢀꢁ
ꢀꢁꢂꢃ
ꢀꢀ
ꢀꢁꢂꢃ ꢂꢄꢅ ꢆꢃꢇꢈ ꢁꢂ
ꢅꢉꢁꢃ ꢊꢁRꢊꢆꢁꢅꢋ
ꢀ0ꢁ
ꢀꢁꢂꢃꢄꢅꢆꢇꢈ
1μF
ꢀꢁꢂꢃ
ꢀꢁꢂꢃꢄ ꢆ ꢇꢈꢆ ꢉꢉꢆ
ꢇꢊꢋꢃꢌꢍ
ꢀꢁ.ꢂꢃ
ꢀ0ꢁꢂ
ꢀ00ꢁ
ꢀꢁꢂꢃ
ꢀ
Rꢀ
ꢁ
ꢀꢁ
ꢄꢅꢄ0
ꢆꢇRꢈꢆꢁR
ꢀꢁꢂ
ꢀꢀ0ꢁꢂ
ꢀꢁ.ꢂꢃ
ꢀꢁ.ꢂꢃ
ꢀꢁꢂꢀꢃ ꢄꢅꢂ
ꢀ
ꢀ ꢁꢂꢃꢄ
ꢀꢁ
ꢀꢁ ꢂꢃꢀꢄ0ꢄ0
Figure 13. 2MHz 5V, 10A Step-Down Converter with Spread Spectrum
ꢀ
ꢀꢁ
ꢀ
ꢀꢁꢂ
ꢀꢁ
ꢀ.ꢁꢂ ꢃꢄ ꢅꢆꢂ
ꢀ.ꢁꢂꢃ
0.ꢀꢁꢂ
0.ꢀꢁꢂꢃ
ꢀꢁꢂꢃꢄ
ꢀ
ꢀꢁꢂ
ꢀꢁ
ꢀ.ꢀꢁ
ꢀꢁꢂꢃ ꢂꢄꢅ ꢆꢃꢇꢈ ꢁꢂ
ꢅꢉꢁꢃ ꢊꢁRꢊꢆꢁꢅꢋ
ꢀꢁꢂꢃꢄ ꢆ ꢇꢈꢆ ꢉꢉꢆ
ꢇꢊꢋꢃꢌꢍ
ꢀꢁꢂꢃ
ꢀꢀ
ꢀ0ꢁ
ꢀꢁꢂꢃꢄꢅꢆꢇꢈ
1μF
ꢀꢁꢂꢃ
ꢀꢁ.ꢀꢂ
ꢀꢁꢂꢃꢄꢂꢅ
ꢀꢁꢂꢃ
ꢀ00ꢁ
ꢀꢁꢂꢃ
ꢄꢅ
ꢀ
ꢁ
ꢀꢁ
Rꢀ
ꢆꢅꢆ0
ꢇꢈRꢉꢇꢁR
ꢀꢁꢂ
ꢀꢀ0ꢁꢂ
ꢀꢀ.ꢁꢂ
ꢀꢁ.ꢂꢃ
ꢀꢁꢂꢀꢃ ꢄꢅꢆ
ꢀ
ꢀ ꢁꢂꢃꢄ
ꢀꢁ
ꢀꢁ ꢂꢃꢀꢄ0ꢅ0
Figure 14. 2MHz 3.3V, 10A Step-Down Converter with Spread Spectrum
V
IN
V
BST
IN
12.4V TO 42V
0.1μF
3.3µH
EN/UV
4.7µF
14.7k
V
12V
10A
OUT
SW
BIAS
INTV
CC
1μF
10pF
100k
LT8638S
V
C
47µF
x2
1210
X5R/X7R
RT
FB
PINS NOT USED IN THIS CIRCUIT:
CLKOUT,
470pF
38.3k
5.23k
GND
PG, SYNC/MODE, SS,
PHMODE
8638S F15
f
= 1MHz
SW
L: XAL1010
Figure 15. 1MHz 12V, 10A Step-Down Converter
Rev. 0
26
For more information www.analog.com
LT8638S
PACKAGE DESCRIPTION
ꢜ
ꢝ
ꢟ
ꢢ ꢄ ꢄ ꢄ
ꢮ ꢮ ꢮ
× ꢠ ꢎ
ꢟ
ꢟ
ꢟ
ꢪ ꢪ ꢥ ꢥ ꢥ
ꢟ
ꢏ . ꢠ ꢌ 0 0
0 . ꢑ ꢌ 0 0
0 . ꢠ ꢌ 0 0
0 . 0 0 0 0
0 . ꢠ ꢌ 0 0
0 . ꢑ ꢌ 0 0
ꢏ . ꢠ ꢌ 0 0
ꢞ ꢞ ꢞ
ꢟ
× ꢠ
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
27
subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
LT8638S
TYPICAL APPLICATIONS
V
IN
2.8V TO 22V
V
BST
IN
(42V TRANSIENT)
4.7µF
11.5k
0.1µF
EXTERNAL
0.33µH
V
1.8V
10A
OUT
EN/UV
SW
BIAS
SOURCE >3.1V
OR GND
INTV
CC
1µF
1µF
LT8638S
22pF
100k
V
C
47μF
x3
PINS NOT USED IN THIS CIRCUIT:
CLKOUT,
RT
FB
1210
X5R/X7R
PG, SYNC/MODE, SS,
PHMODE
470µF
16.9k
49.9k
GND
8638S F16
f
= 2MHz
SW
L: XAL6030
Figure 16. 2MHz 1.8V, 10A Step-Down Converter
RELATED PARTS
PART
DESCRIPTION
COMMENTS
= 3V to 42V, V
LT8648S
42V, 15A Synchronous Step-Down Silent Switcher 2
V
= 0.6V, I = 100µA, I = 6µA,
OUT(MIN) Q SD
IN
7mm × 4mm LQFN-36
LT8636
42V, 5A Synchronous Step-Down Silent Switcher with I = 2.5µA
V
= 3.4V to 42V, V
= 0.97V, I = 2.5µA, I < 1µA,
OUT(MIN) Q SD
Q
IN
4mm × 3mm LQFN-20
LT8640S/
LT8643S
42V, 6A Synchronous Step-Down Silent Switcher 2 with I = 2.5µA
V
Q
= 3.4V to 42V, V
= 0.97V,
OUT(MIN)
Q
IN
I = 2.5μA, I < 1μA, 4mm × 4mm LQFN-24
SD
LT8640/
LT8640-1
42V, 5A, 96% Efficiency, 3MHz Synchronous MicroPower Step-Down DC/
V
= 3.4V to 42V, V
= 0.97V, I = 2.5µA, I < 1µA,
OUT(MIN) Q SD
IN
DC Converter with I = 2.5μA
3mm × 4mm QFN-18
Q
LT8650S
LT8653S
LT8652S
42V, Dual 4A Synchronous Step-Down Silent Switcher 2 with I = 6.2µA
V
= 3V to 42V, V
= 0.8V, I = 6.2µA, I = 1.7µA,
Q
IN
OUT(MIN) Q SD
4mm × 6mm LQFN-32
42V, Dual 2A Synchronous Step-Down Silent Switcher 2 with I = 6.2µA
V
= 3V to 42V, V
= 0.8V, I = 6.2µA, I = 1.7µA,
OUT(MIN) Q SD
Q
IN
4mm × 3mm LQFN-20
18V, Dual 8.5A Synchronous Step-Down Silent Switcher 2 with I = 16µA
V
= 3V to 18V, V
= 0.6V, I = 16µA, I = 6µA,
OUT(MIN) Q SD
Q
IN
4mm × 7mm LQFN-36
LT8645S/
LT8646S
65V, 8A, Synchronous Step-Down Silent Switcher 2 with I = 2.5μA
V
= 3.4V to 65V, V
= 0.97V, I = 2.5µA, I < 1µA,
Q
IN
OUT(MIN)
Q
SD
4mm × 6mm LQFN-32
LT8641
65V, 3.5A, 95% Efficiency, 3MHz Synchronous MicroPower Step-Down
V
SD
= 3V, V
= 65V, V
= 0.81V, I = 2.5µA,
OUT(MIN) Q
IN(MIN)
IN(MAX)
DC/DC Converter with I = 2.5μA
I
< 1µA, 3mm × 4mm QFN-18
Q
LT8609S
42V, 2A Synchronous Step-Down Silent Switcher 2 with I = 2.5µA
V
= 3V to 42V, V
= 0.774V, I = 2.5µA, I < 1µA,
OUT(MIN) Q SD
Q
IN
3mm × 3mm LQFN-16
LT8609/
LT8609A
42V, 2A, 94% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
V
SD
= 3V to 42V, V
= 0.782V, I = 2.5µA,
IN
OUT(MIN) Q
DC/DC Converter with I = 2.5µA
I
< 1µA, MSOP-10E, 3mm × 3mm DFN-18
Q
LT8610A/
LT8610AB
42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
V
= 3.4V to 42V, V
= 0.97V, I = 2.5µA, I < 1µA,
OUT(MIN) Q SD
IN
DC/DC Converter with I = 2.5µA
MSOP-16E
Q
LT8602
42V, Quad Output (2.5A + 1.5A + 1.5A + 1.5A) 95% Efficiency, 2.2MHz
V
SD
= 3V to 42V, V
= 0.8V, I = 2.5µA,
IN
OUT(MIN) Q
Synchronous MicroPower Step-Down DC/DC Converter with I = 25µA
I
< 1µA, 6mm × 6mm QFN-40
Q
Rev. 0
04/21
www.analog.com
ANALOG DEVICES, INC. 2021
28
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