LT1612_1 [Linear]
Synchronous, Step-Down 800kHz PWM DC/DC Converter; 同步整流，降压型PWM 800kHz的DC / DC转换器
| 型号： | LT1612_1 |
| 厂家： | 
Linear |
| 描述： | Synchronous, Step-Down 800kHz PWM DC/DC Converter |
| 文件： | 总12页 (文件大小：248K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT1612
Synchronous, Step-Down
800kHz PWM
DC/DC Converter
U
DESCRIPTIO
FEATURES
■
The LT®1612 is an 800kHz, synchronous step-down DC/
DC converter that operates from an input voltage as low
as 2V. Internal 0.45Ω switches deliver output currents up
to 500mA, and the 800kHz switching frequency allows the
use of small, low value ceramic input and output capaci-
tors. Input voltage ranges from 5.5V down to 2V and
output voltage can be set as low as the 620mV reference.
The device features Burst ModeTM operation, keeping
efficiency high at light loads. Burst Mode operation can be
defeated by pulling the MODE pin high, enabling constant
switching throughout the load range for low noise.
Operates from Input Voltage As Low As 2V
■
■
■
■
■
■
■
Internal 0.7A Synchronous Switches
Uses Ceramic Input and Output Capacitors
620mV Reference Voltage
800kHz Fixed Frequency Switching
Programmable Burst Mode Operation
Low Quiescent Current: 160µA
8-Lead MSOP or SO Package
U
APPLICATIO S
■
Portable Devices
No-loadquiescentcurrentis160µAandshutdowncurrent
is less than 1µA. The device is available in 8-lead SO and
MSOP packages.
■
Lithium-Ion Step-Down Converters
■
5V to 3.3V Conversion
2-Cell Alkaline Step-Down Converters
■
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
U
TYPICAL APPLICATION
Efficiency for LT1612 vs Linear Regulator
VOUT = 1.2V
0.1µF
90
V
IN
L1
BOOST
SW
V
IN
2V
10µH
V
OUT
SHDN
1.2V
80
70
60
50
40
30
V
V
= 2V
= 3V
IN
IN
500mA
LT1612
100pF
MODE
FB
C1
10µF
V
C
GND
V
V
= 2V (LINEAR)
IN
R1
215k
1%
C2
R2
232k
1%
33.2k
330pF
68µF
3.15V
= 3V (LINEAR)
100
IN
C1: TAIYO-YUDEN JMK325BJ106MN
C2: PANASONIC EEFCDOF680R
L1: SUMIDA CD43-100
1612 F01a
10
500
LOAD CURRENT (mA)
1612 • F01b
Figure 1. 2V to 1.2V Converter
sn1612 1612fs
1
LT1612
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VIN) ............................................... 5.5V
SW Pin Voltage....................................................... 5.5V
FB Pin Voltage ............................................... VIN + 0.3V
VC Pin Voltage ........................................................... 2V
SHDN Pin Voltage................................................... 5.5V
MODE Pin Voltage .................................................. 5.5V
W W
U W
(Note 1)
BOOST Pin Voltage ....................................... VIN + 5.5V
Junction Temperature........................................... 125°C
Operating Temperature Range (Note 2) ... –40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
U
W U
PACKAGE/ORDER INFORMATION
ORDER PART
ORDER PART
TOP VIEW
NUMBER
NUMBER
TOP VIEW
V
1
2
3
4
8
7
6
5
SHDN
MODE
BOOST
SW
C
V
FB 2
1
8 SHDN
7 MODE
6 BOOST
5 SW
C
FB
LT1612EMS8
LT1612ES8
S8 PART MARKING
1612
V
3
V
IN
GND 4
IN
GND
MS8 PACKAGE
8-LEAD PLASTIC MSOP
MS8 PART MARKING
LTMS
S8 PACKAGE
8-LEAD PLASTIC SO
TJMAX = 125°C, θJA = 200°C/ W
TJMAX = 125°C, θJA = 120°C/ W
Consult factory for parts specified with wider operating temperature ranges.
The ● denotes specifications which apply over the full operating
ELECTRICAL CHARACTERISTICS
temperature range, otherwise specifications are T = 25°C, V = V
= 3V
A
IN
SHDN
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
Quiescent Current
MODE = 5V
MODE = 0V, Not Switching
SHDN = 0V
●
●
●
1
160
2
220
1
mA
µA
µA
Q
V
FB Voltage
0.605
0.60
0.62
0.62
0.635
0.635
V
V
FB
●
●
●
FB Line Regulation
0.02
7
0.15
50
%/V
nA
FB Pin Bias Current (Note 3)
Error Amplifier Transconductance
g
250
µmhos
m
Min Input Voltage
Max Input Voltage
2
V
V
5.5
f
Oscillator Frequency
700
550
800
900
1100
kHz
kHz
OSC
●
●
f
Line Regulation
1
%/V
OSC
Maximum Duty Cycle
85
80
90
%
%
Shutdown Threshold
Minimum Voltage for Active
Maximum Voltage for Shutdown
●
●
2
V
V
0.2
sn1612 1612fs
2
LT1612
ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are T = 25°C, V = V
= 3V
A
IN
SHDN
SYMBOL PARAMETER
SHDN Pin Current
CONDITIONS
MIN
TYP
MAX
UNITS
SHDN = 2V
SHDN = 5V
●
●
10
30
15
45
µA
µA
BOOST Pin Current
BOOST = V + 2V
4
mA
mA
IN
Switch Current Limit (Note 4)
Duty Cycle = 0%
MODE = OV
MODE = 5V
600
550
710
900
900
650
180
200
300
mA
mA
mV
mV
µA
Burst Mode Operation Current Limit
Switch Voltage Drop
MODE = 0V
I
I
= 500mA
280
400
1
SW
Rectifier Voltage Drop
SW Pin Leakage
= 500mA
RECT
V
= 5V, V
= 0V
●
SW
SHDN
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 2: The LT1612E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
Note 3: Bias current flows out of the FB pin.
Note 4: Duty cycle affects current limit due to slope compensation.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
FB Voltage vs Temperature
Quiescent Current vs Temperature
FB Pin Bias Current
0.64
0.63
0.62
0.61
0.60
0.59
190
180
170
160
150
140
130
120
110
100
0
–2
MODE = 0V
–4
–6
–8
–10
–12
–14
–16
–18
–20
–50
0
25
50
75
100
–25
–50 –25
0
25
75
100
–50 –25
25
50
75
100
50
0
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
1612 • G01
1612 • G02
1612 • G03
sn1612 1612fs
3
LT1612
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency vs
Temperature
Switch Current Limit vs
Temperature
SHDN Pin Bias Current
800
700
600
500
400
300
200
1000
900
800
700
600
500
400
80
70
60
50
40
30
20
10
0
MODE = 0V
MODE = 5V
1
2
4
0
5
3
–50
0
25
50
75
100
–50
0
25
50
75
100
–25
–25
TEMPERATURE (°C)
TEMPERATURE (°C)
SHDN PIN VOLTAGE (V)
1612 • G04
1612 • G05
1612 • G06
Maximum Duty Cycle vs
Temperature
Switch Voltage Drop
Rectifier Voltage Drop
94
90
86
82
78
74
70
600
500
400
300
200
100
0
600
500
400
300
200
100
0
–50
0
25
50
75
100
–25
0
200
300
400
500
600
0
200
300
400
500
600
100
100
TEMPERATURE (°C)
SWITCH CURRENT (mA)
RECTIFIER CURRENT (mA)
1612 • G07
1612 • G08
1612 • G09
sn1612 1612fs
4
LT1612
U
U
U
PIN FUNCTIONS
VC (Pin 1): Compensation Pin. This is the current sink/
source output of the error amplifier. By connecting an RC
network from this pin to ground, frequency response can
be tuned for a wide range of circuit configurations. The
voltage at this pin also sets the current limit, and if
grounded, the switch will remain in the OFF state.
SW (Pin 5): Switch Pin. Connect inductor and boost
capacitor here. Minimize trace area at this pin to keep EMI
down.
BOOST (Pin 6): This is the supply pin for the switch driver
and must be above VIN by 1.5V for proper switch opera-
tion. Connect the boost capacitor to this pin.
FB (Pin 2): Feedback Pin. This pin is the negative input to
the error amplifier. Connect the resistor divider tap to this
point which sets VOUT according to:
MODE (Pin 7): Burst Mode Operation Disable Pin. For
continuous switching operation (low noise), pull this pin
above 2V. For Burst Mode operation which gives better
light load efficiency, tie to ground. Output ripple voltage in
Burst Mode operation is typically 30mVP-P. See applica-
tions section for more information about this function.
VOUT = 0.62V (1 + R1/R2)
VIN (Pin 3): Supply Pin. Bypass capacitor C1 must be right
next to this pin.
SHDN (Pin 8): Shutdown Pin. Pull this pin low for shut-
down mode. Tie to a voltage between 2V and 5.5V for
normal operation.
GND (Pin 4): Ground Pin. Connect directly to local ground
plane.
W
BLOCK DIAGRA
R
SENSE
0.08Ω
BOOST DIODE
6
BOOST
V
3
1
2
IN
V
C
–
+
+
–
FB
SLOPE
COMPENSATION
A2
A1
V/I
0.62V
SWITCH
SWITCH
DRIVER
+
–
7
SW
5
MODE
A3
0.7V
FLIP-FLOP
RECTIFIER
DRIVE
R
Q
RECTIFIER
ENABLE
S
SHUTDOWN
8
SHDN
OSCILLATOR
GND
4
1612 BD
sn1612 1612fs
5
LT1612
U
OPERATIO
Layout Hints
The LT1612 employs fixed frequency, current mode con-
trol. This type of control uses two feedback loops. The
main control loop sets output voltage and operates as
follows: A load step causes VOUT and the FB voltage to be
perturbed slightly. The error amplifier responds to this
change in FB by driving the VC pin either higher or lower.
Because switch current is proportional to the VC pin
voltage, this change causes the switch current to be
adjusted until VOUT is once again satisfied. Loop compen-
sation is taken care of by an RC network from the VC pin
to ground.
The LT1612 switches current at high speed, mandating
careful attention to layout for proper performance. You
will not get advertised performance with careless layout.
Figure 2 shows recommended component placement for
a buck (step-down) converter. Follow this closely in your
PC layout. Note the direct path of the switching loops.
Input capacitor C1 must be placed close (<5mm) to the IC
package. As little as 10mm of wire or PC trace from CIN to
VIN will cause problems such as inability to regulate or
oscillation.
Inside this main loop is another that sets current limit on
acycle-by-cyclebasis. Thislooputilizescurrentcompara-
tor A2 to control peak current. The oscillator runs at
800kHz and issues a set pulse to the flip-flop at the
beginning of each cycle, turning the switch on. With the
switch now in the ON state the SW pin is effectively
connectedtoVIN. Currentrampsupintheinductorlinearly
at a rate of (VIN – VOUT)/L. Switch current is set by the VC
pin voltage and when the voltage across RSENSE trips the
currentcomparator,aresetpulsewillbegeneratedandthe
switch will be turned off. Since the inductor is now loaded
up with current, the SW pin will fly low and trigger the
rectifier to turn on. Current will flow through the rectifier
decreasing at a rate of VOUT/L until the oscillator issues a
new set pulse, causing the cycle to repeat.
The ground terminal of input capacitor C1 should tie close
to Pin 4 of the LT1612. Doing this reduces dI/dt in the
ground copper which keeps high frequency spikes to a
minimum. The DC/DC converter ground should tie to the
PC board ground plane at one place only, to avoid intro-
ducing dI/dt in the ground plane.
C
C
R1
R2
R
C
1
2
3
4
8
7
6
5
SHDN
MODE
LT1612
V
IN
C3
C1
If the load is light and VC decreases below A3’s trip point,
the device will enter the Burst Mode operation region (the
MODE pin must be at ground or floating). In this state the
oscillator and all other circuitry except the reference and
comparator A3 are switched on and off at low frequency.
This mode of operation increases efficiency at light loads
but introduces low frequency voltage ripple at the output.
For continuous switching and no low frequency output
voltage ripple, pull the MODE pin high. This will disable
comparator A3 which forces the oscillator to run
continuously.
C2
L1
MULTIPLE
VIAs
1612 F02
GND
V
OUT
Figure 2. Recommended Component Placement. Traces
Carrying High Current are Direct. Trace Area at FB Pin and VC
Pin Is Kept Low. Lead Length to Battery Should Be Kept Short
sn1612 1612fs
6
LT1612
U
OPERATIO
Burst Mode Operation Defeat
consumptionbutalsoaddslowfrequencyvoltagerippleto
the output. Figure 3 shows switching waveforms for a 5V
to3.3VconverterrunninginBurstModeoperation.Output
voltage ripple is approximately 20mVP-P. If the MODE pin
is pulled high, Burst Mode operation will be inhibited and
the oscillator runs continuously with no low frequency
ripple at the output. See Figures 4 and 5.
To maintain high efficiency at light loads, the LT1612 will
automatically shift into Burst Mode operation (MODE = 0V
or floating). In this mode of operation the oscillator and
switch drive circuitry is alternately turned on and off,
reducing quiescent current to 160µA. This reduces power
VOUT
200mV/DIV
AC COUPLED
VOUT
20mV/DIV
AC COUPLED
IL
200mA/DIV
IL
200mA/DIV
ILOAD
10mA TO 310mA
5µs/DIV
1612 F03
1612 F04
0.1ms/DIV
Figure 3. Output Voltage Ripple is 20mVP-P for
the Circuit of Figure 1
Figure 4. Transient Response for the Circuit of Figure 1
with the MODE Pin Tied to Ground or Floating
VOUT
200mV/DIV
AC COUPLED
IL
200mA/DIV
ILOAD
10mA TO 300mA
0.1ms/DIV
1612 F05
Figure 5. With the MODE Pin Tied High, Low
Frequency Output Voltage Ripple Is No Longer Present
sn1612 1612fs
7
LT1612
U
TYPICAL APPLICATIONS
Single Li-Ion to 2V Converter
Li-Ion to 2V Converter Efficiency
85
80
75
70
65
60
55
50
0.1µF
V
IN
2.7V TO 4.2V
L1
BOOST
SW
V
IN
10µH
V
OUT
V = 2.8V
IN
SHDN
2V
500mA
LT1612
20pF
V
= 4.2V
IN
MODE
V
FB
C
GND
1M
1%
V
IN
= 3.5V
10µF
CERAMIC
30.1k
680pF
22µF
CERAMIC
453k
1%
C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100
1612 TA02
1
10
100
1000
LOAD CURRENT (mA)
1612 TA04
Transient Response
Burst Mode Operation
VOUT 20mV/DIV
VOUT 50mV/DIV
IL 200mA/DIV
IL 100mA/DIV
LOAD STEP
125mA TO 300mA
1612 TA03
MODE = LOW
VIN = 4V
OUT = 2V
MODE = HIGH
5µs/DIV
1612 TA05
100µs/DIV
V
Inrush Current at Start-Up
VOUT 2V/DIV
INRUSH
CURRENT
200mA/DIV
VSHDN 5V/DIV
0.2ms/DIV
1612 TA06
sn1612 1612fs
8
LT1612
U
TYPICAL APPLICATIONS
5V to 2.5V Converter
5V to 2.5V Converter Efficiency
85
80
75
70
65
60
55
50
0.1µF
V
IN
L1
BOOST
SW
V
IN
5V
10µH
V
OUT
SHDN
2.5V
500mA
LT1612
20pF
MODE
V
FB
C
GND
C1
1M
10µF
CERAMIC
C2
22µF
CERAMIC
30.1k
680pF
332k
C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100
1
10
100
1000
1612 TA07
LOAD CURRENT (mA)
1612 TA08
2V to 0.9V Converter
Efficiency for LT1612 vs Linear Regulator.
VOUT = 0.9V.
0.1µF
V
80
IN
L1
BOOST
SW
V
IN
2V
10µH
V
V
= 2V
= 3V
OUT
IN
0.9V
SHDN
70
60
50
40
30
20
10
500mA
LT1612
100pF
V
IN
MODE
FB
C1
V
C
V
= 2V (LINEAR)
GND
10µF
IN
R1
105k
C2
33.2k
330pF
R2
232k
68µF
V
= 3V (LINEAR)
IN
3.15V
C1: TAIYO-YUDEN JMK325BJ106MN
C2: PANASONIC EEFCDOF680R
L1: SUMIDA CD43-100
1612 TA09
1
10
100
1000
LOAD CURRENT (mA)
1612 TA10
sn1612 1612fs
9
LT1612
U
PACKAGE DESCRIPTION Dimension in inches (millimeters) unless otherwise noted.
MS8 Package
8-Lead Plastic MSOP
(LTC DWG # 05-08-1660)
0.118 ± 0.004*
(3.00 ± 0.102)
8
7
6
5
0.118 ± 0.004**
(3.00 ± 0.102)
0.193 ± 0.006
(4.90 ± 0.15)
1
2
3
4
0.043
(1.10)
MAX
0.034
(0.86)
REF
0.007
(0.18)
0° – 6° TYP
SEATING
PLANE
0.009 – 0.015
(0.22 – 0.38)
0.021 ± 0.006
(0.53 ± 0.015)
0.005 ± 0.002
(0.13 ± 0.05)
0.0256
(0.65)
BSC
MSOP (MS8) 1100
* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH,
PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
sn1612 1612fs
10
LT1612
U
PACKAGE DESCRIPTION Dimension in inches (millimeters) unless otherwise noted.
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 – 0.197*
(4.801 – 5.004)
7
5
8
6
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
SO8 1298
1
3
4
2
0.010 – 0.020
(0.254 – 0.508)
× 45°
0.053 – 0.069
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
0.008 – 0.010
(0.203 – 0.254)
0°– 8° TYP
0.016 – 0.050
(0.406 – 1.270)
0.050
(1.270)
BSC
0.014 – 0.019
(0.355 – 0.483)
TYP
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
sn1612 1612fs
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
11
LT1612
U
TYPICAL APPLICATIO
5V to 3.3V Converter
Efficiency
C3
85
80
75
70
65
60
55
50
V
V
= 5V
0.1µF
IN
OUT
= 3.3V
V
IN
L1
10µH
BOOST
SW
V
IN
5V
V
OUT
SHDN
3.3V
500mA
LT1612
20pF
MODE
FB
C1
10µF
V
C
GND
R1
1M
1%
R3
R2
232k
1%
C2
22µF
33.2k
C4
680pF
1
10
100
1000
C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100
1612 TA01a
LOAD CURRENT (mA)
1612 TA01b
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Q
sn1612 1612fs
LT/TP 1100 4K • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
12
●
●
LINEAR TECHNOLOGY CORPORATION 1999
(408)432-1900 FAX:(408)434-0507 www.linear-tech.com
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