LTC3520EUF-PBF [Linear]
Synchronous 1A Buck-Boost and 600mA Buck Converters; 1A同步降压 - 升压和600mA buck转换器
| 型号： | LTC3520EUF-PBF |
| 厂家： | 
Linear |
| 描述： | Synchronous 1A Buck-Boost and 600mA Buck Converters |
| 文件： | 总24页 (文件大小：367K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3520
Synchronous 1A
Buck-Boost and 600mA
Buck Converters
FEATURES
DESCRIPTION
The LTC®3520 combines 1A buck-boost and 600mA
synchronous buck DC/DC converters in a tiny 4mm ×
4mm package. A programmable switching frequency
allows the efficiency to be optimized while minimizing
the solution footprint. Both converters feature soft-start
and current limit protection. The uncommitted gain block
can be configured as an LDO or utilized as a battery-good
comparator.
■
Dual High Efficiency DC/DC Converters:
Buck-Boost (V : 2.2V to 5.25V, I
= 1A at
OUT
IN
OUT
V
= 3.3V, V ≥ 3V)
OUT
Buck (V : 0.8V to V , I
= 600mA)
OUT
IN OUT
■
■
■
2.2V to 5.5V Input Voltage Range
Pin-Selectable Burst Mode® Operation
Uncommitted Gain Block for LDO Controller,
Battery Good Indication or Sequencing
Programmable 100kHz to 2MHz Switching Frequency
55µA Total Quiescent Current for Both Converters in
Burst Mode Operation
■
■
Thebuckconverteriscurrentmodecontrolledwithinternal
synchronousrectificationtoimproveefficiency.Pin-select-
ableBurstModeoperationcanbeenabledtoimprovelight
load efficiency, or the buck converter can be operated in
low noise PWM mode for sensitive applications.
■
■
■
Thermal and Overcurrent Protection
<1µA Quiescent Current in Shutdown
24-Lead 4mm × 4mm QFN Package
The buck-boost converter provides continuous conduc-
tion operation to maximize efficiency and minimize noise.
At light loads, use of Burst Mode operation will improve
efficiency.
APPLICATIONS
■
Portable Media Players
■
Digital Cameras
The LTC3520 provides a <1µA shutdown mode and over-
temperature shutdown on both converters. The LTC3520
is available in a low profile (0.75mm) 24-lead 4mm ×
4mm QFN package.
■
Handheld PCs, PDAs
GPS Receivers
■
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners. Protected by
U.S. Patents, including 5481178, 6166527, 6304066, 6404251, 6580258.
TYPICAL APPLICATION
3.3V at 500mA, 1.8V at 600mA and 1.5V at 200mA Converter
Efficiency vs V
IN
V
IN
100
95
90
85
80
75
70
4.7µH
2.2V TO
5.5V
22µF
PV
IN1
PV
IN2
PV
IN3
SV SW1A
IN
BUCK-BOOST
I = 150mA
OUT
V
3.3V
4.7µH
OUT1
SW1B
V
OUT2
1.8V
600mA
SW2
V
OUT1
470pF
500mA
1A FOR
255k
27pF
56pF
10k
BUCK
OUT
10µF
47µF
V
C1
V
IN
≥ 3V
1M
I
= 250mA
FB2
SS2
15k
0.01µF
200k
FB1
SS1
0.01µF
309k
LTC3520
54.9k
R
T
V
OUT2
PWM1
PWM2
SD3
A
OUT
V
BURST
PWM
OUT
1.5V
200mA
33pF
100k
110k
SD2
OFF ON
4.7µF
SD1
A
IN
2.7
3.2
4.2
(V)
4.7
5.2
2.2
3.7
PGND1 SGND PGND2
V
IN
3520 TA01
3520f
1
LTC3520
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
PV , PV , PV , SV Voltage................–0.3V to 6V
IN1
IN2
IN3
IN
SW1A, SW1B, SW2 Voltage
DC............................................................–0.3V to 6V
Pulsed <100ns…………….………….......–1V to 7V
Voltage, All Other Pins .................................–0.3V to 6V
Operating Temperature Range (Note 2) ...–40°C to 85°C
Maximum Junction Temperature (Note 5) ............ 125°C
Storage Temperature Range...................–65°C to 150°C
24 23 22 21 20 19
SV
1
2
3
4
5
6
18 FB1
IN
A
SS1
17
16
15
14
OUT
A
SGND
IN
25
R
T
V
C1
PWM1
SD1
FB2
13 SS2
7
8
9 10 11 12
UF PACKAGE
24-LEAD (4mm × 4mm) PLASTIC QFN
= 125°C, θ = 37°C/W
T
JMAX
JA
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
24-Lead (4mm × 4mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 85°C
LTC3520EUF#PBF
LTC3520EUF#TRPBF
3520
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS The
otherwise noted.
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T = 25°C, S = P
= P
= P
= 3.6V, V
= 3.3V, R = 54.9k, unless
A
VIN
VIN1
VIN2
VIN3
OUT1
T
PARAMETER
CONDITIONS
MIN
TYP
MAX
5.5
1
UNITS
V
●
●
Input Voltage
2.2
0.8
Quiescent Current in Shutdown
Undervoltage Lockout
Burst Mode Quiescent Current, Both Converters
Oscillator Frequency
V
SD1
= V
= V = 0V
SD3
0.01
2
µA
SD2
SV Rising
IN
2.2
V
V
FB1
= V = 0.88V, V
= 0V
55
1
µA
FB2
SD3
●
R = 54.9k
T
1.2
MHz
Buck Converter
Ω
Ω
PMOS Switch Resistance
NMOS Switch Resistance
NMOS Switch Leakage
PMOS Switch Leakage
0.32
0.18
0.1
V
V
= 5V, S = P
= P
= P
= 5V
5
µA
µA
SW2
VIN
VIN1
VIN2
VIN2
VIN3
VIN3
= 0V, S = P
= P
VIN1
= P
= 5V
0.1
10
SW2
VIN
3520f
2
LTC3520
ELECTRICAL CHARACTERISTICS The
otherwise noted.
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T = 25°C, S = P
= P
= P
= 3.6V, V
= 3.3V, R = 54.9k, unless
A
VIN
VIN1
VIN2
VIN3
OUT1 T
PARAMETER
CONDITIONS
MIN
TYP
0.790
1
MAX
0.809
50
UNITS
V
●
Feedback Voltage (FB2 Pin)
Feedback Input Current (FB2 Pin)
PMOS Current Limit
(Note 4)
0.771
nA
A
●
●
●
(Note 3)
0.8
1.25
Maximum Duty Cycle
V
= 0.72V
= 0.88V
100
%
FB2
FB2
Minimum Duty Cycle
V
0
%
Soft-Start Charging Current
SD2 Input High Voltage
SD2 Input Low Voltage
SD2 Input Current
6
µA
V
1.4
2.2
0.4
1
V
0.01
µA
Buck-Boost Converter
Output Voltage
●
5.25
V
Ω
PMOS Switch Resistance
NMOS Switch Resistance
NMOS Switch Leakage
PMOS Switch Leakage
Feedback Voltage (FB1 Pin)
Feedback Input Current (FB1 Pin)
Forward Current Limit
Reverse Current Limit
0.20
0.15
0.1
0.1
0.782
1
Ω
V
V
= V
= V
= 5V, S = P
= P
= P
= 5V
5
10
µA
µA
V
SW1A
SW1B
SW1B
VIN
VIN1
VIN2
VIN3
= 0V, S = P
= P
= P
VIN2
= 5V
SW1A
VIN
VIN1
VIN3
●
●
0.766
1.4
0.798
50
nA
A
(Note 3)
(Note 3)
(Note 3)
2
560
325
80
mA
mA
dB
µA
µA
Burst Mode Operation Current Limit
Error Amplifier Gain
Error Amplifier Sink Current
Error Amplifier Source Current
Maximum Duty Cycle
500
14
●
●
Boost (% Switch C is On)
Buck (% Switch A is On)
70
100
80
%
%
Minimum Duty Cycle
0
%
µA
V
Soft-Start Charging Current
SD1, PWM1 Input High Voltage
SD1, PWM1 Input Low Voltage
SD1, PWM1 Input Current
Gain Block
6
1.4
0.4
1
V
0.01
µA
Quiescent Current
V
= 0.88V, V
= V = 0V
SD2
45
0.786
1
µA
V
AIN
SD1
●
A
A
A
A
A
Pin Threshold Voltage
Pin Input Bias Current
0.770
0.802
50
IN
nA
mA
µA
mV
dB
IN
Sink Current
Source Current
Pin Voltage
V
AIN
= 0.72V, V
= 0.88V, V
= 1.8V
= 1.8V
= 1mA
17
OUT
OUT
OUT
AOUT
AOUT
AOUT
V
AIN
18
V
AIN
= 0.72V, I
25
150
Open Loop Gain
80
3520f
3
LTC3520
ELECTRICAL CHARACTERISTICS The
●
denotes the specifications which apply over the full operating temp-
= 3.3V, V = 1.8V, R = 54.9k, unless otherwise noted.
erature range, otherwise specifications are at T = 25°C, V = 3.6V, V
A
IN
OUT1
OUT2
T
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
µs
Propagation Delay
SD3 Input High Voltage
SD3 Input Low Voltage
SD3 Input Current
A
Falling
11
OUT
1.4
V
0.4
1
V
0.01
µA
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 2: The LTC3520 is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 4: The LTC3520 is tested in a proprietary non-switching test mode
that internally connects the FB2 pin to the output of the buck converter
error amplifier.
Note 5: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 3: Current measurements are performed when the LTC3520 is not
switching. The current limit values in operation will be somewhat higher
due to the propagation delay of the comparators.
(T = 25°C, unless otherwise specified)
A
TYPICAL PERFORMANCE CHARACTERISTICS
Buck-Boost Efficiency
Lithium-Ion to 3.3V
Buck Efficiency
Lithium-Ion to 2.7V
100
90
80
70
60
50
40
30
20
10
0
300
250
200
150
100
50
800
700
600
500
400
300
200
100
0
100
90
80
70
60
50
40
30
20
10
0
V
IN
V
IN
= 4.2V
= 2.7V
Burst Mode OPERATION
PWM MODE
Burst Mode
OPERATION
PWM MODE
Burst Mode
OPERATION
POWER LOSS
Burst Mode
OPERATION
POWER LOSS
L = COILCRAFT
MSS6132-4.7µH
L = SUMIDA
CDRH3D16NP-4R7N
0
1
10
100
1000
1
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
3520 G01
3520 G02
3520f
4
LTC3520
TYPICAL PERFORMANCE CHARACTERISTICS
Buck Efficiency
Lithium-Ion to 1.8V
Switching Frequency vs R
T
100
90
80
70
60
50
40
30
20
10
0
800
700
600
500
400
300
200
100
0
10000
1000
100
Burst Mode
OPERATION
PWM MODE
V
IN
V
IN
= 4.2V
= 3V
Burst Mode
OPERATION
POWER LOSS
L = SUMIDA
CDRH3D16NP-4R7N
R
BURST
= 249k
1
10
100
1000
10
100
1000
LOAD CURRENT (mA)
R
(k)
T
3520 G03
3520 G04
LDO Load
Transient Response
Buck-Boost Load
Transient Response
V
OUT
LDO V
OUT
V
= 5V
IN
100mV/DIV
200mV/DIV
V
OUT
V
= 2.2V
IN
500mV/DIV
LOAD
CURRENT
100mA/DIV
(20mA TO
210mA STEP)
LOAD CURRENT
500mA/DIV
3520 G05
3520 G06
50µs/DIV
= 4.7µF
OUT
200µs/DIV
C = 47µF
OUT
V
V
= 3.6V
OUT
C
IN
L = 4.7µH
= 3.3V
= 1.5V LDO INPUT VOLTAGE = 1.8V
V
IN
Buck Load Transient
Response (PWM Mode)
Buck Load Transient
Response (Burst Mode Operation)
BUCK V
BUCK V
OUT
100mV/DIV
OUT
100mV/DIV
LOAD CURRENT
500mA/DIV
LOAD CURRENT
200mA/DIV
(50mA TO
(5mA TO
500mA STEP)
300mA STEP)
3520 G07
3520 G08
100µs/DIV
100µs/DIV
V
V
C
= 3.6V
IN
V
V
= 3.6V
C
R
= 22µF
OUT
BURST
IN
OUT
= 1.8V
OUT
OUT
= 1.8V
= 249k
= 10µF
3520f
5
LTC3520
TYPICAL PERFORMANCE CHARACTERISTICS
Buck Burst Mode Threshold
= 1.2V
Buck Burst Mode Threshold
= 1.8V
V
V
Buck-Boost R
OUT
OUT
DS(ON)
45
40
35
30
25
20
15
10
5
250
200
150
100
50
45
40
35
30
25
20
15
10
5
L = 3.3µH
L = 4.7µH
PMOS
(SWITCHES A AND D)
R
BURST
R
= 249k
NMOS
BURST
= 274k
(SWITCHES B AND C)
R
= 249k
BURST
R
= 301k
BURST
4.2
R
= 301k
BURST
0
0
0
2.7
3.2
4.7
5.2
2.2
3.7
2.7
3.2
4.2
4.7
5.2
–25
–5
35
55
75
2.2
3.7
–45
15
V
(V)
V
(V)
TEMPERATURE (°C)
IN
IN
3520 G09b
3520 G09
3520 G10
Buck R
DS(ON)
Switching Frequency
Buck-Boost Feedback Voltage
400
2.0
1.0
0.8
350
300
PMOS
NMOS
1.5
1.0
0.6
0.4
0.2
250
200
150
100
50
0.5
0
0
–0.2
–0.5
–1.0
–1.5
–0.4
–0.6
–0.8
–1.0
0
–25
–5
35
55
75
–45
15
–2.0
–25
–5
35
55
75
–45
15
–20
0
40
60
80
–40
20
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
3520 G11
3520 G12
3520 G13
Buck Feedback Voltage
No Load Input Current
100
90
80
70
60
50
40
30
20
10
0
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
BUCK AND BUCK-BOOST
CONVERTERS ENABLED,
Burst Mode OPERATION
–20
0
40
60
80
5.2
–40
20
4.2
2.2
3.2
3.7
4.7
2.7
V
(V)
TEMPERATURE (°C)
IN
3520 G15
3520 G14
3520f
6
LTC3520
TYPICAL PERFORMANCE CHARACTERISTICS
Buck-Boost Maximum
Output Current, PWM Mode
Buck-Boost Maximum Output
Current, Burst Mode Operation
1800
1600
1400
1200
1000
800
600
400
200
0
140
120
100
80
V
= 3.3V
OUT
V
= 3.3V
OUT
V
= 5V
OUT
V
= 5V
OUT
60
40
20
0
2.2
2.7
3.2
4.2
4.7
5.2
2.2
3.7
2.7
3.2
4.2
4.7
5.2
3.7
V
V
(V)
(V)
IN
IN
3520 G16
3520 G17
Buck-Boost PWM Mode
Efficiency vs Frequency
Buck Efficiency vs Frequency
100
100
L = 8.2µH COILCRAFT
L = 4.7µH SUMIDA
MSS6132
CDRH3D16NP
L = 4.7µH COILCRAFT
95
90
85
80
75
70
95
90
85
80
75
70
MSS7341
L = 8.2µH
COILCRAFT
L = 2.2µH
MSS6132
SUMIDA
L = 2.2µH
COILCRAFT
1812PS
CDRH3D16NP
V
V
I
= 2.5V
V
V
I
= 3.6V
IN
OUT
IN
OUT
= 1.8V
= 3.3V
= 100mA
= 200mA
LOAD
LOAD
0.4
0.8
1
1.2 1.4 1.6 1.8
2
0.6
0.4
0.8
1
1.2 1.4 1.6 1.8
2
0.6
SWITCHING FREQUENCY (MHz)
SWITCHING FREQUENCY (MHz)
3520 G19
3520 G18
Buck-Boost
Burst Mode Operation
Buck Burst Mode Operation
BUCK V
OUT
50mV/DIV
V
OUT
50mV/DIV
INDUCTOR
CURRENT
200mA/DIV
INDUCTOR
CURRENT
200mA/DIV
3520 G22
3520 G21
10µs/DIV
20µs/DIV
V
V
LOAD
= 3.6V
R
C
= 249k
BURST
C
= 22µF
I
= 25mA
LOAD
IN
OUT
= 1.8V
= 10µF
OUT
L = 4.7µH
V
= 3.6V
IN
OUT
I
= 10mA
3520f
7
LTC3520
PIN FUNCTIONS
SV (Pin 1): Small Signal Power Supply Connection.
amplifier. Forcing this pin below 0.4V disables the uncom-
mitted amplifier. This pin cannot be left floating.
IN
This pin is used to power the internal circuitry of the
LTC3520. This pin should be bypassed using a 0.1µF or
larger ceramic capacitor placed as close as possible to
PV (Pin 9): High Current Power Supply Connection
IN2
Used to Supply the Buck Converter PMOS Power Device.
This pin should be bypassed by a 22µF or larger ceramic
capacitor. The bypass capacitor should be placed as close
to the pin as possible and should have a short return path
the pin with a short return path to ground. Pins PV
,
IN1
PV , PV , and SV must be connected together in
IN2
IN3
IN
the application circuit.
A
(Pin 2): Uncommitted Amplifier Output. This pin
to ground. Pins PV , PV , PV , and SV must be
OUT
IN1 IN2 IN3 IN
should be connected to the base of an external PNP
transistor for use as an LDO regulator. If used as a
battery-good indicator or for supply sequencing, this pin
is the comparator output.
connected together in the application circuit.
SW2(Pin10):BuckConverterSwitchNode. Thispinmust
be connected to one side of the buck inductor.
PGND2 (Pin 11): High Current Ground Connection for the
Buck Converter N-Channel MOSFET Power Device. The
PCB trace connecting this pin to ground should be made
as short and wide as possible.
A
(Pin 3): Non-Inverting Input to the Uncommitted
IN
Amplifier. In LDO applications, this pin is connected to
the LDO feedback voltage.
R (Pin4):ProgramstheFrequencyoftheInternalOscilla-
T
PWM2(Pin12):Burst/PWMModeControlPinfortheBuck
Converter. This pin can be used in the following ways:
tor. Thispinmustbetiedtogroundviaanexternalresistor.
The value of the resistor controls the oscillator frequency.
For details on choosing the value of this resistor see the
Applications Information section of this datasheet.
PWM2 forced high: With PWM2 forced high, the buck
converter will be forced into low noise fixed frequency
operation. The buck converter will remain in this mode
unlesstheloadcurrentislowenoughthattheminimum
on-time is reached at which point the converter will
begin pulse-skipping to maintain regulation.
PWM1(Pin5):LogicInputUsedtoChooseBetweenBurst
and PWM Mode for the Buck-Boost Converter. This pin
cannot be left floating.
PWM1 = Low: The buck-boost converter will operate in
variable frequency mode to improve efficiency at light
loads. In this mode, the LTC3520 can only supply a
reduced load current (typically 50mA).
PWM2 connected to ground via resistor: PWM2 can be
connected to ground through a resistor to control the
load current at which Burst Mode operation is entered
and exited. Larger resistor values will cause the buck
converter to enter Burst Mode operation at lower load
currents and will result in lower output voltage ripple
in Burst Mode operation. Smaller resistor values will
causeBurstModeoperationtobeenteredathigherload
currents and the Burst Mode ripple will be larger.
PWM1 = High: The buck-boost converter will remain
in low noise, fixed frequency PWM mode at all load
currents.
SD1 (Pin 6): Buck-Boost Active-Low Shutdown Pin. Forc-
ing this pin above 1.4V enables the buck-boost converter.
Forcing this pin below 0.4V disables the buck-boost
converter. This pin cannot be left floating.
PWM2 forced low: With PWM2 forced to ground, the
buckconverterwilloperateinBurstModeoperationfor
allbutthehighestloadcurrents.Generally,thismodeof
operationisutilizedtoforcethebuckconverterintoBurst
Mode operation when it is known that the load current
will be relatively low (under 75mA) or in applications
that are not sensitive to output voltage ripple.
SD2 (Pin 7): Buck Active-Low Shutdown Pin. Forcing
this pin above 1.4V enables the buck converter. Forcing
this pin below 0.4V disables the buck converter. This pin
cannot be left floating.
SD3(Pin8):UncommittedAmplifierActive-LowShutdown
Pin. Forcing this pin above 1.4V enables the uncommitted
3520f
8
LTC3520
PIN FUNCTIONS
SS2(Pin13):BuckConverterSoft-StartPin.Thispinmust
be connected to a soft-start capacitor. The value of the
capacitor determines the duration of the soft-start period.
Forinformationonchoosingthevalueofthiscapacitor,see
the Applications Information section of this datasheet.
between FB1 and ground and R2 is a resistor between
FB1 and the buck-boost output voltage:
R2
R1
⎛
⎞
VOUT = 0.782V 1+
⎜
⎝
⎟
⎠
V
(Pin 19): Buck-Boost Output Voltage Node. This
FB2 (Pin 14): Feedback Voltage for the Buck Converter.
This pin is derived from a resistor divider on the buck
output voltage. The buck output voltage is given by the
following equation where R1 is a resistor between FB2
and ground and R2 is a resistor between FB2 and the
buck output voltage:
OUT1
pin should be connected to a low ESR buck-boost output
capacitor. The capacitor should be placed as close to the
IC as possible and should have a short return path to
ground.
SW1B (Pin 20): Buck-Boost Switch Node. This pin must
be connected to one side of the buck-boost inductor.
R2
R1
⎛
⎞
VOUT = 0.790V 1+
⎜
⎝
⎟
⎠
PGND1 (Pin 21): High Current Ground Connection for
the Buck-Boost NMOS Power Devices. The PCB trace
connecting this pin to ground should be made as short
and wide as possible.
V
(Pin 15): Buck-Boost Error Amplifier Output. A fre-
C1
quency compensation network is connected to FB1 to
compensate the loop. During Burst Mode operation, V
is driven internally by a clamp circuit.
C1
SW1A (Pin 22): Buck-Boost Switch Node. This pin must
be connected to one side of the buck-boost inductor.
SGND (Pin 16): Small Signal Ground. This pin is used
as a ground reference for the internal circuitry of the
LTC3520.
PV (Pin 23), PV (Pin 24): High Current Power Sup-
IN1
IN3
ply Connections Used to Power the Buck-Boost Converter
PowerSwitchA. Thesepinsshouldbeconnectedtogether
and bypassed by a 22µF or larger ceramic capacitor. The
bypass capacitor should be placed as close to the pin as
possible and should have a short return path to ground.
Pins PV , PV , PV , and SV must be connected
SS1 (Pin 17): Buck-Boost Converter Soft-Start Pin. This
pin must be connected to a soft-start capacitor. The value
of the capacitor determines the duration of the soft-start
period. For information on choosing the value of this
capacitor, see the Applications Information section of
this datasheet.
IN1
IN2
IN3
IN
together in the application circuit.
Exposed Pad (Pin 25): Ground. The Exposed Pad must be
electrically connected to ground and soldered to the PCB.
Pins PGND1, PGND2, SGND, and the Exposed Pad must
be connected together in the application circuit.
FB1 (Pin 18): Feedback Voltage for the Buck-Boost Con-
verter. This pin is derived from a resistor divider on the
buck-boostoutputvoltage.Thebuck-boostoutputvoltage
is given by the following equation where R1 is a resistor
3520f
9
LTC3520
BLOCK DIAGRAM
23
PV
24
PV
22
SW1A
20
SW1B
19
V
OUT1
*
*
IN1
IN3
A
D
0.56A
GATE
DRIVERS
B
C
PGND1
PGND1
+
Gm
2A
–
+
–
+
–
FB1
SS1
–
BUCK-
BOOST
PWM
3A
18
17
+
+
PWM1
SD1
5
6
0.782V
LOGIC
+
–
5µA
SD3
8
3
V
C1
15
BANDGAP
REFERENCE
A
A
IN
+
–
0.786V
OUT
2
1
R
T
THERMAL
SHUTDOWN
OSC
4
9
DISABLE
INTERNAL
V
CC
SV
*
IN
PV
*
IN2
SLOPE
COMPENSATION
+
+
–
1.25A
BUCK
PWM
LOGIC
SW2
FB2
SS2
10
11
+
–
14
13
–
Gm
0.790V
+
+
PGND2
5µA
0A
SGND
16
PGND1
21
PWM2
12
SD2
7
3520 F02
*PINS SV , PV , PV AND PV MUST BE CONNECTED TOGETHER IN THE APPLICATION.
IN
IN1
IN2
IN3
3520f
10
LTC3520
OPERATION
The LTC3520 combines a synchronous buck DC/DC
converter and a four-switch buck-boost DC/DC converter
in a single 4mm x 4mm QFN package. The buck-boost
converter utilizes a proprietary switching algorithm which
allows its output voltage to be regulated above, below, or
equal to the input voltage. The buck converter provides a
high efficiency lower voltage output and supports 100%
duty cycle operation to extend battery life. In Burst Mode
operation, the total quiescent current for both converters
is reduced to 55µA (typical). Both converters operate
synchronously from a common internal oscillator whose
frequency is programmed via an external resistor. In ad-
dition, the LTC3520 contains an uncommitted gain block
which can be configured as a comparator for low battery
detection or as a power-good indicator. Alternatively, the
gain block can be utilized in conjunction with an external
PNP to create an LDO, thereby allowing the LTC3520 to
generate a third low noise output voltage.
the Burst Mode entry threshold are provided in the Typical
Performance Characteristics section of this datasheet.
Under dropout and near dropout conditions, Burst Mode
operation will not be entered.
The value of R
controls the load current at which
BURST
Burst Mode operation will be entered. Larger resistor
values will cause Burst Mode operation to be entered at
lighter load currents. However, if the value of R
is
BURST
too large, then the buck converter will not enter Burst
Mode operation at any current, especially when operating
with V close to the buck output voltage. Conversely, if
IN
R
is too small, the ripple in Burst Mode operation
BURST
may become objectionable, especially at high input volt-
ages. For most applications, choosing R
represents a reasonable compromise.
= 301k
BURST
The output voltage ripple in Burst Mode operation is de-
pendent upon the value of R , the input voltage, the
BURST
output voltage, the inductor value and the output capaci-
tor. The Burst Mode operation output voltage ripple can
be reduced by increasing the size of the output capacitor,
increasing the value of the inductor or increasing the
BUCK CONVERTER OPERATION
PWM Mode Operation
value of R
.
BURST
WhenthePWM2pinisheldhigh,theLTC3520buckconverter
uses a constant-frequency, current mode control architec-
ture. Both the main (P-channel MOSFET) and synchronous
rectifier (N-channel MOSFET) switches are internal. At
the start of each oscillator cycle, the P-channel switch is
turned on and remains on until the current waveform with
superimposed slope compensation ramp exceeds the error
amplifier output. At this point, the synchronous rectifier is
turned on and remains on until the inductor current falls to
zero or a new switching cycle is initiated. As a result, the
buck converter operates with discontinuous inductor cur-
rent at light loads which improves efficiency. At extremely
light loads, the minimum on-time of the main switch will
be reached and the buck converter will begin turning off for
multiple cycles in order to maintain regulation.
Low Dropout Operation
As the input voltage decreases to a value approaching the
output regulation voltage, the duty cycle increases toward
the maximum on-time. Further reduction of the supply
voltage will force the power P-channel MOSFET switch
to remain on for more than one cycle until 100% duty
cycle operation is reached and the power switch remains
on continuously. In this dropout state, the output voltage
will be determined by the input voltage less the resistive
voltage drop across the main switch and series resistance
of the inductor.
Slope Compensation
Currentmodecontrolrequirestheuseofslopecompensa-
tion to prevent subharmonic oscillations in the inductor
current waveform at high duty cycle operation. This is ac-
complishedinternallyontheLTC3520throughtheaddition
of a compensating ramp to the current sense signal. In
some current mode ICs, current limiting is performed by
clamping the error amplifier voltage to a fixed maximum.
Burst Mode Operation
Burst Mode operation is enabled by either connecting
PWM2togroundthrougharesistor,R ,orbyshorting
BURST
PWM2 to ground. The buck converter will automatically
transition between PWM mode at high load current and
Burst Mode operation at light currents. Typical curves for
3520f
11
LTC3520
OPERATION
This leads to a reduced output current capability at large
step-down ratios. In contrast, the LTC3520 performs cur-
rentlimitingpriortotheadditionoftheslopecompensation
ramp and therefore achieves a peak inductor current limit
that is independent of duty cycle.
When the input voltage is significantly greater than the
output voltage, the buck-boost converter operates in
buck mode. Switch D turns on continuously and switch C
remains off. Switches A and B are pulse width modulated
to produce the required duty cycle to support the output
regulation voltage. As the input voltage decreases, switch
A remains on for a larger portion of the switching cycle.
When the duty cycle reaches approximately 85%, the
switch pair AC begins turning on for a small fraction of the
switching period. As the input voltage decreases further,
the AC switch pair remains on for longer durations and
the duration of the BD phase decreases proportionally. As
the input voltage drops below the output voltage, the AC
phase will eventually increase to the point that there is no
longer any BD phase. At this point, switch A remains on
continuously while switch pair CD is pulse width modu-
lated to obtain the desired output voltage. In this case, the
converter is operating solely in boost mode.
Soft-Start
Thebuckconverterincorporatesavoltagemodesoft-start
circuit which is adjustable via the value of an external
soft-start capacitor, C . The typical soft-start duration
is given by the following equation:
SS
t (ms) = 0.15C (nF)
SS
SS
The buck converter remains in regulation during soft-start
and will therefore respond to output load transients which
occurduringthistime. Inaddition, theoutputvoltagerise-
time has minimal dependency on the size of the output
capacitor or load current.
This switching algorithm provides a seamless transition
between operating modes and eliminates discontinuities
in average inductor current, inductor current ripple, and
loop transfer function throughout all three operational
modes. These advantages result in increased efficiency
and stability in comparison to the traditional four-switch
buck-boost converter.
Error Amplifier and Compensation
The LT3520 buck converter utilizes an internal trans-
conductance error amplifier. Compensation of the feed-
back loop is performed internally to reduce the size of the
application circuit and simplify the design process. The
compensation network has been designed to allow use of
a wide range of output capacitors while simultaneously
ensuring a rapid response to load transients.
L
BUCK-BOOST CONVERTER OPERATION
PWM Mode Operation
A
D
When the PWM pin is held high, the LTC3520 buck-boost
converteroperatesinaconstant-frequencyPWMmodeus-
ingvoltagemodecontrol.Aproprietaryswitchingalgorithm
allows the converter to switch between buck, buck-boost,
and boost modes without discontinuity in inductor cur-
rent or loop characteristics. The switch topology for the
buck-boost converter is shown in Figure 1.
B
C
PGND1
PGND1
LTC3520
3520 F01
Figure 1. Buck-Boost Switch Topology
3520f
12
LTC3520
OPERATION
Error Amplifier
designed to improve efficiency at light loads and reduce
the standby current at zero load. In Burst Mode operation,
the inductor is charged with fixed peak amplitude current
pulses. These current pulses are repeated as often as
necessary to maintain the output regulation voltage. The
typicaloutputcurrentwhichcanbesuppliedinBurstMode
operation is dependent upon the input and output voltage
as given by the following formula:
The error amplifier operates in voltage mode. Appropriate
loop compensation components must be utilized around
the amplifier (between the FB1 and V pins) in order
to ensure stable operation. For improved bandwidth, an
additional RC feedforward network can be placed across
the upper feedback divider resistor.
C1
Current Limit Operation
0.13 • V
IN
IOUT(MAX),BURST
=
A
The buck-boost converter has two current limit circuits.
The primary current limit is an average current limit circuit
which injects an amount of current into the feedback node
which is proportional to the extent that the switch A cur-
rent exceeds the current limit value. Due to the high gain
of this loop, the injected current forces the error amplifier
outputtodecreaseuntiltheaveragecurrentthroughswitch
A decreases approximately to the current limit value. The
averagecurrentlimitutilizestheerroramplifierinanactive
state and thereby provides a smooth recovery with little
overshootoncethecurrentlimitfaultconditionisremoved.
Since the current limit is based on the average current
through switch A, the peak inductor current in current limit
will have a dependency on the duty cycle (i.e., on the input
and output voltages in the overcurrent condition).
V + VOUT
IN
InBurstModeoperation,theerroramplifierisnotusedbut
is instead placed in a low current standby mode to reduce
supply current and improve light load efficiency.
Soft-Start
The buck-boost converter incorporates a voltage mode
soft-start circuit which is adjustable via the value of an
external soft-start capacitor, C . The typical soft-start
SS
duration is given by the following equation:
t (ms) = 0.15C (nF)
SS
SS
The converter remains in regulation during soft-start and
will therefore respond to output load transients that occur
during this time. In addition, the output voltage rise time
has minimal dependency on the size of the output capaci-
tor or load. During soft-start, the buck-boost converter is
forced into PWM operation regardless of the state of the
PWM1 pin.
The speed of the average current limit circuit is limited by
thedynamicsoftheerroramplifier. Onahardoutputshort,
it would be possible for the inductor current to increase
substantially beyond current limit before the average cur-
rent limit circuit would react. For this reason, there is a
second current limit circuit which turns off switch A if the
current ever exceeds approximately 150% of the average
current limit value. This provides additional protection in
the case of an instantaneous hard output short.
Transition From Burst to PWM Operation
In Burst Mode operation, the compensation network is
not used and the V pin is disconnected from the error
C1
amplifier. During long periods of Burst Mode operation,
leakage currents in the external components or on the
PCB could cause the compensation capacitor to charge
or discharge resulting in a large output transient when
returning to the fixed frequency mode of operation. To
prevent this from happening, the LTC3520 employs an
Reverse Current Limit
The reverse current comparator on switch D monitors the
inductor current entering the V
pin. If this current
OUT1
exceeds 560mA (typical) switch D is turned off for the
remainder of the switching cycle.
active clamp circuit that holds the voltage on the V pin
C1
to the optimal level during Burst Mode operation. This
minimizes any output transient when returning to fixed
frequency operation.
Burst Mode Operation
With the PWM1 pin held low, the buck-boost converter
operatesutilizingavariablefrequencyswitchingalgorithm
3520f
13
LTC3520
OPERATION
COMMON FUNCTIONS
Alternatively, the gain block can be utilized as an LDO
with the addition of an external PNP as shown in
Figure 3. The LDO is convenient for applications requiring
a third output (possibly a low current 2.5V or a quiet 3V
supply).AnexternalPMOScanbeusedinplaceofthePNP,
but a much larger output capacitor is required to ensure
stability at light load. The gain block has an independent
shutdown pin (SD3) and should be disabled when not in
use to reduce quiescent current.
Oscillator
The buck-boost and buck converters operate from a com-
mon internal oscillator. The switching frequency for both
converters is set by the value of an external resistor, R ,
T
located between the R pin and ground according to the
T
following equation:
54,000
f(kHz) =
Thermal Shutdown
RT(kΩ)
If the die temperature exceeds 150ºC both converters
will be disabled. All power devices will be turned off and
all switch nodes will be high impedance. The soft-start
circuits for both converters are reset during thermal
shutdown to provide a smooth recovery once the over-
temperature condition is eliminated. Both converters will
restart (if enabled) when the die temperature drops to
approximately 140ºC.
Gain Block
The LTC3520 contains a gain block (pins A and A
)
OUT
IN
that can be used as a low battery indicator or power-good
comparator for either the buck or buck-boost output volt-
age. Typical circuits for these applications are shown in
Figure2.Asmall-valuedcapacitorcanbeaddedfromA
OUT
toGNDtoprovidefilteringandpreventglitchingduringslow
transitions through the threshold region. The gain block
is not disabled by the undervoltage lockout. This allows
the uncommitted amplifier to be utilized as a low battery
indicator down to a supply voltage of 1.6V typically.
Undervoltage Lockout
If the supply voltage decreases below 2V (typical) then
both converters will be disabled and all power devices will
be turned off. The soft-start circuits for both converters
are reset during undervoltage lockout to provide a smooth
restartoncetheinputvoltagerisesabovetheundervoltage
lockout threshold.
The A
pin is not an open-drain output. Rather, it is a
OUT
push-pull output that can both sink and source current.
The uncommitted amplifier is internally powered by the
higher of either the SV or V
the maximum voltage on the A
supply voltage or the buck-boost output voltage, which-
ever is larger.
voltages. This restricts
OUT1
IN
pin to either the input
OUT
3.3V
LBO
PGOOD
A
A
A
A
OUT
OUT
A
A
OUT
V
330pF
750k
OUT
V
V
2.5V
BAT
OUT
LTC3520
LTC3520
LTC3520
200mA
2.49M
806k
169k
33pF
IN
IN
4.7µF
IN
402k
76.8k
3520 F02
3520 F05
Figure 2. Gain Block Used as a Comparator
Figure 3. Gain Block Configured as an LDO
3520f
14
LTC3520
APPLICATIONS INFORMATION
The basic LTC3520 application circuit is shown as the
Typical Application on the front page of this datasheet.
The external component selection is determined by the
desired output voltages, output currents, and ripple volt-
age requirements of each particular application. However,
basicguidelinesandconsiderationsforthedesignprocess
are provided in this section.
In particularly space restricted applications it may be
advantageous to use a much smaller value inductor at
the expense of larger ripple current. In such cases, the
converter will operate in discontinuous conduction for a
wider range of output loads and efficiency will be reduced.
In addition, there is a minimum inductor value required
to maintain stability of the current loop (given the fixed
internal slope compensation). Specifically, if the buck
converter is going to be utilized at duty cycles over 40%,
Operating Frequency Selection
the inductance value must be at least L
the following equation:
as given by
MIN
The operating frequency choice is a tradeoff between ef-
ficiencyandapplicationarea.Higheroperatingfrequencies
allow the use of smaller inductors and smaller input and
outputcapacitors,therebyreducingapplicationarea.How-
ever, higheroperatingfrequenciesalsoincreaseswitching
lossesandthereforedecreaseefficiency.Typicalefficiency
versusswitchingfrequencycurvesforbothconvertersare
given in the Typical Performance Characteristics section
of this datasheet.
L
= 1.4 • V µH
OUT
MIN
Table 1 depicts the minimum required inductance for
several common output voltages.
Table 1. Buck Minimum Inductance
OUTPUT VOLTAGE
MINIMUM INDUCTANCE
0.8V
1.2V
2V
1.1µH
1.7µH
2.8µH
3.8µH
4.5µH
Buck Inductor Selection
The choice of buck inductor value influences both the ef-
ficiency and the magnitude of the output voltage ripple.
Largerinductancevalueswillreduceinductorcurrentripple
and will therefore lead to lower output voltage ripple. For a
fixedDCresistance,alargervalueinductorwillyieldhigher
efficiency by lowering the peak current and reducing core
losses. However, a larger inductor within the same family
will generally have a greater series resistance, thereby
offsetting this efficiency advantage.
2.7V
3.3V
Buck Output Capacitor Selection
A low ESR output capacitor should be utilized at the buck
output in order to minimize voltage ripple. Multilayer
ceramic capacitors are an excellent choice as they have
low ESR and are available in small footprints. In addi-
tion to controlling the ripple magnitude, the value of the
output capacitor also sets the loop crossover frequency
and therefore can impact loop stability. There is both a
minimum and maximum capacitance value required to
ensure stability of the loop. If the output capacitance is
too small, the loop crossover frequency will increase to
the point where switching delay and the high frequency
parasitic poles of the error amplifier will degrade the
phase margin. In addition, the wider bandwidth produced
by a small output capacitor will make the loop more sus-
ceptible to switching noise. At the other extreme, if the
output capacitor is too large, the crossover frequency
can decrease too far below the compensation zero and
also lead to degraded phase margin. Table 2 provides a
guideline for the range of allowable values of low ESR
Givenadesiredpeaktopeakcurrentripple,ΔI ,therequired
L
inductor can be calculated via the following expression,
where f represents the switching frequency in MHz:
⎛
⎞
⎟
⎠
1
f∆IL
VOUT
L =
VOUT 1−
µH
⎜
⎝
V
IN
AreasonablechoiceforripplecurrentisΔI =240mAwhich
L
represents40%ofthemaximum600mAloadcurrent. The
DC current rating of the inductor should be at least equal
to the maximum load current plus half the ripple current
in order to prevent core saturation and loss of efficiency
during operation. To optimize efficiency, an inductor with
low series resistance should be utilized.
3520f
15
LTC3520
APPLICATIONS INFORMATION
output capacitors. Larger value output capacitors can
be accommodated provided they have sufficient ESR to
stabilize the loop or by adding a feedforward capacitor in
parallel with the upper feedback resistor.
feedforward capacitor be placed in parallel with R2 in
order to improve the transient response and reduce Burst
Mode ripple.
Buck-Boost Output Voltage Programming
Table 2. Buck Output Capacitor Range
The buck-boost output voltage is set by a resistive divider
according to the following formula:
V
C
C
MAX
OUT
MIN
0.8V
1.2V
1.8V
2.7V
3.3V
30µF
15µF
10µF
7µF
100µF
50µF
30µF
22µF
20µF
R2
R1
⎛
⎞
VOUT = 0.782V 1+
⎜
⎝
⎟
⎠
The external divider is connected to the output as shown in
Figure 5. In addition to setting the output voltage, the value
of R2 plays an integral role in compensation of the buck-
boost control loop. For more details, see the Closing the
Buck-Boost Feedback Loop section of this datasheet.
6µF
Buck Input Capacitor Selection
ThePV pinprovidescurrenttothebuckconverterPMOS
IN2
power switch. It is recommended that a low ESR ceramic
capacitor with a value of at least 22µF be used to bypass
this pin. The capacitor should be placed as close to the
pin as possible and have a short return to ground.
Buck-Boost Inductor Selection
To achieve high efficiency, a low ESR inductor should be
utilized for the buck-boost converter. The inductor must
haveasaturationratinggreaterthantheworstcaseaverage
inductor current plus half the ripple current. The peak-to-
peakinductorcurrentripplewillbelargerinbuckandboost
mode than in the buck-boost region. The peak-to-peak
inductor current ripple for each mode can be calculated
from the following formulas, where f is the frequency in
MHz and L is the inductance in µH:
Buck Output Voltage Programming
The buck converter output voltage is set by a resistive
divider according to the following formula:
R2
R1
⎛
⎞
VOUT = 0.790V 1+
⎜
⎝
⎟
⎠
The external divider is connected to the output as shown
in Figure 4. A reasonable compromise between noise
immunity and quiescent current is provided by choosing
R2 = 249k. The required value for R1 can then be solved
via the formula above. It is recommended that a 27pF
VOUT (V − VOUT
)
1
fL
1
IN
∆IL,P−P,BUCK
=
V
IN
V (VOUT − V )
IN
IN
∆IL,P−P,BOOST
=
fL
VOUT
0.8V ≤ V
≤ 5.25V
OUT
2.2V ≤ V
≤ 5.25V
OUT
LTC3520
FB1
LTC3520
FB2
R2
27pF
R2
R1
R1
GND
GND
3520 F05
3520 F04
Figure 4. Setting the Buck Output Voltage
Figure 5. Setting the Buck-Boost Output Voltage
3520f
16
LTC3520
APPLICATIONS INFORMATION
In addition to affecting output current ripple, the size of
the inductor can also affect the stability of the feedback
loop. In boost mode, the converter transfer function has
a right half plane zero at a frequency that is inversely
proportional to the value of the inductor. As a result, a
large inductor can move this zero to a frequency that is
low enough to degrade the phase margin of the feedback
loop. It is recommended that the inductor value be chosen
less than 10µH if the buck-boost converter is to be used
in the boost region.
capacitor is too small, the bandwidth of the converter
will extend high enough to degrade the phase margin.
To prevent this from happening, it is recommended that
a minimum value of 22µF be used for the buck-boost
output capacitor.
Buck-Boost Input Capacitor Selection
Thesupplycurrenttothebuck-boostconverterisprovided
by the PV and PV pins. It is recommended that a
IN1
IN3
low ESR ceramic capacitor with a value of at least 22µF
be located as close to this pin as possible.
Buck-Boost Output Capacitor Selection
Inductor Style and Core Material
A low ESR output capacitor should be utilized at the buck-
boost converter output in order to minimize output volt-
age ripple. Multilayer ceramic capacitors are an excellent
choice as they have low ESR and are available in small
footprints. The capacitor should be chosen large enough
to reduce the output voltage ripple to acceptable levels.
Neglecting the capacitor ESR and ESL, the peak-to-peak
output voltage ripple can be calculated by the following
Different inductor core materials and styles have an
impact on the size and price of an inductor at any given
peak current rating. Toroid or shielded pot cores in ferrite
or permalloy materials are small and reduce emissions,
but generally cost more than powdered iron core induc-
tors with similar electrical characteristics. The choice of
inductor style depends upon the price, sizing, and EMI
requirements of a particular application. However, the
inductor must also have low ESR to provide acceptable
efficiency and must be able to carry the highest current
required by the application without saturating. Table 3
provides a list of several manufacturers of inductors that
are well suited to LTC3520 applications.
formulas, where f is the frequency in MHz, C
is the
OUT
capacitance in µF, L is the inductance in µH, and I
is
LOAD
the output current in amps.
ILOAD (VOUT − V )
IN
∆VP−P, BOOST
∆VP−P, BUCK
=
COUT VOUT
f
(V − VOUT) VOUT
1
IN
=
Table 3. Inductor Vendor Information
8LCOUTf2
V
IN
MANUFACTURER
Coilcraft
Murata
PHONE
WEB SITE
847-639-6400
814-238-0490
847-956-0702
847-803-6296
847-699-7864
www.coilcraft.com
www.murata.com
www.sumida.com
www.component.tdk.com
www.tokoam.com
Since the output current is discontinuous in boost mode,
the ripple in this mode will generally be much larger than
the magnitude of the ripple in buck mode. In addition to
controlling the ripple magnitude, the value of the output
capacitoralsoaffectsthelocationoftheresonantfrequency
in the open loop converter transfer function. If the output
Sumida
TDK
TOKO
3520f
17
LTC3520
APPLICATIONS INFORMATION
Capacitor Vendor Information
where L is the inductance in henries and C
is the output
OUT
capacitance in farads. The output filter zero is given by:
BoththeinputandoutputcapacitorsusedwiththeLTC3520
must be low ESR and designed to handle the large AC cur-
rents generated by switching converters. The vendors in
Table 4 provide capacitors that are well suited to LTC3520
application circuits.
1
fFILTER_ZERO
=
Hz
2πRESR COUT
whereR istheequivalentseriesresistanceoftheoutput
ESR
capacitor. A challenging aspect of the loop dynamics in
boost mode is the presence of a right half plane zero at
the frequency given by:
Table 4. Capacitor Vendor Information
MANU-
FACTURER WEB SITE
PART NUMBER
2
V
IN
Taiyo Yuden www.t-yuden.com
JMK212BJ226MG-T
22µF, 6.3V
fRHPZ
=
Hz
2πIOUT LVOUT
TDK
www.component.tdk.com C3216X5ROJ106KB
10 F, 6.3V
The loop gain is typically rolled off to below unity gain
before the worst case right half plane zero frequency.
µ
Sanyo
www.secc.co.jp
6APD10M 10
GRM21BR60J226ME39
22 F, 6.3V
µF, 6.3V
Murata
www.murata.com
A simple Type I compensation network as shown in
Figure 6 can be utilized to stabilize the buck-boost
converter. However, this will yield a relatively low band-
width and slow transient response. To ensure sufficient
phase margin using Type I compensation, the loop must
be crossed over a decade before the LC double pole fre-
quency. The unity-gain frequency of the error amplifier
with Type I compensation is given by:
µ
Closing the Buck-Boost Feedback Loop
TheLTC3520buck-boostconverteremploysvoltagemode
PWMcontrol.Thecontroltooutputgainvarieswithopera-
tional region (buck, boost, or buck-boost), but is usually
no greater than 24dB. The output filter exhibits a double
pole response as given by the following equations:
1
fUG
=
Hz
1
2πR1CP1
fFILTER_POLE
=
Hz(Buck Mode)
2π LCOUT
1
fFILTER_POLE
=
Hz(Boost Mode)
2π VOUT LCOUT
V
OUT
0.782V
+
R1
FB1
18
–
C
R2
P1
V
C1
15
3520 F06
Figure 6. Type I Compensation Network
3520f
18
LTC3520
APPLICATIONS INFORMATION
Most applications require a faster transient response than
canbeattainedusingTypeIcompensationinordertoreduce
the size of the output capacitor. To achieve a higher loop
bandwidth, Type III compensation is required, providing
two zeros to compensate for the double pole response of
the output filter. Referring to Figure 7, the location of the
compensation poles and zeros are given as follows:
PCB Layout Considerations
The LTC3520 switches large currents at high frequencies.
Special care should be given to the PCB layout to ensure
stable, noise-free operation. Figure 8 depicts the recom-
mended PCB layout to be utilized for the LTC3520. A few
key guidelines follow:
1. All circulating current paths should be kept as short as
possible.Thiscanbeaccomplishedbykeepingtheroutes
to all bold components in Figure 8 as short and as wide
as possible. Capacitor ground connections should via
downtothegroundplanebytheshortestroutepossible.
1
fPOLE1
fZERO1
fZERO2
fPOLE2
≅
=
=
=
Hz ≅ 0Hz
2π(32000)R1CP1
1
Hz
2πRZCP1
ThebypasscapacitorsonPV ,PV ,andPV should
IN1
IN2
IN3
1
be placed as close to the IC as possible and should have
the shortest possible paths to ground.
Hz
2πR1CZ1
1
2. The small signal ground pad (SGND) should have a
single-point connection to the power ground. A con-
venient way to achieve this is to short the pin directly
to the Exposed Pad as shown in Figure 8.
Hz
2πRZCP2
where all resistances are in ohms and all capacitances
are in farads.
3. The components shown in bold and their connections
should all be placed over a complete ground plane to
reduce the cross-sectional area of circulating current
paths.
V
OUT
0.782V
+
–
C
R1
Z1
4. To prevent large circulating currents from disrupting
the output voltage sensing, the ground for each resistor
divider should be returned directly to the small signal
ground pin (SGND).
FB1
18
C
R2
P1
V
C1
R
Z
15
C
P2
5. Use of vias in the die attach pad will enhance the ther-
mal environment of the converter especially if the vias
extend to a ground plane region on the exposed bottom
surface of the PCB.
3520 F07
Figure 7. Type III Compensation Network
3520f
19
LTC3520
APPLICATIONS INFORMATION
BUCK-BOOST
V
OUT
24 23 22 21 20 19
C
SS1
FB1
SS1
SV
1
2
3
4
5
6
18
17
16
15
14
13
IN
KELVIN DIRECTLY
TO PIN 16
A
OUT
A
SGND
IN
R
T
KELVIN DIRECTLY
TO PIN 16
V
FB2
R
T
C1
SGND
PWM1
SD1
SS2
C
SS2
7
8
9 10 11 12
R
BURST
BUCK V
OUT
3520 F08
UNINTERRUPTED GROUND PLANE MUST EXIST UNDER ALL COMPONENTS
SHOWN IN BOLD AND UNDER TRACES CONNECTING TO THOSE COMPONENTS.
VIA TO GROUND PLANE
Figure 8. LTC3520 Recommended PCB Layout
3520f
20
LTC3520
TYPICAL APPLICATIONS
Sequenced Buck Converter Start-Up
3.3V at 500mA and 1.8V at 600mA Outputs
V
IN
2.2V TO 4.2V
C1
Li-Ion
L1
22µF
4.7µH
L2
PV
PV
PV SV SW1A
IN3 IN
IN1
IN2
V
V
3.3µH
OUT
3.3V
SW1B
OUT
1.8V
600mA
SW2
V
OUT1
470pF
500mA
1A FOR V ≥ 3V
C3
22µF
56pF
10k
255k
27pF
0.022µF
IN
V
C1
C2
22µF
1M
FB2
SS2
15k
200k
FB1
SS1
0.022µF
309k
LTC3520
54.9k
301k
442k
R
T
A
IN
PWM
PWM1
PWM2
SD3
BURST
158k
A
OUT
ON
SD1
OFF
SD2
PGND1 SGND PGND2
470pF
499k
C1, C2, C3: TAIYO YUDEN CERAMIC JMK212BJ226MG-T
L1: TDK RLF7030T-4R7M3R4 4.7µH
L2: TDK RLF7030T-3R3M4R 3.3µH
THE BUCK CONVERTER IS ENABLED WHEN THE
BUCK-BOOST OUTPUT VOLTAGE REACHES 3.0V.
3520 TA02a
Typical Waveforms During Power-Up
SD1, SD3
5V/DIV
BUCK-BOOST
V
OUT
2V/DIV
A
OUT
5V/DIV
BUCK V
OUT
1V/DIV
3520 TA02b
1ms/DIV
3520f
21
LTC3520
TYPICAL APPLICATIONS
Dual 3.3V at 500mA and 1.2V at 600mA Supplies
with Power Good Output
V
IN
2.2V TO 4.2V
C1
Li-Ion
L1
22µF
4.7µH
L2
PV
PV
PV SV SW1A
IN3 IN
IN1
IN2
V
3.3V
500mA
1A FOR V ≥ 3V
V
3.3µH
OUT
SW1B
OUT
1.2V
600mA
SW2
V
OUT1
470pF
27pF
100k
191k
C3
22µF
56pF
10k
IN
V
C1
C2
22µF
1M
FB2
SS2
15k
0.022µF
FB1
SS1
0.022µF
309k
LTC3520
54.9k
301k
442k
R
T
A
IN
PWM
PWM1
PWM2
SD3
BURST
158k
BUCK-BOOST
PGOOD OUTPUT
A
OUT
150pF
SD2
ON
SD1
OFF
PGND1 SGND PGND2
C1, C2, C3: TAIYO YUDEN CERAMIC JMK212BJ226MG-T
L1: TDK RLF7030T-4R7M3R4 4.7µH
L2: TDK RLF7030T-3R3M4R 3.3µH
3520 TA03a
Typical Waveforms During Power-Up
SD1, SD2, SD3
5V/DIV
BUCK-BOOST V
OUT
1.5V/DIV
BUCK V
OUT
0.5V/DIV
A
OUT
(BUCK-BOOST PGOOD)
5V/DIV
3520 TA03b
1ms/DIV
3520f
22
LTC3520
PACKAGE DESCRIPTION
UF Package
24-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1697)
0.70 0.05
4.50 0.05
3.10 0.05
2.45 0.05
(4 SIDES)
PACKAGE OUTLINE
0.25 0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
BOTTOM VIEW—EXPOSED PAD
R = 0.115
PIN 1 NOTCH
R = 0.20 TYP OR
0.35 × 45° CHAMFER
0.75 0.05
4.00 0.10
(4 SIDES)
TYP
23 24
PIN 1
TOP MARK
(NOTE 6)
0.40 0.10
1
2
2.45 0.10
(4-SIDES)
(UF24) QFN 0105
0.200 REF
0.25 0.05
0.00 – 0.05
0.50 BSC
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)—TO BE APPROVED
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE, IF PRESENT
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
3520f
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
23
LTC3520
TYPICAL APPLICATION
Li-Ion to 3.3V at 500mA and 1.8V at 600mA
with Low Battery Detection
V
IN
2.2V TO 4.2V
C1
Li-Ion
L1
22µF
4.7µH
L2
PV
PV
PV SV SW1A
IN3 IN
IN1
IN2
V
V
3.3µH
OUT
SW1B
OUT
3.3V
1.8V
600mA
SW2
V
OUT1
470pF
3.3nF
500mA
C3
22µF
56pF
10k
255k
200k
1A FOR V ≥ 3V
27pF
IN
V
C1
C2
22µF
1M
FB2
SS2
15k
3.3nF
FB1
SS1
V
IN
309k
LTC3520
54.9k
301k
750k
R
T
A
IN
PWM
PWM1
PWM2
SD3
BURST
392k
A
OUT
BAT_LOW
LOW BATTERY OUTPUT
(ACTIVE LOW)
THRESHOLD = 2.3V
SD2
ON
SD1
OFF
C1, C2, C3: TAIYO YUDEN CERAMIC
JMK212BJ226MG-T
L1: TDK RLF7030T-4R7M3R4 4.7µH
L2: TDK RLF7030T-3R3M4R 3.3µH
PGND1 SGND PGND2
3520 TA04
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
LTC3410/
LTC3410B
300mA (I ), 2.25MHz Synchronous Buck V : 2.5V to 5.5V, V
: 0.8V to V , I = 26µA, I < 1µA, SC70 Packages
IN Q SD
OUT
IN
OUT(RANGE)
OUT(RANGE)
OUT(RANGE)
DC/DC Converter
LTC3440
600mA (I ), 2MHz Synchronous Buck-
V : 2.5V to 5.5V, V
IN
: 2.5V to 5.5V, I = 25µA, I < 1µA, MS and DFN Packages
Q SD
OUT
Boost DC/DC Converter
LTC3441
1.2A (I ), 2MHz Synchronous Buck-
V : 2.4V to 5.5V, V
IN
: 2.4V to 5.25V, I = 25µA, I < 1µA, DFN Package
Q SD
OUT
Boost DC/DC Converter
LTC3442
LTC3443
LTC3444
LTC3455
LTC3456
LTC3522
LTC3530
LTC3532
LTC3548
1.2A (I ), 2MHz Synchronous Buck-
V : 2.4V to 5.5V, V
: 2.4V to 5.25V, I = 35µA, I < 1µA, DFN Package
OUT
IN
OUT(RANGE) Q SD
Boost DC/DC Converter
600kHz, 1.2A, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, V : 2.4V to 5.5V, V
: 2.4V to 5.25V, I = 25µA, I < 1µA, DFN
OUT(RANGE) Q SD
IN
Package
1.5MHz, 400mA, Synchronous Buck-Boost V : 2.75V to 5.5V, V
DC/DC Converter
: 0.5V to 5V, I < 1µA, DFN Package
SD
IN
OUT(RANGE)
Dual DC/DC Converter with USB Power
Manager and Li-Ion Battery Charger
96% Efficiency, Seamless Transition Between Inputs, I = 110µA, I < 2µA, QFN Package
Q SD
Two Cell Multi-Output DC/DC Converter
with USB Power Manager
92% Efficiency, Seamless Transition Between Inputs, I = 180µA, I < 1µA, QFN Package
Q
SD
400mA (I ) Synchronous Buck-Boost
V : 2.4V to 5.5V, Buck-Boost V
: 2.2V to 5.25V, Buck V : 0.6V to V ,
OUT(RANGE) IN
OUT
IN
OUT(RANGE)
and 200mA Buck DC/DC Converters
I = 25µA, I < 1µA, QFN Package
Q SD
600mA (I ), 2MHz Synchronous Buck-
V : 1.8V to 5.5V, V
: 1.8V to 5.5V, I = 40µA, I < 1µA, DFN and
Q SD
OUT
IN
OUT(RANGE)
: 2.4V to 5.25V, I = 35µA, I < 1µA, DFN and
OUT(RANGE) Q SD
Boost DC/DC Converter
MSOP Packages
500mA (I ), 2MHz Synchronous Buck-
V : 2.4V to 5.5V, V
OUT
IN
Boost DC/DC Converter
MSOP Packages
400mA/800mA, 2.25MHz Dual
Synchronous Step-Down DC/DC Converter MSOP Packages
95% Efficiency, V : 2.5V to 5.5V, V
= 0.6V, I = 40µA, I < 1µA, DFN and
OUT(MIN) Q SD
IN
3520f
LT 0807 • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
●
●
© LINEAR TECHNOLOGY CORPORATION 2007
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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