LINER LTC3520 Synchronous 1a buck-boost and 600ma buck converter Datasheet

LTC3520
Synchronous 1A
Buck-Boost and 600mA
Buck Converters
FEATURES
DESCRIPTION
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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.
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Dual High Efficiency DC/DC Converters:
Buck-Boost (VOUT: 2.2V to 5.25V, IOUT = 1A at
VOUT = 3.3V, VIN ≥ 3V)
Buck (VOUT: 0.8V to VIN , IOUT = 600mA)
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
Thermal and Overcurrent Protection
<1μA Quiescent Current in Shutdown
24-Lead 4mm × 4mm QFN Package
The buck converter is current mode controlled with internal
synchronous rectification to improve efficiency. Pin-selectable Burst Mode operation can be enabled to improve light
load efficiency, or the buck converter can be operated in
low noise PWM mode for sensitive applications.
The buck-boost converter provides continuous conduction operation to maximize efficiency and minimize noise.
At light loads, use of Burst Mode operation will improve
efficiency.
APPLICATIONS
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Portable Media Players
Digital Cameras
Handheld PCs, PDAs
GPS Receivers
The LTC3520 provides a <1μA shutdown mode and overtemperature shutdown on both converters. The LTC3520
is available in a low profile (0.75mm) 24-lead 4mm ×
4mm QFN package.
L, LT, LTC, LTM, Linear Technology, the Linear logo 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
4.7μH
10μF
255k
BUCK-BOOST
IOUT = 150mA
4.7μH
22μF
PVIN1 PVIN2 PVIN3 SVIN SW1A
SW1B
VOUT1
SW2
27pF
470pF
56pF
VC1
FB2
15k
10k
FB1
SS2
0.01μF
LTC3520
54.9k
309k
SS1
RT
AOUT
PWM2
SD3
OFF ON
95
BUCK
IOUT = 250mA
90
85
80
VOUT2
PWM1
BURST PWM
47μF
1M
0.01μF
200k
VOUT1
3.3V
500mA
1A FOR
VIN ≥ 3V
EFFICIENCY (%)
VIN
2.2V TO
5.5V
VOUT2
1.8V
600mA
Efficiency vs VIN
100
33pF
100k
SD2
SD1
VOUT
1.5V
200mA
70
2.2
4.7μF
AIN
PGND1 SGND PGND2
75
110k
2.7
3.2
4.2
3.7
VIN (V)
4.7
5.2
3520 TA01b
3520 TA01
3520fa
1
LTC3520
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
VOUT1
SW1B
PGND1
SW1A
PVIN3
PVIN1
TOP VIEW
PVIN1, PVIN2, PVIN3, SVIN Voltage................ –0.3V to 6V
SW1A, SW1B, SW2 Voltage
DC............................................................ –0.3V to 6V
Pulsed <100ns ............................................ –1V to 7V
Voltage, All Other Pins ................................. –0.3V to 6V
Operating Junction Temperature Range
(Note 2).................................................. –40°C to 125°C
Maximum Junction Temperature (Note 5) ............ 125°C
Storage Temperature Range .................. –65°C to 150°C
24 23 22 21 20 19
SVIN 1
18 FB1
AOUT 2
17 SS1
AIN 3
16 SGND
25
RT 4
15 VC1
13 SS2
PWM2
PGND2
9 10 11 12
SW2
8
PVIN2
7
SD3
14 FB2
SD1 6
SD2
PWM1 5
UF PACKAGE
24-LEAD (4mm s 4mm) PLASTIC QFN
TJMAX = 125°C, θJA = 37°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3520EUF#PBF
LTC3520EUF#TRPBF
3520
24-Lead (4mm × 4mm) Plastic QFN
–40°C to 85°C
LTC3520IUF#PBF
LTC3520IUF#TRPBF
3520
24-Lead (4mm × 4mm) Plastic QFN
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
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 l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C, SVIN = PVIN1 = PVIN2 = PVIN3 = 3.6V, VOUT1 = 3.3V, RT = 54.9k,
unless otherwise noted.
PARAMETER
CONDITIONS
Quiescent Current in Shutdown
MIN
l
Input Voltage
VSD1 = VSD2 = VSD3 = 0V
LTC3520E
LTC3520I
TYP
2.2
l
l
V
0.01
0.5
1
4
μA
μA
2.2
SVIN Rising
2
Burst Mode Quiescent Current, Both Converters
VFB1 = VFB2 = 0.88V, VSD3 = 0V
55
Oscillator Frequency
RT = 54.9k
0.8
UNITS
5.5
Undervoltage Lockout
l
MAX
1
V
μA
1.2
MHz
Buck Converter
PMOS Switch Resistance
0.32
Ω
NMOS Switch Resistance
0.18
Ω
NMOS Switch Leakage
VSW2 = 5V, SVIN = PVIN1 = PVIN2 = PVIN3 = 5V
0.1
5
μA
PMOS Switch Leakage
VSW2 = 0V, SVIN = PVIN1 = PVIN2 = PVIN3 = 5V
0.1
10
μA
3520fa
2
LTC3520
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C, SVIN = PVIN1 = PVIN2 = PVIN3 = 3.6V, VOUT1 = 3.3V, RT = 54.9k,
unless otherwise noted.
PARAMETER
Feedback Voltage (FB2 Pin)
CONDITIONS
(Note 4)
l
MIN
TYP
MAX
0.771
0.790
0.809
V
1
50
nA
Feedback Input Current (FB2 Pin)
PMOS Current Limit
(Note 3)
l
0.8
Maximum Duty Cycle
VFB2 = 0.72V
l
100
Minimum Duty Cycle
VFB2 = 0.88V
l
1.25
A
%
0
Soft-Start Charging Current
6
SD2 Input High Voltage
%
μA
1.4
V
SD2 Input Low Voltage
SD2 Input Current
UNITS
0.01
0.4
V
1
μA
Buck-Boost Converter
l
Output Voltage
2.2
5.25
V
PMOS Switch Resistance
0.20
Ω
NMOS Switch Resistance
0.15
Ω
NMOS Switch Leakage
VSW1A = VSW1B = 5V, SVIN = PVIN1 = PVIN2 = PVIN3 = 5V
0.1
5
μA
PMOS Switch Leakage
VSW1A = VSW1B = 0V, SVIN = PVIN1 = PVIN2 = PVIN3 = 5V
0.1
10
μA
0.782
0.798
V
1
50
nA
l
Feedback Voltage (FB1 Pin)
0.766
Feedback Input Current (FB1 Pin)
Forward Current Limit
(Note 3)
LTC3520E
LTC3520I
l
l
1.4
1.25
2
1.85
A
A
Reverse Current Limit
(Note 3)
560
mA
Burst Mode Operation Current Limit
(Note 3)
325
mA
Error Amplifier Gain
80
dB
Error Amplifier Sink Current
500
μA
Error Amplifier Source Current
14
μA
80
%
%
Maximum Duty Cycle
Boost (% Switch C is On)
Buck (% Switch A is On)
l
70
100
l
Minimum Duty Cycle
0
Soft-Start Charging Current
6
SD1, PWM1 Input High Voltage
μA
1.4
V
SD1, PWM1 Input Low Voltage
SD1, PWM1 Input Current
%
0.01
0.4
V
1
μA
Gain Block
Quiescent Current
VAIN = 0.88V, VSD1 = VSD2 = 0V
45
l
AIN Pin Threshold Voltage
AIN Pin Input Bias Current
0.770
μA
0.786
0.802
V
1
50
nA
AOUT Sink Current
VAIN = 0.72V, VAOUT = 1.8V
17
mA
AOUT Source Current
VAIN = 0.88V, VAOUT = 1.8V
18
μA
AOUT Pin Voltage
VAIN = 0.72V, IAOUT = 1mA
Open Loop Gain
25
80
150
mV
dB
3520fa
3
LTC3520
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C, VIN = 3.6V, VOUT1 = 3.3V, VOUT2 = 1.8V, RT = 54.9k, unless
otherwise noted.
PARAMETER
CONDITIONS
Propagation Delay
A OUT Falling
MIN
TYP
MAX
11
SD3 Input High Voltage
μs
1.4
V
SD3 Input Low Voltage
SD3 Input Current
0.4
V
1
μA
0.01
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 LTC3520E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
junction temperature range are assured by design, characterization and
correlation with statistical process controls. The LTC3520I is guaranteed
to meet performance specifications over the full –40°C to 125°C operating
junction temperature range.
UNITS
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.
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.
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C, unless otherwise specified)
Buck-Boost Efficiency
Lithium-Ion to 3.3V
VIN = 4.2V
VIN = 2.7V
90
90 Burst Mode OPERATION
PWM MODE
250
200
60
50
150
40
Burst Mode
OPERATION
POWER LOSS
30
20
100
50
L = COILCRAFT
MSS6132-4.7μH
10
0
1
100
10
LOAD CURRENT (mA)
600
70
60
400
50
40
300
30
20
L = SUMIDA
CDRH3D16NP-4R7N
10
0
1000
3520 G01
500
PWM MODE
0
1
Burst Mode
OPERATION
POWER LOSS
100
10
LOAD CURRENT (mA)
POWER LOSS (mW)
Burst Mode
OPERATION
700
80
POWER LOSS (mW)
EFFICIENCY (%)
80
70
800
100
300
EFFICIENCY (%)
100
Buck Efficiency
Lithium-Ion to 2.7V
200
100
0
1000
3520 G02
3520fa
4
LTC3520
TYPICAL PERFORMANCE CHARACTERISTICS
Buck Efficiency
Lithium-Ion to 1.8V
Switching Frequency vs RT
Burst Mode
90 OPERATION
600
VIN = 4.2V
VIN = 3V
60
500
50
400
40
300
30
20
L = SUMIDA
CDRH3D16NP-4R7N
RBURST = 249k
10
0
1
Burst Mode
OPERATION
POWER LOSS
200
POWER LOSS (mW)
70
SWITCHING FREQUENCY (kHz)
700
PWM MODE
80
EFFICIENCY (%)
10000
800
100
1000
100
0
1000
100
10
LOAD CURRENT (mA)
100
10
100
RT (k)
3520 G03
1000
3520 G04
LDO Load
Transient Response
Buck-Boost Load
Transient Response
VOUT
VIN = 5V
200mV/DIV
LDO VOUT
100mV/DIV
VOUT
VIN = 2.2V
500mV/DIV
LOAD
CURRENT
100mA/DIV
(20mA TO
210mA STEP)
LOAD CURRENT
500mA/DIV
3520 G05
50μs/DIV
COUT = 4.7μF
VIN = 3.6V
VOUT = 1.5V LDO INPUT VOLTAGE = 1.8V
COUT = 47μF
L = 4.7μH
VIN = 3.3V
Buck Load Transient
Response (PWM Mode)
BUCK VOUT
100mV/DIV
LOAD CURRENT
500mA/DIV
(50mA TO
500mA STEP)
LOAD CURRENT
200mA/DIV
(5mA TO
300mA STEP)
100μs/DIV
3520 G06
Buck Load Transient
Response (Burst Mode Operation)
BUCK VOUT
100mV/DIV
VIN = 3.6V
VOUT = 1.8V
COUT = 10μF
200μs/DIV
3520 G07
VIN = 3.6V
VOUT = 1.8V
100μs/DIV
COUT = 22μF
RBURST = 249k
3520 G08
3520fa
5
LTC3520
TYPICAL PERFORMANCE CHARACTERISTICS
Buck Burst Mode Threshold
VOUT = 1.2V
Buck Burst Mode Threshold
VOUT = 1.8V
45
40
35
35
30
25
RBURST
= 274k
RBURST = 249k
20
15
10
RBURST = 301k
Buck-Boost RDS(ON)
250
L = 3.3μH
PMOS
(SWITCHES A AND D)
200
30
RDS(ON) (mΩ)
L = 4.7μH
40
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
45
RBURST = 249k
25
20
RBURST = 301k
15
150
NMOS
(SWITCHES B AND C)
100
10
50
5
5
0
2.2
2.7
3.2
4.2
3.7
VIN (V)
4.7
0
2.2
5.2
2.7
3.2
3520 G09
4.2
3.7
VIN (V)
4.7
0
–45
5.2
–25
–5
35
55
15
TEMPERATURE (°C)
75
3520 G10
350
PMOS
RDS(ON) (mΩ)
300
250
NMOS
200
150
100
50
0
–45
–25
55
35
15
TEMPERATURE (°C)
–5
75
Switching Frequency
Buck-Boost Feedback Voltage
1.0
2.0
0.8
1.5
CHANGE FROM T = 25°C (%)
Buck RDS(ON)
400
SWITCHING FREQUENCY, CHANGE FROM 25°C (%)
3520 G09b
1.0
0.5
0
–0.5
–1.0
–0.6
–25
55
35
15
TEMPERATURE (°C)
–5
75
0.6
80
0
–0.2
–0.4
–0.6
–0.8
80
3520 G14
70
60
50
40
30
20 BUCK AND BUCK-BOOST
10 CONVERTERS ENABLED,
Burst Mode OPERATION
0
4.2
3.7
2.2
3.2
2.7
VIN (V)
80
Buck Output Voltage vs
Load Current
CHANGE IN VOLTAGE FROM 100mA LOAD (%)
0.8
90
0.2
0
40
60
20
TEMPERATURE (°C)
3520 G13
No Load Input Current
0.4
–20
3520 G12
NO LOAD CURRENT (μA)
CHANGE FROM T = 25°C (%)
–0.4
–0.8
100
40
60
20
TEMPERATURE (°C)
0
–0.2
–1.0
–40
1.0
0
0.2
–2.0
–45
Buck Feedback Voltage
–20
0.4
–1.5
3520 G11
–1.0
–40
0.6
4.7
5.2
3520 G15
3
TA = 125°C
2
1
TA = 85°C
TA = –40°C, 25°C
0
–1
–2
–3
0
400
200
LOAD CURRENT (mA)
600
3520 G23
3520fa
6
LTC3520
TYPICAL PERFORMANCE CHARACTERISTICS
LDO Output Voltage vs
Load Current
0.4
0.3
TA = –40°C
0.2
TA = 25°C
0.1
0
–0.1
TA = 125°C
–0.2
–0.3
–0.4
–0.5
200
400
600
800 1000
LOAD CURRENT (mA)
1200
0.20
1800
0.15
1600
0.10
0.05
TA = 125°C
0
TA = –40°C
–0.05
TA = 25°C
–0.10
–0.15
–0.20
95
VOUT = 3.3V
100
EFFICIENCY (%)
OUTPUT CURRENT (mA)
120
20
0
2.2
2.7
3.2
4.2
3.7
VIN (V)
4.7
5.2
800
600
400
2.7
3.2
4.2
3.7
VIN (V)
4.7
5.2
3520 G16
Buck Efficiency vs Frequency
100
100
40
VOUT = 5V
1000
Buck-Boost PWM Mode
Efficiency vs Frequency
140
60
1200
3520 G25
Buck-Boost Maximum Output
Current, Burst Mode Operation
VOUT = 5V
1400
0
2.2
300
200
100
LOAD CURRENT (mA)
0
3520 G24
80
VOUT = 3.3V
200
L = 8.2μH COILCRAFT
MSS6132
L = 4.7μH COILCRAFT
MSS7341
90
L = 2.2μH
COILCRAFT
1812PS
85
80
75 VIN = 3.6V
VOUT = 3.3V
ILOAD = 200mA
70
0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
SWITCHING FREQUENCY (MHz)
3520 G17
L = 4.7μH SUMIDA
CDRH3D16NP
95
EFFICIENCY (%)
0
Buck-Boost Maximum
Output Current, PWM Mode
OUTPUT CURRENT (mA)
0.5
CHANGE IN VOLTAGE FROM ZERO LOAD (%)
CHANGE IN VOLTAGE FROM ZERO LOAD (%)
Buck-Boost Output Voltage vs
Load Current
2
L = 8.2μH
COILCRAFT
90 MSS6132
L = 2.2μH
SUMIDA
CDRH3D16NP
85
80
75 VIN = 2.5V
VOUT = 1.8V
ILOAD = 100mA
70
0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
SWITCHING FREQUENCY (MHz)
3520 G19
3520 G18
Buck-Boost Burst Mode Operation
2
Buck Burst Mode Operation
BUCK VOUT
50mV/DIV
VOUT
50mV/DIV
INDUCTOR
CURRENT
200mA/DIV
INDUCTOR
CURRENT
200mA/DIV
COUT = 22μF
L = 4.7μH
20μs/DIV
ILOAD = 25mA
VIN = 3.6V
3520 G21
VIN = 3.6V
VOUT = 1.8V
ILOAD = 10mA
10μs/DIV
RBURST = 249k
COUT = 10μF
3520 G22
3520fa
7
LTC3520
PIN FUNCTIONS
SVIN (Pin 1): Small Signal Power Supply Connection.
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
the pin with a short return path to ground. Pins PVIN1,
PVIN2, PVIN3, and SVIN must be connected together in
the application circuit.
AOUT (Pin 2): Uncommitted Amplifier Output. This pin
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.
AIN (Pin 3): Non-Inverting Input to the Uncommitted
Amplifier. In LDO applications, this pin is connected to
the LDO feedback voltage.
RT (Pin 4): Programs the Frequency of the Internal Oscillator. This pin must be tied to ground via an external resistor.
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.
PWM1 (Pin 5): Logic Input Used to Choose Between Burst
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).
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. Forcing 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.
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 (Pin 8): Uncommitted Amplifier Active-Low Shutdown
Pin. Forcing this pin above 1.4V enables the uncommitted
amplifier. Forcing this pin below 0.4V disables the uncommitted amplifier. This pin cannot be left floating.
PVIN2 (Pin 9): High Current Power Supply Connection
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
to ground. Pins PVIN1, PVIN2, PVIN3, and SVIN must be
connected together in the application circuit.
SW2 (Pin 10): Buck Converter Switch Node. This pin must
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.
PWM2 (Pin 12): Burst/PWM Mode Control Pin for the Buck
Converter. This pin can be used in the following ways:
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
unless the load current is low enough that the minimum
on-time is reached at which point the converter will
begin pulse-skipping to maintain regulation.
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
cause Burst Mode operation to be entered at higher load
currents and the Burst Mode ripple will be larger.
PWM2 forced low: With PWM2 forced to ground, the
buck converter will operate in Burst Mode operation for
all but the highest load currents. Generally, this mode of
operation is utilized to force the buck converter into Burst
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.
3520fa
8
LTC3520
PIN FUNCTIONS
SS2 (Pin 13): Buck 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.
between FB1 and ground and R2 is a resistor between
FB1 and the buck-boost output voltage:
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:
VOUT1 (Pin 19): Buck-Boost Output Voltage Node. This
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.
VOUT
⎛ R2 ⎞
= 0 . 790 V ⎜ 1 + ⎟
⎝
R1⎠
VC1 (Pin 15): Buck-Boost Error Amplifier Output. A frequency compensation network is connected to FB1 to
compensate the loop. During Burst Mode operation, VC1
is driven internally by a clamp circuit.
SGND (Pin 16): Small Signal Ground. This pin is used
as a ground reference for the internal circuitry of the
LTC3520.
⎛ R2 ⎞
VOUT = 0 . 782V ⎜ 1 + ⎟
⎝
R1⎠
SW1B (Pin 20): Buck-Boost Switch Node. This pin must
be connected to one side of the buck-boost inductor.
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.
SW1A (Pin 22): Buck-Boost Switch Node. This pin must
be connected to one side of the buck-boost inductor.
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.
PVIN1 (Pin 23), PVIN3 (Pin 24): High Current Power Supply Connections Used to Power the Buck-Boost Converter
Power Switch A. These pins should be connected together
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 PVIN1, PVIN2, PVIN3, and SVIN must be connected
together in the application circuit.
FB1 (Pin 18): Feedback Voltage for the Buck-Boost Converter. This pin is derived from a resistor divider on the
buck-boost output voltage. The buck-boost output voltage
is given by the following equation where R1 is a resistor
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.
3520fa
9
LTC3520
BLOCK DIAGRAM
23
PVIN1*
24
PVIN3*
22
SW1A
20
SW1B
19
VOUT1
A
D
0.56A
+
–
GATE
DRIVERS
B
+
C
PGND1
PGND1
Gm
–
5
6
8
3
3A
PWM1
+
–
SD3
FB1
0.782V
SS1
AIN
17
15
BANDGAP
REFERENCE
+
–
AOUT
SVIN*
18
5μA
VC1
THERMAL
SHUTDOWN
INTERNAL
VCC
1
+
–
BUCKBOOST
PWM
LOGIC
SD1
0.786V
2
+
–
+
+
–
2A
RT
OSC
DISABLE
PVIN2*
SLOPE
COMPENSATION
4
9
+
1.25A
14
FB2
13
+
+
BUCK
PWM
LOGIC
+
–
–
0.790V
SS2
+
–
Gm
SW2
PGND2
5μA
11
+
–
0A
10
SGND
PGND1
16
21
PWM2
SD2
12
7
3520 F02
*PINS SVIN, PVIN1, PVIN2 AND PVIN3 MUST BE CONNECTED TOGETHER IN THE APPLICATION.
3520fa
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 × 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 addition, 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.
BUCK CONVERTER OPERATION
PWM Mode Operation
When the PWM2 pin is held high, the LTC3520 buck converter
uses a constant-frequency, current mode control architecture. 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 current 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.
Burst Mode Operation
Burst Mode operation is enabled by either connecting
PWM2 to ground through a resistor, RBURST, or by shorting
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
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 RBURST controls the load current at which
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 RBURST is
too large, then the buck converter will not enter Burst
Mode operation at any current, especially when operating
with VIN close to the buck output voltage. Conversely, if
RBURST is too small, the ripple in Burst Mode operation
may become objectionable, especially at high input voltages. For most applications, choosing RBURST = 301k
represents a reasonable compromise.
The output voltage ripple in Burst Mode operation is dependent upon the value of RBURST, the input voltage, the
output voltage, the inductor value and the output capacitor. 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
value of RBURST.
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
Current mode control requires the use of slope compensation to prevent subharmonic oscillations in the inductor
current waveform at high duty cycle operation. This is accomplished internally on the LTC3520 through the addition
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.
3520fa
11
LTC3520
OPERATION
This leads to a reduced output current capability at large
step-down ratios. In contrast, the LTC3520 performs current limiting prior to the addition of the slope compensation
ramp and therefore achieves a peak inductor current limit
that is independent of duty cycle.
Soft-Start
The buck converter incorporates a voltage mode soft-start
circuit which is adjustable via the value of an external
soft-start capacitor, CSS. The typical soft-start duration is
given by the following equation:
tSS(ms) = 0.15CSS(nF)
The buck converter remains in regulation during soft-start
and will therefore respond to output load transients which
occur during this time. In addition, the output voltage risetime has minimal dependency on the size of the output
capacitor or load current.
Error Amplifier and Compensation
The LT3520 buck converter utilizes an internal transconductance error amplifier. Compensation of the feedback 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.
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 modulated to obtain the desired output voltage. In this case, the
converter is operating solely in boost mode.
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.
L
SW1A
PVIN1
BUCK-BOOST CONVERTER OPERATION
SW1B
A
VOUT1
D
PWM Mode Operation
When the PWM pin is held high, the LTC3520 buck-boost
converter operates in a constant-frequency PWM mode using voltage mode control. A proprietary switching algorithm
allows the converter to switch between buck, buck-boost,
and boost modes without discontinuity in inductor current or loop characteristics. The switch topology for the
buck-boost converter is shown in Figure 1.
B
LTC3520
PGND1
C
PGND1
3520 F01
Figure 1. Buck-Boost Switch Topology
3520fa
12
LTC3520
OPERATION
Error Amplifier
The error amplifier operates in voltage mode. Appropriate
loop compensation components must be utilized around
the amplifier (between the FB1 and VC1 pins) in order
to ensure stable operation. For improved bandwidth, an
additional RC feedforward network can be placed across
the upper feedback divider resistor.
Current Limit Operation
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 current exceeds the current limit value. Due to the high gain
of this loop, the injected current forces the error amplifier
output to decrease until the average current through switch
A decreases approximately to the current limit value. The
average current limit utilizes the error amplifier in an active
state and thereby provides a smooth recovery with little
overshoot once the current limit fault condition is removed.
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).
The speed of the average current limit circuit is limited by
the dynamics of the error amplifier. On a hard output short,
it would be possible for the inductor current to increase
substantially beyond current limit before the average current 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.
Reverse Current Limit
The reverse current comparator on switch D monitors the
inductor current entering the VOUT1 pin. If this current
exceeds 560mA (typical) switch D is turned off for the
remainder of the switching cycle.
Burst Mode Operation
With the PWM1 pin held low, the buck-boost converter
operates utilizing a variable frequency switching algorithm
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
typical output current which can be supplied in Burst Mode
operation is dependent upon the input and output voltage
as given by the following formula:
IOUT(MAX ),BURST =
0 . 13 • VIN
A
VIN + VOUT
In Burst Mode operation, the error amplifier is not used but
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, CSS. The typical soft-start
duration is given by the following equation:
tSS(ms) = 0.15CSS(nF)
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 capacitor or load. During soft-start, the buck-boost converter is
forced into PWM operation regardless of the state of the
PWM1 pin.
Transition From Burst to PWM Operation
In Burst Mode operation, the compensation network is
not used and the VC1 pin is disconnected from the error
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
active clamp circuit that holds the voltage on the VC1 pin
to the optimal level during Burst Mode operation. This
minimizes any output transient when returning to fixed
frequency operation.
3520fa
13
LTC3520
OPERATION
COMMON FUNCTIONS
Oscillator
The buck-boost and buck converters operate from a common internal oscillator. The switching frequency for both
converters is set by the value of an external resistor, RT,
located between the RT pin and ground according to the
following equation:
f(kHz) =
54, 000
R T (kΩ)
Thermal Shutdown
Gain Block
The LTC3520 contains a gain block (pins AIN and AOUT)
that can be used as a low battery indicator or power-good
comparator for either the buck or buck-boost output voltage. Typical circuits for these applications are shown in
Figure 2. A small-valued capacitor can be added from AOUT
to GND to provide filtering and prevent glitching during slow
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.
The AOUT pin is not an open-drain output. Rather, it is a
push-pull output that can both sink and source current.
The uncommitted amplifier is internally powered by the
higher of either the SVIN or VOUT1 voltages. This restricts
the maximum voltage on the AOUT pin to either the input
supply voltage or the buck-boost output voltage, whichever is larger.
LBO
AOUT
VBAT
LTC3520
2.49M
PGOOD
VOUT
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 overtemperature condition is eliminated. Both converters will
restart (if enabled) when the die temperature drops to
approximately 140ºC.
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
restart once the input voltage rises above the undervoltage
lockout threshold.
3.3V
AOUT
VOUT
2.5V
200mA
AOUT
LTC3520
169k
330pF
LTC3520
750k
AIN
806k
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). An external PMOS can be used in place of the PNP,
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.
33pF
AIN
4.7μF
76.8k
AIN
3520 F05
402k
Figure 3. Gain Block Configured as an LDO
3520 F02
Figure 2. Gain Block Used as a Comparator
3520fa
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 voltage requirements of each particular application. However,
basic guidelines and considerations for the design process
are provided in this section.
Operating Frequency Selection
The operating frequency choice is a tradeoff between efficiency and application area. Higher operating frequencies
allow the use of smaller inductors and smaller input and
output capacitors, thereby reducing application area. However, higher operating frequencies also increase switching
losses and therefore decrease efficiency. Typical efficiency
versus switching frequency curves for both converters are
given in the Typical Performance Characteristics section
of this datasheet.
Buck Inductor Selection
The choice of buck inductor value influences both the efficiency and the magnitude of the output voltage ripple.
Larger inductance values will reduce inductor current ripple
and will therefore lead to lower output voltage ripple. For a
fixed DC resistance, a larger value inductor will yield higher
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.
Given a desired peak to peak current ripple, ΔIL, the required
inductor can be calculated via the following expression,
where f represents the switching frequency in MHz:
L=
⎛
⎞
1
V
VOUT ⎜ 1 − OUT ⎟ μ H
fΔ IL
VIN ⎠
⎝
A reasonable choice for ripple current is ΔIL = 240mA which
represents 40% of the maximum 600mA load current. 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.
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%,
the inductance value must be at least LMIN as given by
the following equation:
LMIN = 1.4 • VOUT μH
Table 1 depicts the minimum required inductance for
several common output voltages.
Table 1. Buck Minimum Inductance
OUTPUT VOLTAGE
MINIMUM INDUCTANCE
0.8V
1.1μH
1.2V
1.7μH
2V
2.8μH
2.7V
3.8μH
3.3V
4.5μH
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 addition 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 susceptible 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
3520fa
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
VOUT
CMIN
CMAX
0.8V
30μF
100μF
1.2V
15μF
50μF
1.8V
10μF
30μF
2.7V
7μF
22μF
3.3V
6μF
20μF
Buck Input Capacitor Selection
The PVIN2 pin provides current to the buck converter PMOS
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 Output Voltage Programming
The buck converter output voltage is set by a resistive
divider according to the following formula:
⎛ R2 ⎞
VOUT = 0 . 790 V ⎜ 1 + ⎟
⎝
R1⎠
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
The buck-boost output voltage is set by a resistive divider
according to the following formula:
⎛ R2 ⎞
VOUT = 0 . 782V ⎜ 1 + ⎟
⎝
R1⎠
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 buckboost control loop. For more details, see the Closing the
Buck-Boost Feedback Loop section of this datasheet.
Buck-Boost Inductor Selection
To achieve high efficiency, a low ESR inductor should be
utilized for the buck-boost converter. The inductor must
have a saturation rating greater than the worst case average
inductor current plus half the ripple current. The peak-topeak inductor current ripple will be larger in buck and boost
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:
1 VOUT ( VIN − VOUT )
fL
VIN
1 VIN ( VOUT − VIN )
=
fL
VOUT
Δ IL,P −P,BUCK =
Δ IL,P −P,BOOST
0.8V ≤ VOUT ≤ 5.25V
2.2V ≤ VOUT ≤ 5.25V
LTC3520
R2
27pF
LTC3520
FB2
R2
FB1
R1
R1
GND
GND
3520 F04
Figure 4. Setting the Buck Output Voltage
3520 F05
Figure 5. Setting the Buck-Boost Output Voltage
3520fa
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.
Buck-Boost Output Capacitor Selection
A low ESR output capacitor should be utilized at the buckboost converter output in order to minimize output voltage 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
formulas, where f is the frequency in MHz, COUT is the
capacitance in μF, L is the inductance in μH, and ILOAD is
the output current in amps.
Δ VP −P, BOOST =
Δ VP −P, BUCK =
ILOAD ( VOUT − VIN )
COUT VOUT f
1
8LCOUT
f2
( VIN − VOUT ) VOUT
VIN
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
capacitor also affects the location of the resonant frequency
in the open loop converter transfer function. If the output
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
The supply current to the buck-boost converter is provided
by the PVIN1 and PVIN3 pins. It is recommended that a
low ESR ceramic capacitor with a value of at least 22μF
be located as close to this pin as possible.
Inductor Style and Core Material
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 inductors 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.
Table 3. Inductor Vendor Information
MANUFACTURER
PHONE
WEB SITE
Coilcraft
847-639-6400
www.coilcraft.com
Murata
814-238-0490
www.murata.com
Sumida
847-956-0702
www.sumida.com
TDK
847-803-6296
www.component.tdk.com
TOKO
847-699-7864
www.tokoam.com
3520fa
17
LTC3520
APPLICATIONS INFORMATION
Capacitor Vendor Information
Both the input and output capacitors used with the LTC3520
must be low ESR and designed to handle the large AC currents generated by switching converters. The vendors in
Table 4 provide capacitors that are well suited to LTC3520
application circuits.
Table 4. Capacitor Vendor Information
MANUFACTURER WEB SITE
PART NUMBER
Taiyo Yuden
JMK212BJ226MG-T
22μF, 6.3V
www.t-yuden.com
www.component.tdk.com
C3216X5ROJ106KB
10μF, 6.3V
Sanyo
www.secc.co.jp
6APD10M 10μF, 6.3V
Murata
www.murata.com
GRM21BR60J226ME39
22μF, 6.3V
Closing the Buck-Boost Feedback Loop
The LTC3520 buck-boost converter employs voltage mode
PWM control. The control to output gain varies with operational 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:
f FILTER _ POL E =
1
2π LCOUT
f FILTER _ ZERO =
2π VOUT LCOUT
2π RESR COUT
Hz
VIN 2
Hz
2π IOUT LVOUT
The loop gain is typically rolled off to below unity gain
before the worst case right half plane zero frequency.
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 bandwidth 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 frequency. The unity-gain frequency of the error amplifier
with Type I compensation is given by:
Hz (Buck Mode)
1
1
where RESR is the equivalent series resistance of the output
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:
f RHPZ =
TDK
f FILTER _ POLE =
where L is the inductance in henries and COUT is the output
capacitance in farads. The output filter zero is given by:
f UG =
1
Hz
2π R1 CP1
Hz (Boost Mode)
VOUT
+
0.782V
R1
FB1
–
18
VC1
15
CP1
R2
3520 F06
Figure 6. Type I Compensation Network
3520fa
18
LTC3520
APPLICATIONS INFORMATION
Most applications require a faster transient response than
can be attained using Type I compensation in order to reduce
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:
1
Hz
2π R1C Z1
1
Hz
=
2π R Z CP2
f ZERO2 =
where all resistances are in ohms and all capacitances
are in farads.
VOUT
+
0.782V
R1
FB1
–
CZ1
18
VC1
CP1
RZ
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 recommended 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. This can be accomplished by keeping the
routes to all bold components in Figure 8 as short
and as wide as possible. Capacitor ground connections should via down to the ground plane by the
shortest route possible. The bypass capacitors on
PVIN1, PVIN2 , and PVIN3 should be placed as close
to the IC as possible and should have the shortest
possible paths to ground.
1
fPOLE1 ≅
Hz ≅ 0Hz
2π (32000)R1CP1
1
Hz
f ZERO1 =
2π R Z CP 1
fPOLE2
PCB Layout Considerations
R2
15
CP2
3520 F07
Figure 7. Type III Compensation Network
2. The small signal ground pad (SGND) should have a
single-point connection to the power ground. A convenient way to achieve this is to short the pin directly
to the Exposed Pad as shown in Figure 8.
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.
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).
5. Use of vias in the die attach pad will enhance the thermal environment of the converter especially if the vias
extend to a ground plane region on the exposed bottom
surface of the PCB.
3520fa
19
LTC3520
BUCK-BOOST
VOUT
VOUT1
SV1B
PGND1
SW1A
PVIN1
PVIN3
APPLICATIONS INFORMATION
24 23 22 21 20 19
18
AOUT
2
17
AIN
3
16 SGND
RT
4
PWM1
5
SD1
6
15 VC1
FB2
14
SS2
13
PWM2
PGND2
9 10 11 12
SW2
8
PVIN2
7
SD3
SGND
1
SD2
RT
FB1
SVIN
CSS1
KELVIN DIRECTLY
TO PIN 16
SS1
KELVIN DIRECTLY
TO PIN 16
CSS2
RBURST
BUCK VOUT
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
3520fa
20
LTC3520
TYPICAL APPLICATIONS
Sequenced Buck Converter Start-Up
3.3V at 500mA and 1.8V at 600mA Outputs
VIN
2.2V TO 4.2V
C1
22μF
Li-Ion
L1
4.7μH
L2
3.3μH
VOUT
1.8V
600mA
C2
22μF
255k
PVIN1 PVIN2 PVIN3 SVIN SW1A
SW1B
VOUT1
SW2
27pF
470pF
56pF
VC1
1M
FB2
200k
15k
0.022μF
C3
22μF
VOUT
3.3V
500mA
1A FOR VIN ≥ 3V
10k
FB1
SS2
0.022μF
LTC3520
54.9k
309k
SS1
442k
RT
AIN
BURST PWM
301k
PWM1
PWM2
158k
AOUT
SD3
SD1
OFF ON
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
VOUT
2V/DIV
AOUT
5V/DIV
BUCK VOUT
1V/DIV
1ms/DIV
3520 TA02b
3520fa
21
LTC3520
TYPICAL APPLICATIONS
Dual 3.3V at 500mA and 1.2V at 600mA Supplies
with Power Good Output
VIN
2.2V TO 4.2V
C1
22μF
Li-Ion
L1
4.7μH
L2
3.3μH
VOUT
1.2V
600mA
27pF
PVIN1 PVIN2 PVIN3 SVIN SW1A
SW1B
VOUT1
SW2
100k
470pF
56pF
VC1
C2
22μF
1M
FB2
191k
10k
15k
0.022μF
C3
22μF
VOUT
3.3V
500mA
1A FOR VIN ≥ 3V
FB1
SS2
0.022μF
LTC3520
54.9k
309k
SS1
RT
442k
AIN
BURST PWM
301k
PWM1
PWM2
SD3
158k
150pF
SD2
OFF ON
BUCK-BOOST
PGOOD OUTPUT
AOUT
SD1
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 VOUT
1.5V/DIV
BUCK VOUT
0.5V/DIV
AOUT
(BUCK-BOOST PGOOD)
5V/DIV
1ms/DIV
3520 TA03b
3520fa
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
2.45 ± 0.05
3.10 ± 0.05 (4 SIDES)
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
4.00 ± 0.10
(4 SIDES)
BOTTOM VIEW—EXPOSED PAD
0.75 ± 0.05
R = 0.115
TYP
PIN 1 NOTCH
R = 0.20 TYP OR
0.35 × 45° CHAMFER
23 24
0.40 ± 0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
2.45 ± 0.10
(4-SIDES)
(UF24) QFN 0105
0.200 REF
0.00 – 0.05
0.25 ± 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
3520fa
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 representation 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
VIN
2.2V TO 4.2V
C1
22μF
Li-Ion
L1
4.7μH
L2
3.3μH
VOUT
1.8V
600mA
C2
22μF
255k
PVIN1 PVIN2 PVIN3 SVIN SW1A
SW1B
VOUT1
SW2
27pF
470pF
1M
FB2
15k
3.3nF
200k
10k
FB1
SS2
VIN
3.3nF
LTC3520
54.9k
C3
22μF
56pF
VC1
VOUT
3.3V
500mA
1A FOR VIN ≥ 3V
309k
SS1
750k
RT
AIN
BURST PWM
301k
PWM1
PWM2
AOUT
SD3
392k
LOW BATTERY OUTPUT
(ACTIVE LOW)
THRESHOLD = 2.3V
SD2
C1, C2, C3: TAIYO YUDEN CERAMIC
JMK212BJ226MG-T
L1: TDK RLF7030T-4R7M3R4 4.7μH
L2: TDK RLF7030T-3R3M4R 3.3μH
BAT_LOW
SD1
OFF ON
PGND1 SGND PGND2
3520 TA04
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC3410/
LTC3410B
300mA (IOUT), 2.25MHz Synchronous Buck VIN: 2.5V to 5.5V, VOUT(RANGE) : 0.8V to VIN, IQ = 26μA, ISD < 1μA, SC70 Packages
DC/DC Converter
LTC3440
600mA (IOUT), 2MHz Synchronous BuckBoost DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(RANGE) : 2.5V to 5.5V, IQ = 25μA, ISD < 1μA, MS and DFN Packages
LTC3441
1.2A (IOUT), 2MHz Synchronous BuckBoost DC/DC Converter
VIN: 2.4V to 5.5V, VOUT(RANGE) : 2.4V to 5.25V, IQ = 25μA, ISD < 1μA, DFN Package
LTC3442
1.2A (IOUT), 2MHz Synchronous BuckBoost DC/DC Converter
VIN: 2.4V to 5.5V, VOUT(RANGE) : 2.4V to 5.25V, IQ = 35μA, ISD < 1μA, DFN Package
LTC3443
600kHz, 1.2A, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, VIN: 2.4V to 5.5V, VOUT(RANGE): 2.4V to 5.25V, IQ = 25μA, ISD < 1μA, DFN
Package
LTC3444
1.5MHz, 400mA, Synchronous Buck-Boost VIN: 2.75V to 5.5V, VOUT(RANGE) : 0.5V to 5V, ISD < 1μA, DFN Package
DC/DC Converter
LTC3455
Dual DC/DC Converter with USB Power
Manager and Li-Ion Battery Charger
96% Efficiency, Seamless Transition Between Inputs, IQ = 110μA, ISD < 2μA, QFN Package
LTC3456
Two Cell Multi-Output DC/DC Converter
with USB Power Manager
92% Efficiency, Seamless Transition Between Inputs, IQ = 180μA, ISD < 1μA, QFN Package
LTC3522
400mA (IOUT) Synchronous Buck-Boost
and 200mA Buck DC/DC Converters
VIN: 2.4V to 5.5V, Buck-Boost VOUT(RANGE) : 2.2V to 5.25V, Buck VOUT(RANGE) : 0.6V to VIN ,
IQ = 25μA, ISD < 1μA, QFN Package
LTC3530
600mA (IOUT), 2MHz Synchronous BuckBoost DC/DC Converter
VIN: 1.8V to 5.5V, VOUT(RANGE) : 1.8V to 5.5V, IQ = 40μA, ISD < 1μA, DFN and
MSOP Packages
LTC3532
500mA (IOUT), 2MHz Synchronous BuckBoost DC/DC Converter
VIN: 2.4V to 5.5V, VOUT(RANGE) : 2.4V to 5.25V, IQ = 35μA, ISD < 1μA, DFN and
MSOP Packages
LTC3548
400mA/800mA, 2.25MHz Dual
Synchronous Step-Down DC/DC Converter
95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40μA, ISD < 1μA, DFN and
MSOP Packages
3520fa
24 Linear Technology Corporation
LT 0609 REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007