LTC3789
High Efficiency, Synchronous,
4-Switch Buck-Boost Controller
Features
Description
Single Inductor Architecture Allows VIN Above,
Below or Equal to the Regulated VOUT
n Programmable Input or Output Current
n Wide V Range: 4V to 38V
IN
n 1% Output Voltage Accuracy: 0.8V < V
OUT < 38V
n Synchronous Rectification: Up to 98% Efficiency
n Current Mode Control
n Phase-Lockable Fixed Frequency: 200kHz to 600kHz
n No Reverse Current During Start-Up
n Power Good Output Voltage Monitor
n Internal 5.5V LDO
n Quad N-Channel MOSFET Synchronous Drive
n V
OUT Disconnected from VIN During Shutdown
n True Soft-Start and V
OUT Short Protection, Even in
Boost Mode
n Available in 28-Lead QFN (4mm × 5mm) and
28-Lead SSOP Packages
The LTC®3789 is a high performance buck-boost switching regulator controller that operates from input voltages above, below or equal to the output voltage. The
constant-frequency, current mode architecture allows
a phase-lockable frequency of up to 600kHz, while an
output current feedback loop provides support for battery
charging. With a wide 4V to 38V (40V maximum) input and
output range and seamless, low noise transitions between
operating regions, the LTC3789 is ideal for automotive,
telecom and battery-powered systems.
n
The operating mode of the controller is determined through
the MODE/PLLIN pin. The MODE/PLLIN pin can select
between pulse-skipping mode and forced continuous
mode operation and allows the IC to be synchronized to an
external clock. Pulse-skipping mode offers high efficiency
and low ripple at light loads, while forced continuous
mode operates at a constant frequency for noise-sensitive
applications.
Applications
n
n
n
n
A PGOOD pin indicates when the output is within 10% of
its designed set point. The LTC3789 is available in low profile 28-pin 4mm × 5mm QFN and narrow SSOP packages.
Automotive Systems
Distributed DC Power Systems
High Power Battery-Operated Devices
Industrial Control
L, LT, LTC, LTM, Linear Technology, the Linear logo, µModule and Burst Mode are registered
trademarks and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners. Protected by U.S. Patents, including 5408150,
5481178, 5929620, 6580258, 7365525, 7394231.
Typical Application
1µF
CER
4.7µF
ILIM
VIN
4V TO
38V
PGOOD
VIN
VINSNS
TG1
B
2200pF
8k
2.2µF
+
IOSENSE
EXTVCC
VOUTSNS
TG2
LTC3789
BOOST1
BOOST2
SW2
SW1
0.1µF
1000pF
INTVCC IOSENSE
0.01µF
BG1
ITH
SS
SGND
BG2
MODE/PLLIN
RUN
VFB
FREQ
SENSE+ SENSE– PGND
0.1µF
D
100Ω
0.010Ω
10µF
16V
CER
+
C
ON/OFF
121k
VOUT
12V
5A
330µF
16V
105k, 1%
7.5k
1%
100
12
95
10
90
8
85
6
80
4
75
70
5
10
15
20
VIN (V)
0.010Ω
2
VOUT = 12V
ILOAD = 5A
0
25
30
35
POWER LOSS (W)
22µF
50V A
CER
Efficiency and Power Loss
100Ω
–
EFFICIENCY (%)
+
40
0
3789 TA01b
4.7µH
3789 TA01
3789fc
For more information www.linear.com/LTC3789
1
LTC3789
Absolute Maximum Ratings (Note 1)
Input Supply Voltage (VIN).......................... 40V to –0.3V
Topside Driver Voltages
(BOOST1, BOOST2)................................... 46V to –0.3V
Switch Voltage (SW1, SW2)........................... 40V to –5V
Current Sense Voltages (IOSENSE+, IOSENSE–).. 40V to –0.3V
BOOST1, BOOST2 – SW1, SW2.................... 6V to –0.3V
TG1, TG2 – SW1, SW2.................................. 6V to –0.3V
EXTVCC Voltage.......................................... 14V to –0.3V
INTVCC Voltage............................................. 6V to –0.3V
SENSE+, SENSE– Voltages..................... INTVCC to –0.3V
MODE/PLLIN, SS Voltages ................... INTVCC to –0.3V
VINSNS, VOUTSNS......................................... 40V to –0.3V
TG1, TG2, BG1, BG2 Voltages ............................ (Note 6)
ITH, FREQ, ILIM Voltages........................ INTVCC to –0.3V
VFB Voltage................................................ 2.7V to –0.3V
RUN, PGOOD Voltage ................................... 6V to –0.3V
Operating Junction Temperature Range
(Notes 2, 3)............................................. –40°C to 125°C
Storage Temperature Range................... –65°C to 125°C
INTVCC Peak Output Current.................................100mA
Lead Temperature (Soldering, 10 sec.)
GN Package....................................................... 300°C
Pin Configuration
TOP VIEW
26 TG1
SENSE –
4
25 BOOST1
24 PGND
23 BG1
8
21 INTVCC
RUN
9
20 EXTVCC
FREQ 5
16 SW2
IOSENSE– 14
15 TRIM
15 BOOST2
9 10 11 12 13 14
ILIM
IOSENSE+ 13
16 BG2
VOUTSNS 8
18 BOOST2
17 TG2
17 EXTVCC
VINSNS 7
19 BG2
ILIM 12
18 INTVCC
RUN 6
TG2
FREQ
19 VIN
SW2
22 VIN
TRIM
7
VOUTSNS 11
20 BG1
29
SGND
MODE/PLLIN 4
MODE/PLLIN
VINSNS 10
21 PGND
SGND 3
IOSENSE
6
22 BOOST1
ITH 2
–
SGND
5
28 27 26 25 24 23
SENSE– 1
IOSENSE+
ITH
TG1
27 SW1
3
SW1
2
PGOOD
SS
SENSE+
VFB
28 PGOOD
SS
1
SENSE+
TOP VIEW
VFB
UFD PACKAGE
28-LEAD (4mm × 5mm) PLASTIC QFN
GN PACKAGE
28-LEAD NARROW PLASTIC SSOP
TJMAX = 125°C, θJA = 34°C/W
EXPOSED PAD (PIN 29) IS SGND, MUST BE SOLDERED TO PCB
TJMAX = 125°C, θJA = 80°C/W
order information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3789EGN#PBF
LTC3789EGN#TRPBF
LTC3789
28-Lead Narrow Plastic SSOP
–40°C to 125°C
LTC3789IGN#PBF
LTC3789IGN#TRPBF
LTC3789
28-Lead Narrow Plastic SSOP
–40°C to 125°C
LTC3789EUFD#PBF
LTC3789EUFD#TRPBF
3789
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 125°C
LTC3789IUFD#PBF
LTC3789IUFD#TRPBF
3789
28-Lead (4mm × 5mm) 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.
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/
2
3789fc
For more information www.linear.com/LTC3789
LTC3789
Electrical
Characteristics
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V, unless otherwise noted.
SYMBOL
PARAMETER
VIN
Input Supply Voltage
CONDITIONS
VOUT
Output Voltage
VFB
Regulated Feedback Voltage
ITH Voltage = 1.2V (Note 4), TA = –40°C to 85°C
ITH = 1.2V, TA = 125°C, TA = –40°C to 125°C
IFB
Feedback Current
(Note 4)
VREFLNREG
Reference Voltage Line Regulation
VIN = 4V to 38V (Note 4)
VLOADREG
Output Voltage Load Regulation
(Note 4)
Measured in Servo Loop, ∆ITH Voltage = 1.4V to 2V
Measured in Servo Loop, ∆ITH Voltage = 2V to 2.5V
MIN
l
l
TYP
MAX
UNITS
4
38
V
0.8
38
V
0.808
0.812
V
V
0.792
0.788
l
l
0.800
0.800
–15
–50
nA
0.002
0.02
%/V
0.01
–0.01
0.1
–0.1
%
%
gm
Transconductance Amplifier gm
ITH = 1.2V, Sink/Source 5µA (Note 4)
1.5
IQ
Input DC Supply Current
Normal Mode
Shutdown
(Note 5)
VRUN = 0V
3
40
60
mA
µA
UVLO
Undervoltage Lockout
INTVCC Ramping Down
3.4
3.6
V
UVLO Hyst
Undervoltage Hysteresis
mmho
0.4
ISENSE+
ISENSE–
–=V
+
SENSE = 0V
SENSE Pins Current
VSENSE
IIOSENSE+
IIOSENSE–
IOSENSE Pins Current
VIOSENSE– = VIOSENSE+ = 10V
ISS
Soft-Start Charge Current
VSS = 0V
VRUN Rising
2
V
0.2
±1
µA
10
14
µA
3
4
µA
VRUN(ON)
RUN Pin On-Threshold
1.22
V
VRUN(HYS)
RUN Pin On-Hysteresis
150
mV
IRUN
RUN Pin Source Current
1.2
µA
IRUN(HYS)
RUN Pin Hysteresis Current
5
µA
VSENSE(MAX)
Maximum Current Sense Threshold
Buck Region, (IL Valley)
VFB = 0.7V
VFB = 0.7V
Boost Region, (IL Peak)
VSENSE(IAVG)
Maximum Input/Output Average
Current Sense Threshold
ILIM = 0V
ILIM Floating
ILIM = INTVCC
l
l
73
123
90
140
107
157
mV
mV
48
90
130
50
100
145
52.5
106
160
mV
mV
mV
RDSPFET(ON)
Driver Pull-Up On-Resistance
2.6
Ω
RDSNFET(ON)
Driver Pull-Down On-Resistance
1.5
Ω
TG tr
TG tf
Top Gate Rise Time
Top Gate Fall Time
25
25
ns
ns
BG tr
BG tf
Bottom Gate Rise Time
Bottom Gate Fall Time
25
25
ns
ns
TG/BG t1D
Top Gate Off to Bottom Gate On
Delay Synchronous Switch-On
Delay Time
CLOAD = 3300pF Each Driver (Note 6)
60
ns
BG/TG t1D
Bottom Gate Off to Top Gate On
Delay Top Switch-On Delay Time
CLOAD = 3300pF Each Driver (Note 6)
60
ns
DFMAX,BOOST
Maximum Duty Factor
% Switch C On
90
%
DON(MIN,BOOST)
Minimum Duty Factor for Main
Switch in Boost Operation
% Switch C On
9
%
DON(MIN,BUCK)
Minimum Duty Factor for
Synchronous Switch in Buck
Operation
% Switch B On
9
%
3789fc
For more information www.linear.com/LTC3789
3
LTC3789
Electrical
Characteristics
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
5.2
5.5
5.8
V
0.2
1.0
%
5.5
5.8
V
0.2
1.0
%
INTVCC Linear Regulator
VINTVCCVIN
Internal VCC Voltage
6.5V < VIN < 40V, VEXTVCC = 0V
VLDOVIN
INTVCC Load Regulation
ICC = 0mA to 20mA, VEXTVCC = 0V
VINTVCCEXT
Internal VCC Voltage
6.5V < VEXTVCC < 14V
5.2
VLDOEXT
INTVCC Load Regulation
ICC = 0mA to 20mA, VEXTVCC = 12V
VEXTVCC
EXTVCC Switchover Voltage
ICC = 0mA to 20mA, EXTVCC Ramping Positive
VLDOHYS
EXTVCC Hysteresis
4.7
4.8
V
0.25
V
Oscillator and Phase-Locked Loop
fNOM
Nominal Frequency
VFREQ = 1.2V
350
400
440
kHz
fLOW
Low Fixed Frequency
VFREQ = 0V
175
200
225
kHz
fHIGH
High Fixed Frequency
VFREQ = 2.4V
570
640
710
kHz
fSYNC
Synchronizable Frequency
MODE/PLLIN = External Clock
600
kHz
RMODE/PLLIN
MODE/PLLIN Input Resistance
IFREQ
Frequency Setting Current
l
200
220
8
kΩ
10
12
µA
0.1
0.3
V
±1
µA
PGOOD Output
VPGL
PGOOD Voltage Low
IPGOOD = 2mA
IPGOOD
PGOOD Leakage Current
VPGOOD = 5V
VPG
PGOOD Trip Level
VFB with Respect to Set Output Voltage
VFB Ramping Negative
VFB Ramping Positive
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 LTC3789 is tested under pulse load conditions such that
TJ ≈ TA. The LTC3789E is guaranteed to meet performance specifications
from 0°C to 85°C operating junction temperature. Specifications over
the –40°C to 125°C operating junction temperature range are assured by
design, characterization and correlation with statistical process controls.
The LTC3789I is guaranteed to meet performance specifications over the
full –40°C to 125°C operating junction temperature range.
4
–10
10
%
%
Note 3: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
LTC3789GN: TJ = TA + (PD • 80°C/W)
LTC3789UFD: TJ = TA + (PD • 34°C/W)
Note 4: The LTC3789 is tested in a feedback loop that servos VITH to a
specified voltage and measures the resultant VFB.
Note 5: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See the Applications Information
section.
Note 6: Do not apply a voltage or current to these pins. They must be
connected to capacitive loads only, otherwise permanent damage may
occur.
3789fc
For more information www.linear.com/LTC3789
LTC3789
Typical Performance Characteristics TA = 25°C unless otherwise noted.
Efficiency vs Output Current
(Boost Region)
100
100
VIN = 12V
VOUT = 12V
90
80
70
70
50
40
30
60
50
40
30
60
50
40
30
20
20
10
0
EFFICIENCY (%)
80
70
60
DCM
FCM
CIRCUIT OF FIGURE 13
10
99
100
1000
LOAD CURRENT (mA)
20
10
0
10000
CIRCUIT OF FIGURE 13
DCM
FCM
100
1000
LOAD CURRENT (mA)
10
3789 G01
10
0
10000
94
FREQUENCY
200kHz
300kHz
400kHz
520kHz
93
92
91
5
0
10
20
VIN (V)
30
4
5.0
INTVCC (V)
INTVCC VOLTAGE (V)
EFFICIENCY (%)
95
4.5
3.5
1
4
9
24
19
14
29
INPUT VOLTAGE (V)
3789 G04
INTVCC and EXTVCC Switch
Voltage vs Temperature
FALLING
3
2
1
0
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
3.0
2.5
2.0
1.5
1.0
0.5
0
80
100
4
9
24
14
19
29
INPUT VOLTAGE (V)
34
3789 G08
5
6
7
8
9
10 11 12 13 14
3789 G06
RUN Pin Threshold
vs Temperature
UNDERVOLTAGE RESET VOLTAGE AT RUN (V)
3.5
RISING
4
EXTVCC (V)
3789 G05
4.0
SUPPLY CURRENT (mA)
INTVCC AND EXTVCC SWITCH VOLTAGE (V)
4
0
34
Supply Current vs Input Voltage
6
5
3
2
4.0
40
3789 G03
EXTVCC LDO Line Regulation
5.5
96
10000
6
98
97
100
1000
LOAD CURRENT (mA)
10
3789 G02
6.0
CIRCUIT OF FIGURE 13
DCM
FCM
CIRCUIT OF FIGURE 13
Internal 5.5V LDO Line
Regulation
Efficiency vs VIN
VIN = 18V
VOUT = 12V
90
80
EFFICIENCY (%)
EFFICIENCY (%)
100
VIN = 6V
VOUT = 12V
90
Efficiency vs Output Current
(Buck Region)
Efficiency vs Output Current
(Buck-Boost Region)
1.5
1.4
1.3
1.2
1.1
RISING
FALLING
1.0
0.9
0.8
0.7
0.6
0.5
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
3789 G09
3789 G07
3789fc
For more information www.linear.com/LTC3789
5
LTC3789
Typical Performance Characteristics TA = 25°C unless otherwise noted.
Forced Continuous Mode
Forced Continuous Mode
SW1
10V/DIV
SW1
10V/DIV
SW2
10V/DIV
SW2
10V/DIV
Forced Continuous Mode
SW1
10V/DIV
SW2
10V/DIV
IL
1A/DIV
IL
1A/DIV
IL
1A/DIV
3789 G10
VIN = 12V
VOUT = 12V
Pulse-Skipping Mode
SW1
10V/DIV
SW2
10V/DIV
SW2
10V/DIV
IL
1A/DIV
IL
1A/DIV
VIN = 6V
VOUT = 12V
UNDERVOLTAGE (V)
OSCILLATOR FREQUENCY (kHz)
300
VFREQ = 0V
100
50
100
TEMPERATURE (°C)
150
3789 G16
6
3789 G14
VIN = 18V
VOUT = 12V
3789 G15
2µs/DIV
Undervoltage Threshold at VIN
vs Temperature
5.0
4.5
4.5
VFREQ = 1.2V
0
2µs/DIV
5.0
500
0
–50
IL
1A/DIV
Undervoltage Threshold at INTVCC
vs Temperature
600
200
Pulse-Skipping Mode
SW2
10V/DIV
VIN = 12V
VOUT = 12V
VFREQ = 2.4V
400
VIN = 18V
VOUT = 12V
3789 G12
4µs/DIV
SW1
10V/DIV
3789 G13
Oscillator Frequency
vs Temperature
700
3789 G11
Pulse-Skipping Mode
SW1
10V/DIV
4µs/DIV
4µs/DIV
4.0
RISING
4.0
3.5
FALLING
3.5
UNDERVOLTAGE (V)
VIN = 6V
VOUT = 12V
4µs/DIV
3.0
2.5
2.0
1.5
2.5
2.0
1.5
1.0
0.5
0.5
100
80
3789 G17
FALLING
3.0
1.0
0
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
RISING
0
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
3789 G18
3789fc
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LTC3789
Typical Performance Characteristics TA = 25°C unless otherwise noted.
Maximum Current Sense
Threshold vs Duty Factor (Buck)
150
140
140
140
130
130
110
100
90
80
70
120
110
100
90
80
70
60
60
50
50
0
20
60
40
DUTY FACTOR (%)
80
100
0
20
60
40
DUTY FACTOR (%)
150
3789 G20
50
0
–50
–100
1.5
VITH (V)
2
80
2.5
3
100
50
0
3789 G22
–100
BOOST
140
–50
–150
BUCK
90
Current Foldback Limit
CURRENT LIMIT (mV)
CURRENT LIMIT (mV)
100
1
100
160
150
0.5
110
60
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
3789 G21
100
80
200
0
130
120
Valley Current Threshold
vs VITH (Buck)
200
BOOST
70
3789 G19
Peak Current Threshold
vs VITH (Boost)
–200
MAXIMUM CURRENT LIMIT (mV)
150
120
CURRENT LIMIT (mV)
Maximum Current Limit
vs Temperature
150
CURRENT LIMIT (mV)
CURRENT LIMIT (mV)
Maximum Current Sense
Threshold vs Duty Factor (Boost)
120
100
BUCK
80
60
40
20
0
0.5
1
1.5
VITH (V)
2.5
2
3789 G23
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
VFB (V)
3789 G24
3789fc
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7
LTC3789
Typical Performance Characteristics TA = 25°C unless otherwise noted.
Load Step
Load Step
Load Step
VOUT
200mV/DIV
VOUT
200mV/DIV
VOUT
200mV/DIV
IL
2A/DIV
IL
2A/DIV
IL
2A/DIV
400µs/DIV
3789 G25
VIN = 6V
VOUT = 12V
LOAD STEP = 200mA TO 2A
3789 G26
400µs/DIV
VIN = 12V
VOUT = 12V
LOAD STEP = 300mA TO 3A
Line Transient
400µs/DIV
VIN = 18V
VOUT = 12V
LOAD STEP = 300mA TO 3A
3789 G27
Line Transient
VIN
30V TO 5V
VITH
VITH
VIN
5V TO 30V
VOUT (AC)
500mV/DIV
IL
2A/DIV
VOUT (AC)
500mV/DIV
IL
2A/DIV
1ms/DIV
8
3789 G28
1ms/DIV
3789 G29
3789fc
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LTC3789
Pin Functions
(SSOP/QFN)
VFB (Pin 1/Pin 26): Error Amplifier Feedback Pin. Receives
the feedback voltage for the controller from an external
resistive divider across the output.
SS (Pin 2/Pin 27): External Soft-Start Input. The LTC3789
regulates the VFB voltage to the smaller of 0.8V or the voltage on the SS pin. A internal 3µA pull-up current source
is connected to this pin. A capacitor to ground at this
pin sets the ramp time to final regulated output voltage.
SENSE+ (Pin 3/Pin 28): The (+) Input to the Current Sense
Comparator. The ITH pin voltage and controlled offsets
between the SENSE– and SENSE+ pins, in conjunction
with RSENSE , set the current trip threshold.
SENSE– (Pin 4/Pin 1): The (–) Input to the Current Sense
Comparator.
ITH (Pin 5/Pin 2): Error Amplifier Output and Switching Regulator Compensation Point. The channel’s
current comparator trip point increases with this control
voltage.
SGND (Pin 6/Pins 3, Exposed Pad Pin 29): Small
Signal Ground. Must be routed separately from high
current grounds to the common (–) terminals of the
CIN capacitors. In the QFN package, the exposed pad
is SGND. It must be soldered to PCB ground for rated
thermal performance.
MODE/PLLIN (Pin 7/Pin 4): Mode Selection or External
Synchronization Input to Phase Detector. This is a dualpurpose pin. When external frequency synchronization
is not used, this pin selects the operating mode. The
pin can be tied to SGND or INTVCC. SGND or below
0.8V enables forced continuous mode. INTVCC enables
pulse-skipping mode. For external sync, apply a clock
signal to this pin. The internal PLL will synchronize the
internal oscillator to the clock, and forced continuous
mode will be enabled. The PLL composition network is
integrated into the IC.
FREQ (Pin 8/Pin 5): Frequency Set Pin. There is a precision
10µA current flowing out of this pin. A resistor to ground
sets a voltage which, in turn, programs the frequency.
Alternatively, this pin can be driven with a DC voltage to
vary the frequency of the internal oscillator.
RUN (Pin 9/Pin 6): Run Control Input. Forcing the pin
below 0.5V shuts down the controller, reducing quiescent current. There are 1.2µA pull-up currents for this
pin. Once the RUN pin rises above 1.22V, the IC is turned
on, and an additional 5µA pull-up current is added to
the pin.
VINSNS (Pin 10/Pin 7): VIN Sense Input to the Buck-Boost
Transition Comparator. Connect this pin to the drain of the
top N-channel MOSFET on the input side.
VOUTSNS (Pin 11/Pin 8): VOUT Sense Input to the BuckBoost Transition Comparator. Connect this pin to the VOUT.
ILIM (Pin 12/Pin 9): Input/Output Average Current
Sense Range Input. This pin tied to SGND, INTVCC or
left floating, sets the maximum average current sense
threshold.
IOSENSE+ (Pin 13/Pin 10): The (+) Input to the Input/Output
Average Current Sense Amplifier.
IOSENSE– (Pin 14/Pin 11): The (–) Input to the Input/Output
Average Current Sense Amplifier.
TRIM (Pin 15/Pin 12): Tie this pin to GND for normal
operation. Do not allow this pin to float.
EXTVCC (Pin 20/Pin 17): External Power Input to an
Internal LDO Connected to INTVCC. This LDO supplies
INTVCC power, bypassing the internal LDO powered from
VIN whenever EXTVCC is higher than 4.8V. See EXTVCC
Connection in the Applications Information section. Do
not exceed 14V on this pin.
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9
LTC3789
Pin Functions
(SSOP/QFN)
INTVCC (Pin 21/Pin 18): Output of the Internal Linear Low
Dropout Regulator. The driver and control circuits are
powered from this voltage source. Must be bypassed to
power ground with a minimum of 4.7µF tantalum, ceramic,
or other low ESR capacitor.
VIN (Pin 22/Pin 19): Main Supply Pin. A bypass capacitor
should be tied between this pin and the power ground pin.
BG1, BG2 (Pins 23, 19/Pins 20, 16): High Current
Gate Drives for Bottom (Synchronous) N-Channel
MOSFETs. Voltage swing at these pins is from ground to
INTVCC.
PGND (Pin 24/Pin 21): Driver Power Ground. Connects
to COUT and RSENSE (–) terminal(s) of CIN.
BOOST1, BOOST2 (Pins 25, 18/Pins 22, 15): Bootstrapped
Supplies to the Top Side Floating Drivers. Capacitors
are connected between the BOOST and SW pins and
Schottky diodes are tied between the BOOST and INTVCC
pins. Voltage swing at the BOOST1 pin is from INTVCC
10
to (VIN + INTVCC). Voltage swing at the BOOST2 pin is
from INTVCC to (VOUT + INTVCC).
TG1, TG2 (Pins 26, 17/Pins 23, 14): High Current
Gate Drives for Top N-Channel MOSFETs. These are the
outputs of floating drivers with a voltage swing equal
to INTVCC – 0.5V superimposed on the switch node
voltage SW.
SW1, SW2 (Pins 27, 16/Pins 24, 13): Switch Node
Connections to Inductors. Voltage swing at the SW1
pin is from a Schottky diode (external) voltage drop
below ground to VIN. Voltage swing at the SW2 pin is
from a Schottky diode voltage drop below ground to
VOUT.
PGOOD (Pin 28/Pin 25): Open-Drain Logic Output.
PGOOD is pulled to ground when the voltage on the VFB
pin is not within ±10% of its regulation window, after the
internal 20µs power-bad mask timer expires.
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LTC3789
Block Diagram
INTVCC
VIN
BOOST1
CHARGE
PUMP
BOOST1
+
VIN
CHARGE
PUMP
BOOST2
TG1
FCB
IDREV
BUCK
LOGIC
SW1
INTVCC
–
SW1
+
BG1
PGND
IREV
RSENSE
BG2
–
FCB
BOOST
LOGIC
VFLD
+
INTVCC
TG2
ICMP
BOOST2
–
OV
IOSENSE+
+
1.2µA
RUN
SHDN
SLOPE
EA
FIN
3µA
INTVCC
RSENSE2
VOUT
IOSENSE–
–
–
IOS
MODE/
PLLIN
SW2
+
ILIM
–
VFB
–
+
+
0.80V SS
PHASE DET
220k
10µA
ITH
FREQ
SENSE+
OSCILLATOR
SENSE–
VIN
EXTVCC
VIN
4.8V
EXTVCC
0.86V
+
–
5.5V
LDO
REG
5.5V
LDO
REG
–
+
OV
PGOOD
VFB
–
5.5V
+
INTVCC
SGND
INTERNAL
SUPPLY
0.74V
+
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11
LTC3789
Operation
Main Control Loop
The LTC3789 is a current mode controller that provides
an output voltage above, equal to or below the input voltage. The LTC proprietary topology and control architecture
employs a current-sensing resistor. The inductor current
is controlled by the voltage on the ITH pin, which is the
output of the error amplifier EA. The VFB pin receives the
voltage feedback signal, which is compared to the internal
reference voltage by the EA. If the input/output current
regulation loop is implemented, the sensed inductor current is controlled by either the sensed feedback voltage
or the input/output current.
INTVCC/EXTVCC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTVCC pin.
When the EXTVCC is left open or tied to a voltage less
than 4.5V, an internal 5.5V low dropout (LDO) regulator
supplies INTVCC power from VIN. If EXTVCC is taken above
4.8V, the 5.5V regulator is turned off, and another LDO
regulates INTVCC from EXTVCC. The EXTVCC LDO allows
the INTVCC power to be derived from a high efficiency
external source such as the LTC3789 regulator output
to reduce IC power dissipation. The absolute maximum
voltage on EXTVCC is 14V.
Internal Charge Pump
Each top MOSFET driver is biased from the floating bootstrap capacitors CA and CB, which are normally recharged
by INTVCC through an external diode when the top MOSFET
is turned off. When the LTC3789 operates exclusively in
the buck or boost regions, one of the top MOSFETs is
constantly on. An internal charge pump recharges the
bootstrap capacitor to compensate for the small leakage
current through the bootstrap diode so that the MOSFET
can be kept on. However, if a high leakage diode is used
such that the internal charge pump cannot provide sufficient
12
charges to the external bootstrap capacitor, an internal
UVLO comparator, which constantly monitors the drop
across the capacitor, will sense the (BOOST – SW) voltage
when it is below 3.6V. It will turn off the top MOSFET for
about one-twelfth of the clock period every four cycles to
allow CA or CB to recharge.
Shutdown and Start-Up
The controller can be shut down by pulling the RUN
pin low. When the RUN pin voltage is below 0.5V, the
LTC3789 goes into low quiescent current mode. Releasing RUN allows an internal 1.2µA current to pull up the
pin and enable the controller. When RUN is above the
accurate threshold of 1.22V, the internal LDO will power
up the INTVCC. At the same time, a 6µA pull-up current
will kick in to provide more RUN pin hysteresis. The RUN
pin may be externally pulled up or driven directly by logic.
Be careful not to exceed the absolute maximum rating of
6V on this pin.
The start-up of the controller’s output voltage VOUT is
controlled by the voltage on the SS pin. When the voltage
on the SS pin is less than the 0.8V internal reference, the
LTC3789 regulates the VFB voltage to the SS voltage instead
of the 0.8V reference. This allows the SS pin to be used
to program soft-start by connecting an external capacitor
from the SS pin to SGND. An internal 3µA pull-up current
charges this capacitor, creating a voltage ramp on the SS
pin. As the SS voltage rises linearly from 0V to 0.8V (and
beyond), the output voltage VOUT rises smoothly from zero
to its final value. Alternatively, the SS pin can be used to
cause the start-up of VOUT to track that of another supply.
When RUN is pulled low to disable the controller, or when
INTVCC is below the undervoltage lockout threshold of
3.4V, the SS pin is pulled low by an internal MOSFET. In
undervoltage lockout, the controller is disabled and the
external MOSFETs are held off.
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LTC3789
Operation
Power switch Control
Figure 1 shows a simplified diagram of how the four
power switches are connected to the inductor, VIN, VOUT
and GND. Figure 2 shows the regions of operation for
the LTC3789 as a function of duty cycle, D. The power
switches are properly controlled so the transfer between
regions is continuous.
Switch D is always on and switch C is always off in this
region. At the start of every cycle, synchronous switch
B is turned on first. Inductor current is sensed when
synchronous switch B is turned on. After the sensed
inductor valley current falls below a reference voltage,
which is proportional to VITH, synchronous switch B is
turned off and switch A is turned on for the remainder of
the cycle. Switches A and B will alternate, behaving like
a typical synchronous buck regulator. The duty cycle of
switch A increases until the maximum duty cycle of the
converter reaches DMAX_BUCK, given by:
1
DMAX _ BUCK = 1− • 100% = 91.67%
12
VIN
VOUT
A
SW1
BG1
D
L
TG2
SW2
B
C
BG2
RSENSE
3789 F01
Figure 1. Simplified Diagram of the Output Switches
90%
DMAX
OOST
DMIN
BOOST
DMAX
BUCK
DMIN
BUCK
CLOCK
SWITCH A
SWITCH B
LOW
SWITCH C
Buck Region (VIN >> VOUT)
TG1
Figure 3 shows typical buck region waveforms. If VIN
approaches VOUT, the buck-boost region is reached.
A ON, B OFF
PWM C, D SWITCHES
BOOST REGION
FOUR SWITCH PWM
BUCK/BOOST REGION
D ON, C OFF
PWM A, B SWITCHES
BUCK REGION
HIGH
SWITCH D
IL
3780 F03
Figure 3. Buck Region (VIN >> VOUT)
Buck-Boost Region (VIN ≈ VOUT)
When VIN is close to VOUT , the controller enters buckboost region. Figure 4 shows the typical waveforms in this
region. At the beginning of a clock cycle, if the controller
starts with B and D on, the controller first operates as a
buck region. When ICMP trips, switch B is turned off, and
switch A is turned on. At 120° clock phase, switch C is
turned on. The LTC3789 starts to operate as a boost until
ICMP trips. Then, switch D is turned on for the remainder
of the clock period. If the controller starts with switches
A and C on, the controller first operates as a boost, until
ICMP trips and switch D is turned on. At 120°, switch B is
turned on, making it operate as a buck. Then, ICMP trips,
turning switch B off and switch A on for the remainder of
the clock period.
Boost Region (VIN
(
)
VOUT • VIN(MAX) – VOUT • 100
f • IOUT(MAX) • % Ripple • VIN(MAX)
H,
H
where:
f is operating frequency, Hz
% Ripple is allowable inductor current ripple
VIN(MIN) is minimum input voltage, V
VIN(MAX) is maximum input voltage, V
VOUT is output voltage, V
IOUT(MAX) is maximum output load current, A
For high efficiency, choose an inductor with low core
loss, such as ferrite. Also, the inductor should have low
DC resistance to reduce the I2R losses, and must be able
to handle the peak inductor current without saturating. To
minimize radiated noise, use a toroid, pot core or shielded
bobbin inductor.
CIN and COUT Selection
In the boost region, input current is continuous. In the
buck region, input current is discontinuous. In the buck
region, the selection of input capacitor CIN is driven by
the need to filter the input square wave current. Use a low
ESR capacitor sized to handle the maximum RMS current.
For buck operation, the input RMS current is given by:
IRMS ≈ IOUT(MAX) •
VOUT
VIN
•
–1
VIN
VOUT
Inductor Selection
The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use
of smaller inductor and capacitor values. The inductor
value has a direct effect on ripple current. The inductor
current ripple ∆IL is typically set to 20% to 40% of the
maximum inductor current in the boost region at VIN(MIN).
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT(MAX)/2. This simple worst-case condition
is commonly used for design because even significant
deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based
on only 2000 hours of life which makes it advisable to
derate the capacitor.
18
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APPLICATIONS INFORMATION
In the boost region, the discontinuous current shifts
from the input to the output, so COUT must be capable
of reducing the output voltage ripple. The effects of ESR
(equivalent series resistance) and the bulk capacitance
must be considered when choosing the right capacitor
for a given output ripple voltage. The steady ripple due to
charging and discharging the bulk capacitance is given by:
Ripple (Boost,Cap) =
(
IOUT(MAX) • VOUT – VIN(MIN)
COUT • VOUT • f
)V
where COUT is the output filter capacitor.
The steady ripple due to the voltage drop across the ESR
is given by:
In order to select the power MOSFETs, the power dissipated
by the device must be known. For switch A, the maximum
power dissipation happens in the boost region, when it
remains on all the time. Its maximum power dissipation
at maximum output current is given by:
2
PA,BOOST
V
= OUT • IOUT(MAX) • ρt • RDS(ON)
V
IN
where ρt is a normalization factor (unity at 25°C) accounting for the significant variation in on-resistance
with temperature, typically about 0.4%/°C, as shown in
Figure 11. For a maximum junction temperature of 125°C,
using a value ρt = 1.5 is reasonable.
∆VBOOST,ESR = IOUT(MAX,BOOST) • ESR
ρT NORMALIZED ON-RESISTANCE (Ω)
2.0
In buck mode, VOUT ripple is given by:
∆VOUT ≤ ∆IL • (ESR + 1 / (8 • f • COUT)
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic
and ceramic capacitors are all available in surface mount
packages. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient.
Capacitors are now available with low ESR and high ripple
current ratings, such as OS-CON and POSCAP.
Power MOSFET Selection and
Efficiency Considerations
The LTC3789 requires four external N-channel power MOSFETs, two for the top switches (switches A and D, shown
in Figure 1) and two for the bottom switches (switches
B and C, shown in Figure 1). Important parameters for
the power MOSFETs are the breakdown voltage VBR,DSS,
threshold voltage VGS,TH, on-resistance RDS(ON), reverse
transfer capacitance CRSS and maximum current IDS(MAX).
1.5
1.0
0.5
0
–50
50
100
0
JUNCTION TEMPERATURE (°C)
150
3789 F11
Figure 11. Normalized RDS(ON) vs Temperature
Switch B operates in the buck region as the synchronous
rectifier. Its power dissipation at maximum output current
is given by:
PB,BUCK =
VIN − VOUT
• IOUT(MAX) 2 • ρτ • RDS(ON)
VIN
Switch C operates in the boost region as the control switch.
Its power dissipation at maximum current is given by:
The drive voltage is set by the 5.5V INTVCC supply. Consequently, logic-level threshold MOSFETs must be used
in LTC3789 applications.
PC,BOOST =
( VOUT – VIN ) VOUT
VIN
2
• RDS(ON) + k • VOUT 3 •
• IOUT(MAX) 2 • ρt
IOUT(MAX)
VIN
• CRSS • f
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LTC3789
APPLICATIONS INFORMATION
where CRSS is usually specified by the MOSFET manufacturers. The constant k, which accounts for the loss caused
by reverse recovery current, is inversely proportional to
the gate drive current and has an empirical value of 1.7.
For switch D, the maximum power dissipation happens
in the boost region, when its duty cycle is higher than
50%. Its maximum power dissipation at maximum output
current is given by:
PD,BOOST
V
= IN
VOUT
2
V
• OUT • IOUT(MAX) • ρt • RDS(ON)
V
IN
For the same output voltage and current, switch A has the
highest power dissipation and switch B has the lowest
power dissipation unless a short occurs at the output.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
TJ = TA + P • RTH(JA)
The RTH(JA) to be used in the equation normally includes
the RTH(JC) for the device plus the thermal resistance from
the case to the ambient temperature (RTH(JC)). This value
of TJ can then be compared to the original, assumed value
used in the iterative calculation process.
Schottky Diode (D1, D2) Selection
The Schottky diodes, D1 and D2, shown in Figure 13,
conduct during the dead time between the conduction
of the power MOSFET switches. They are intended to
prevent the body diode of synchronous switches B and D
from turning on and storing charge during the dead time.
In particular, D2 significantly reduces reverse recovery
current between switch D turn-off and switch C turn-on,
which improves converter efficiency and reduces switch
C voltage stress. In order for the diode to be effective, the
inductance between it and the synchronous switch must
be as small as possible, mandating that these components
be placed adjacently.
INTVCC Regulators and EXTVCC
The LTC3789 features a true PMOS LDO that supplies
power to INTVCC from the VIN supply. INTVCC powers the
20
gate drivers and much of the LTC3789’s internal circuitry.
The linear regulator regulates the voltage at the INTVCC pin
to 5.5V when VIN is greater than 6.5V. EXTVCC can supply
the needed power when its voltage is higher than 4.8V
through another on-chip PMOS LDO. Each of these can
supply a peak current of 100mA and must be bypassed to
ground with a minimum of 1µF ceramic capacitor or low
ESR electrolytic capacitor. No matter what type of bulk
capacitor is used, an additional 0.1µF ceramic capacitor
placed directly adjacent to the INTVCC and PGND pins is
highly recommended. Good bypassing is needed to supply
the high transient current required by the MOSFET gate
drivers and to prevent interaction between the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maximum junction temperature rating for the LTC3789 to be
exceeded. The INTVCC current, which is dominated by the
gate charge current, may be supplied by either the 5.5V
linear regulator from VIN or the 5.5V LDO from EXTVCC .
When the voltage on the EXTVCC pin is less than 4.5V, the
linear regulator from VIN is enabled. Power dissipation for
the IC in this case is highest and is equal to VIN • IINTVCC. The
gate charge current is dependent on operating frequency,
as discussed in the Efficiency Considerations section. The
junction temperature can be estimated by using the equations given in Note 3 of the Electrical Characteristics. For
example, the LTC3789 INTVCC current is limited to less
than 24mA from a 24V supply in the SSOP package and
not using the EXTVCC supply:
TJ = 70°C + (28mA)(24V)(80°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (MODE/PLLIN
= SGND) at maximum VIN. When the voltage applied to
EXTVCC rises above 4.8V, the INTVCC linear regulator from
VIN is turned off and the linear regulator from EXTVCC is
turned on and remains on as long as the voltage applied
to EXTVCC remains above 4.5V. Using EXTVCC allows the
MOSFET driver and control power to be derived from the
LTC3789’s switching regulator output during normal
operation and from the VIN when the output is out of
regulation (e.g., start-up, short-circuit). Do not apply
more than 14V to EXTVCC.
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LTC3789
APPLICATIONS INFORMATION
Significant efficiency and thermal gains can be realized
by powering EXTVCC from the output, since the VIN current resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
Tying the EXTVCC pin to a 12V output reduces the junction
temperature in the previous example from 125°C to 97°C:
TJ = 70°C + (28mA)(12V)(80°C/W) = 97°C
Powering EXTVCC from the output can also provide
enough gate drive when VIN drops below 5V. This allows
a wider operating range for VIN after the controller start
into regulation.
The following list summarizes the three possible connections for EXTVCC:
1. EXTVCC left open (or grounded). This will cause INTVCC
to be powered from the internal 5.5V regulator at the
cost of a small efficiency penalty.
2. EXTVCC connected directly to VOUT (4.7V < VOUT < 14V).
This is the normal connection for the 5.5V regulator and
provides the highest efficiency.
3. EXTVCC connected to an external supply. If an external
supply is available in the 4.7V to 14V range, it may be
used to power EXTVCC provided it is compatible with
the MOSFET gate drive requirements.
Note that there is an internal body diode from INTVCC to
VIN. When INTVCC is powered from EXTVCC and VIN drops
lower than 4.5V, the diode will create a back-feeding path
from EXTVCC to VIN. To limit this back-feeding current, a
10Ω ~ 15Ω resistor is recommended between the system
VIN voltage and the chip VIN pin. To truly eliminate this
back-feeding current, a blocking Schottky diode should
be connected between the system VIN and the chip VIN.
Output Voltage
The LTC3789 output voltage is set by an external feedback resistive divider carefully placed across the output
capacitor. The resultant feedback signal is compared with
the internal precision 0.8V voltage reference by the error
amplifier. The output voltage is given by the equation:
R2
VOUT = 0.8V • 1+
R1
where R1 and R2 are defined in Figure 13.
Topside MOSFET Driver Supply (CA, DA, CB, DB)
Referring to Figure 13, the external bootstrap capacitors
CA and CB connected to the BOOST1 and BOOST2 pins
supply the gate drive voltage for the topside MOSFET
switches A and D. When the top switch A turns on, the
switch node SW1 rises to VIN and the BOOST1 pin rises
to approximately VIN + INTVCC. When the bottom switch
B turns on, the switch node SW1 drops to low and the
boost capacitor CA is charged through DA from INTVCC.
When the top switch D turns on, the switch node SW2
rises to VOUT and the BOOST2 pin rises to approximately
VOUT + INTVCC. When the bottom switch C turns on, the
switch node SW2 drops to low and the boost capacitor CB
is charged through DA from INTVCC. The boost capacitors
CA and CB need to store about 100 times the gate charge
required by the top switches A and D. In most applications, a 0.1µF to 0.47µF, X5R or X7R dielectric capacitor
is adequate.
Undervoltage Lockout
The LTC3789 has two functions that help protect the
controller in case of undervoltage conditions. A precision
UVLO comparator constantly monitors the INTVCC voltage
to ensure that an adequate gate-drive voltage is present.
It locks out the switching action when INTVCC is below
3.4V. To prevent oscillation when there is a disturbance
on the INTVCC, the UVLO comparator has 400mV of precision hysteresis.
Another way to detect an undervoltage condition is to monitor the VIN supply. Because the RUN pin has a precision
turn-on reference of 1.22V, one can use a resistor divider
to VIN to turn on the IC when VIN is high enough. An extra
5µA of current flows out of the RUN pin once its voltage
passes 1.22V. One can program the hysteresis of the run
comparator by adjusting the values of the resistive divider.
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LTC3789
APPLICATIONS INFORMATION
Soft-Start Function
Efficiency Considerations
When a capacitor is connected to the SS pin, a soft-start
current of 3µA starts to charge the capacitor. A soft-start
function is achieved by controlling the output ramp voltage according to the ramp rate on the SS pin. Current
foldback is disabled during this phase to ensure smooth
soft-start. When the chip is in the shutdown state with its
RUN pin voltage below 1.22V, the SS pin is actively pulled
to ground. The soft-start range is defined to be the voltage
range from 0V to 0.8V on the SS pin. The total soft-start
time can be calculated as:
The percent efficiency of a switching regulator is equal
to the output power divided by the input power times
100%. It is often useful to analyze individual losses
to determine what is limiting the efficiency and which
change would produce the most improvement. Although
all dissipative elements in circuit produce losses, four
main sources account for most of the losses in LTC3789
circuits:
tSOFTSTART = 0.8 •
CSS
3µA
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator will always start in pulse-skipping mode up
to SS = 0.8V.
Fault Conditions: Current Limit and Current Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage. In
the boost region, maximum sense voltage and the sense
resistance determine the maximum allowed inductor peak
current, which is:
140mV
IL(MAX,BOOST) =
RSENSE
In the buck region, maximum sense voltage and the sense
resistance determine the maximum allowed inductor valley
current, which is:
IL(MAX,BUCK) =
90mV
RSENSE
To further limit current in the event of a short circuit to
ground, the LTC3789 includes foldback current limiting.
If the output falls by more than 50%, then the maximum
sense voltage is progressively lowered to about one-third
of its full value.
22
1. DC I2R losses. These arise from the resistances of the
MOSFETs, sensing resistor, inductor and PC board traces
and cause the efficiency to drop at high output currents.
2. MOSFET Transition loss. This loss arises from the
brief amount of time switch A or switch C spends in
the saturated region during switch node transitions. It
depends upon the input voltage, load current, driver
strength and MOSFET capacitance, among other factors.
3. INTVCC current. This is the sum of the MOSFET driver
and control currents. This loss can be reduced by supplying INTVCC current through the EXTVCC pin from a
high efficiency source, such as the output (if 4.7V <
VOUT < 14V) or alternate supply if available.
4. CIN and COUT loss. The input capacitor has the difficult
job of filtering the large RMS input current to the regulator in buck mode. The output capacitor has the more
difficult job of filtering the large RMS output current in
boost mode. Both CIN and COUT are required to have
low ESR to minimize the AC I2R loss and sufficient
capacitance to prevent the RMS current from causing
additional upstream losses in fuses or batteries.
5. Other losses. Schottky diodes D1 and D2 are responsible
for conduction losses during dead time and light load
conduction periods. Inductor core loss should also be
considered. Switch C causes reverse recovery current
loss in boost mode.
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If
one makes a change and the input current decreases, then
the efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
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APPLICATIONS INFORMATION
Design Example
Output voltage is 12V. Select R1 as 20k. R2 is:
VIN = 5V to 18V
VOUT = 12V
IOUT(MAX) = 5A
f = 400kHz
Maximum ambient temperature = 60°C
VOUT
f •L
IRIPPLE,BUCK =
Selecting MOSFET Switches
V
• 1 – OUT
VIN
∆IL,BUCK • 100
IOUT
%
VIN
V
• 1 – IN
f •L
VOUT
IRIPPLE,BOOST =
A 6.8µH inductor will produce 11% ripple in the boost region
(VIN = 6V) and 29% ripple in the buck region (VIN = 18V).
The RSENSE resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances.
RSENSE =
If we design for a maximum junction temperature, TJ(MAX)
= 125°C, the maximum RDS(ON) value can be calculated.
First, calculate the maximum power dissipation:
TJ(MAX) − TA(MAX)
PD(MAX) =
R(j− a)
∆I L,BOOST • 100
%
I IN
The highest value of ripple current occurs at VIN = VOUT/2.
The MOSFETs are selected based on voltage rating and
RDS(ON) value. It is important to ensure that the part is
specified for operation with the available gate voltage amplitude. In this case, the amplitude is 5.5V and MOSFETs
with an RDS(ON) value specified at VGS = 4.5V can be used.
Select QA and QB. With 18V maximum input voltage MOSFETs with a rating of at least 30V are used. As we do not yet
know the actual thermal resistance (circuit board design and
airflow have a major impact) we assume that the MOSFET
thermal resistance from junction to ambient is 50°C/W.
The highest value of ripple current occurs at the maximum
input voltage. In the boost region, the ripple current is:
∆IL,BOOST =
VOUT • R1
– R1
0.8
Select R2 as 280k. Both R1 and R2 should have a tolerance of no more than 1%.
Set the frequency at 400kHz by applying 1.2V on the FREQ
pin (see Figure 7). The 10µA current flowing out of the
FREQ pin will give 1.2V across a 120k resistor to GND. The
inductance value is chosen first based on a 30% ripple current assumption. In the buck region, the ripple current is:
∆IL,BUCK =
R2 =
2 • 140mV • VIN(MIN)
PD(MAX) =
(125 − 60)
= 1.3W
50
The maximum dissipation in QA occurs at minimum input
voltage when the circuit operates in the boost region and
QA is on continuously. The input current is then:
VOUT • IOUT(MAX)
, OR 12A
VIN(MIN)
We calculate a maximum value for RDS(ON):
2 • IOUT(MAX,BOOST) • VOUT + ∆IL,BOOST • VIN(MIN)
RDS(ON) (125°C) <
Select an RSENSE of 10mΩ.
RDS(ON) (125°C) <
PD(MAX)
IIN(MAX) 2
1.3W
(12A)2
= 0.009Ω
3789fc
For more information www.linear.com/LTC3789
23
LTC3789
APPLICATIONS INFORMATION
The Vishay SiR422DP has a typical RDS(ON) of 0.010Ω at
TJ = 125°C and VGS = 4.5V.
The maximum dissipation in QB occurs at maximum input
voltage when the circuit is operating in the buck region.
The dissipation is:
P B,BUCK =
VIN − VOUT
• IOUT(MAX) 2 • ρt • RDS(ON)
VIN
1.3W
R DS(ON)(125°C) <
= 0.156Ω
18V TO −12V
2
• (5A)
18V
This seems to indicate that a quite small MOSFET can be
used for QB if we only look at power loss. However, with
5A current the voltage drop across 0.156Ω is 0.78V, which
means the MOSFET body diode is conducting. To avoid
body diode current flow we should keep the maximum
voltage drop well below 0.5V, using, for example, Vishay
Si4840BDY in the SO-8 package (RDSON(MAX) = 0.012Ω).
Select QC and QD. With 12V output voltage we need
MOSFETs with 20V or higher rating.
The highest dissipation occurs at minimum input voltage
when the inductor current is highest. For switch QC the
dissipation is:
PC,BOOST =
• IOUT(MAX)
(VOUT − VIN )VOUT
VIN
2
IOUT(MAX)
VIN
P D,BOOST
V
V
= IN • OUT • IOUT(MAX)
VOUT
VIN
2
• ρt • RDS(ON)
Vishay SiR484OY is a possible choice for QC and QD. The
calculated power loss at 5V input voltage is then 1.3W for
QC and 0.84W for QD.
CIN is chosen to filter the square current in the buck region.
In this mode, the maximum input current peak is:
29%
IIN,PEAK(MAX,BUCK) = 5A • 1+
= 5.7A
2
A low ESR (10mΩ) capacitor is selected. Input voltage
ripple is 57mV (assuming ESR dominates the ripple).
COUT is chosen to filter the square current in the boost
region. In this mode, the maximum output current peak is:
IOUT,PEAK(MAX,BOOST) =
11%
12
• 5 • 1+
= 10.6A
5
2
A low ESR (5mΩ) capacitor is suggested. This capacitor
will limit output voltage ripple to 53mV (assuming ESR
dominates the ripple).
PC Board Layout Checklist
2
The basic PC board layout requires a dedicated ground
plane layer. Also, for high current, a multilayer board
provides heat sinking for power components.
• ρt • RDS(ON)
+ k • VOUT 3 •
The dissipation in switch QD is:
• CRSS • f
where CRSS is usually specified by the MOSFET manufacturers. The constant k, which accounts for the loss caused
by reverse recovery current, is inversely proportional to
the gate drive current and has an empirical value of 1.7.
• The ground plane layer should not have any traces and
should be as close as possible to the layer with power
MOSFETs.
• Place CIN, switch A, switch B and D1 in one compact area. Place COUT, switch C, switch D and D2 in
one compact area. One layout example is shown in
Figure 12.
• Use immediate vias to connect the components (including the LTC3789’s SGND and PGND pins) to the
ground plane. Use several large vias for each power
component.
24
3789fc
For more information www.linear.com/LTC3789
LTC3789
APPLICATIONS INFORMATION
• Use planes for VIN and VOUT to maintain good voltage
filtering and to keep power losses low.
• Flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of
power components. Connect the copper areas to any
DC net (VIN or GND). When laying out the printed circuit
board, the following checklist should be used to ensure
proper operation of the LTC3789. These items are also
illustrated in Figure 13.
• Segregate the signal and power grounds. All smallsignal components should return to the SGND pin at
one point, which is then tied to the PGND pin close to
the inductor current sense resistor RSENSE.
• Place switch B and switch C as close to the controller as
possible, keeping the PGND, BG and SW traces short.
• Keep the high dV/dT SW1, SW2, BOOST1, BOOST2,
TG1 and TG2 nodes away from sensitive small-signal
nodes.
• The path formed by switch A, switch B, D1 and the CIN
capacitor should have short leads and PC trace lengths.
The path formed by switch C, switch D, D2 and the
COUT capacitor also should have short leads and PC
trace lengths.
SW2
VIN
SW1
L
VOUT
D2
QA
QD
D1
QB
QC
CIN
COUT
• The output capacitor (–) terminals should be connected
as closely as possible to the (–) terminals of the input
capacitor.
• Connect the top driver boost capacitor CA closely to the
BOOST1 and SW1 pins. Connect the top driver boost
capacitor CB closely to the BOOST2 and SW2 pins.
• Connect the input capacitors CIN and output capacitors
COUT closely to the power MOSFETs. These capacitors
carry the MOSFET AC current in the boost and buck
region.
• Connect VFB pin resistive dividers to the (+) terminals of
COUT and signal ground. A small VFB bypass capacitor
may be connected closely to the LTC3789 SGND pin.
The R2 connection should not be along the high current
or noise paths, such as the input capacitors.
• Route SENSE– and SENSE+ leads together with minimum PC trace spacing. Avoid having sense lines pass
through noisy areas, such as switch nodes. The filter
capacitor between SENSE+ and SENSE– should be as
close as possible to the IC. Ensure accurate current
sensing with Kelvin connections at the sense resistor.
One layout example is shown in Figure 14.
• Connect the ITH pin compensation network closely to
the IC, between ITH and the signal ground pins. The
capacitor helps to filter the effects of PCB noise and
output voltage ripple voltage from the compensation loop.
• Connect the INTVCC bypass capacitor, CVCC, closely
to the IC, between the INTVCC and the power ground
pins. This capacitor carries the MOSFET drivers’ current
peaks. An additional 1µF ceramic capacitor placed immediately next to the INTVCC and PGND pins can help
improve noise performance substantially.
RSENSE
LTC3789
CKT
GND
3789 F12
Figure 12. Switches Layout
3789fc
For more information www.linear.com/LTC3789
25
LTC3789
APPLICATIONS INFORMATION
R1
20k
R2
280k
CSS
6.8nF
VPULLUP
1
2
CC2
1000pF
CC1
3300pF
3
RC
68k
4
5
6
7
121k
8
ON/OFF
VIN
VOUT
10Ω
VOUT
9
10
11
12
13
VFB PGOOD
SW1
SS
TG1
BOOST1
SENSE+
PGND
SENSE–
BG1
ITH
SGND
VIN
LTC3789
MODE/PLLIN
INTVCC
FREQ
EXTVCC
RUN
BG2
VINSNS
BOOST2
VOUTSNS
27
CIN
47µF
26
QA
SiR422DP
25
24
DA
DFLS160
23
VOUT
19
TG2
SW2
TRIM
L
6.8µH
D1
B240A
CVCC 4.7µF
21 INTVCC
20
QB
SiR422DP
CF 1µF
22
10mΩ
DB
DFLS160
QC
SiR422DP
18
CB
0.22µF
ILIM
IOSENSE+
2.2µF 14 I
–
OSENSE
VIN
5V TO 38VMAX
CA
0.22µF
17
QD
SiR422DP
16
D2
B240A
COUT
2.2µF
10mΩ
15
+
100Ω
VOUT
12V, 5A
330µF
100Ω
3789 F13
1k
1k
Figure 13. LTC3789 12V/5A, Buck-Boost Regulator
28
27
26
25
24
23
22
21
20
19
18
17
16
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
RSENSE
PGND
C
R
R
SGND
3789 F14
Figure 14. Sense Lines Layout
26
3789fc
For more information www.linear.com/LTC3789
LTC3789
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
GN Package
28-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
.386 – .393*
(9.804 – 9.982)
.045 .005
28 27 26 25 24 23 22 21 20 19 18 17 1615
.254 MIN
.033
(0.838)
REF
.150 – .165
.229 – .244
(5.817 – 6.198)
.0165 .0015
.150 – .157**
(3.810 – 3.988)
.0250 BSC
1
RECOMMENDED SOLDER PAD LAYOUT
.015 .004
¥ 45∞
(0.38 0.10)
.0075 – .0098
(0.19 – 0.25)
2 3
4
5 6
7
8
9 10 11 12 13 14
.0532 – .0688
(1.35 – 1.75)
.004 – .0098
(0.102 – 0.249)
0 – 8 TYP
.016 – .050
(0.406 – 1.270)
.008 – .012
(0.203 – 0.305)
TYP
NOTE:
1. CONTROLLING DIMENSION: INCHES
INCHES
2. DIMENSIONS ARE IN
(MILLIMETERS)
3. DRAWING NOT TO SCALE
.0250
(0.635)
BSC
GN28 (SSOP) 0204
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
3789fc
For more information www.linear.com/LTC3789
27
LTC3789
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
UFD Package
28-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1712 Rev B)
0.70 ±0.05
4.50 ± 0.05
3.10 ± 0.05
2.50 REF
2.65 ± 0.05
3.65 ± 0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
3.50 REF
4.10 ± 0.05
5.50 ± 0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
4.00 ± 0.10
(2 SIDES)
0.75 ± 0.05
R = 0.05
TYP
PIN 1 NOTCH
R = 0.20 OR 0.35
× 45° CHAMFER
2.50 REF
R = 0.115
TYP
27
28
0.40 ± 0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
5.00 ± 0.10
(2 SIDES)
3.50 REF
3.65 ± 0.10
2.65 ± 0.10
(UFD28) QFN 0506 REV B
0.200 REF
0.00 – 0.05
0.25 ± 0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X).
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
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
28
3789fc
For more information www.linear.com/LTC3789
LTC3789
Revision History
REV
DATE
DESCRIPTION
A
9/11
Updated Features, Description and Typical Application.
1
Updated Electrical Characteristics section.
3
B
07/14
PAGE NUMBER
Updated text in MODE/PLLIN, BOOST1, BOOST2, SW1, SW2 in Pin Functions section.
9, 10
Updated text in Operation section.
12-15
Updated text in Applications Information section.
16-25
Updated Figure 13.
26
Updated Typical Application and Related Parts.
30
Updated Application Schematic
1
Updated Nominal Frequency Resistor
4
Updated VOUTSNS and VIN Sections
C
11/14
9, 10
Updated LBOOST equation
18
Added TG1, TG2 Absolute Maximum Ratings
2
Added Note 6
4
Replaced Figure 9
17
Added Text
21
3789fc
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.
For more
information
www.linear.com/LTC3789
29
LTC3789
Typical Application
24V/5A Buck-Boost Regulator
VIN
R5
10Ω, 0805
VOS+
INTVCC
RFB1
232k
RFB2
8.06k
1
2
0.01µF
R9, 1.24k
3
R10, 1.24k
C11, OPT
CC2, 0.01µF
4
VIN
R21
121k, 1%
D8
BZX84C5V1
7
8
9
C7, 0.1µF
R31
12.1k
10
C8, 0.1µF
11
12
R25
0Ω
R13, 100Ω
R14
100Ω
VOUT
VIN
13
C10, 2.2µF 14
SW1
TG1
SS
SENSE1+
BOOST1
SENSE1–
PGND1
CC1, 1000pF 5
ITH
RC, 15k
6
R30
+ 68.1k
28
C15
1µF
50V
1210
R7
100k
PGOOD
VFB
BG1
LTC3789EGN
SGND
VIN
EXTVCC
RUN
BG2
VINSNS
VOUTSNS
27
BOOST2
26
R11, 0Ω
25
24
IOSENSE–
TG2
SW2
TRIM
CIN1
270µF
50V
OPT
+
CIN2
270µF
50V
21
VOUT
C1
R2
2.2µF, 0.010Ω
50V
2%
X5R
+
J1
COUT2
330µF
34V
VOUT
24V AT
R8 5A
10Ω
VOS+
C3
2.2µF
50V
X5R
D6
B240A
L1
5.5µH
Q4
SiR422DP
22
VIN
V
9V TO OUT1
35V
Q3
SiR422DP
C4
0.22µF, 16V
D4
DFLS160
23
D5
B240A
C18
10µF, 1206
20
19
D2
D7
DFLS160 3 BAS16 1
18
R1, 5.6Ω
ILIM
IOSENSE+
+
Q2
SiR422DP
INTVCC
MODE/PLLIN INTVCC
FREQ
J3
C6
3.3µF
50V
1210
17
Q5
SiR422DP
C22
0.22µF, 16V
16
15
R18
8mΩ
2%
R4, 100Ω
R3, 100Ω
3789 TA02
L1: WÜRTH 7443630550
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
LTC3780
High Efficiency (Up to 98%) Synchronous, 4-Switch Buck-Boost
DC/DC Controller
4V ≤ VIN ≤ 36V, 0.8V ≤ VOUT ≤ 30V, 5mm × 5mm QFN-32 and
SSOP-24 Packages
LTC3785
High Efficiency (Up to 98%) Synchronous, 4-Switch Buck-Boost
DC/DC Controller
2.7V ≤ VIN ≤ 10V, 2.7V ≤ VOUT ≤ 10V, 4mm × 4mm
QFN-24 Package
LTM4605
High Efficiency Buck-Boost DC/DC µModule™ Regulator
Complete Power Supply
4.5V ≤ VIN ≤ 20V, 0.8V ≤ VOUT ≤ 16V, 15mm × 15mm × 2.8mm
LGA Package
LTM4607
High Efficiency Buck-Boost DC/DC µModule Regulator
Complete Power Supply
4.5V ≤ VIN ≤ 36V, 0.8V ≤ VOUT ≤ 25V, 15mm × 15mm × 2.8mm
LGA Package
LTM4609
High Efficiency Buck-Boost DC/DC µModule Regulator
Complete Power Supply
4.5V ≤ VIN ≤ 36V, 0.8V ≤ VOUT ≤ 34V, 15mm × 15mm × 2.8mm
LGA Package
LTC3112
2.5A Synchronous Buck-Boost DC/DC Converter
2.7V ≤ VIN ≤ 15V, 2.5V ≤ VOUT ≤ 14V, 4mm × 5mm DFN-16 and
TSSOP-20 Packages
LTC3533
2A Synchronous Buck-Boost Monolithic DC/DC Converter
1.8V ≤ VIN ≤ 5.5V, 1.8V ≤ VOUT ≤ 5.25V, IQ = 40µA,
ISD < 1µA, 3mm × 4mm DFN-14 Package
30 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LTC3789
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LTC3789
3789fc
LT 1114 REV B • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2010
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