60V, 4-Switch Synchronous Buck-Boost Controller Regulates
Voltage from Wide Ranging Inputs and Charges Batteries at
98.5% Efficiency at 100W+
Keith Szolusha
The LT®3791-1 is a 4-switch synchronous buck-boost DC/DC converter that regulates
both constant voltage and constant current at up to 98.5% efficiency using only a
single inductor. It can deliver well over a hundred watts and features a 60V input
and output rating, making it an ideal DC/DC voltage regulator and battery charger
when both step-up and step-down conversion is needed. In addition to the high
voltage, power and efficiency, it features short-circuit protection, a SYNC pin for
synchronization to an external clock, a CLKOUT pin for driving an external SYNC
pin or for parallel operation, OVLO (overvoltage lockout), SHORT output flag, C/10
detection and output flag for battery chargers, and a CCM pin for discontinuous or
continuous conduction mode. The inclusion of DCM (discontinuous conduction mode)
increases light load efficiency and prevents reverse current when it is undesirable.
120W, 24V 5A OUTPUT BUCK-BOOST
VOLTAGE REGULATOR
operates from an input voltage range
of 12V to 58V. Adjustable undervoltage and overvoltage lockout protect
the circuit. It has short-circuit protection and the SHORT output flag indicates
The buck-boost converter shown in
Figure 1 regulates 24V with 0A–5A load
at up to 98.5% efficiency (Figure 2). It
VIN
12V TO
58V
0.003Ω
VIN
1µF
51Ω
499k
INTVCC
0.1µF
IVINP
BST1
499k
TG1
EN/UVLO
27.4k
100k
18.7k
BG1
INTVCC
LT3791-1
200k
0.1µF
VOUT
24V
5A
715k
M3
13.7k
BG2
VC
1000pF
RT
15k
10nF
22 | October 2012 : LT Journal of Analog Innovation
SW2
TG2
ISP
ISN
FB
SGND
147k
200kHz
LTspice IV
38.3k
SNSN
PGND
SS SYNC
33nF
L1
10µH
M4
COUT
220µF
35V ROUT
×2 7.5mΩ
4.7µF
50V
×2
Figure 1. 120W 24V 5A output buckboost voltage regulator accepts a
12V–58V input
C1
47µF
80V
0.004Ω
PWM
100k
M2
0.1µF
+
SNSP
SHORT
C/10
CCM
IVINMON
ISMON
CLKOUT
CTRL
VREF
33nF
M1
SWI
OVLO
56.2k
+
BST2
IVINN
4.7µF
100V
4.7µF
D1 D2
470nF
10k
when there is a short circuit on the
output. It features DCM operation at
light load for lowest power consumption and reverse current protection.
ROUT limits the output current during both
D1, D2: NXP BAT46WJ
L1: COILCRAFT SER2915L-103KL 10µH
M1, M2: RENESAS RJK0651DPB 60Vds
M3, M4: RENESAS RJK0451DPB 40Vds
C1: NIPPON CHEMICON EMZA800ADA470MJAOG
COUT: SUNCON 35HVT220M 35V 220µF ×2
circuits.linear.com/589
design features
The LT3791-1 can regulate both constant voltage and constant current. Large
capacitive loads such as supercapacitors and batteries require constant current
charging until they are charged up to a termination voltage, at which point they
require constant voltage regulation. The LT3791-1 easily satisfies this requirement.
100
VIN = 14V
IIN = 8.87A
VOUT = 24V
IOUT = 5A
95
EFFICIENCY (%)
90
85
80
75
70
VIN = 12V
VIN = 24V
VIN = 54V
65
Figure 2. Efficiency and worst
case thermal results for the 24V
converter in Figure 1
60
0
1
2
3
LOAD CURRENT (A)
4
5
a short circuit and an overload situation, making this a robust application.
The 14V, 10A voltage regulator in Figure 3
takes a slightly different approach. It runs
in CCM throughout its entire load current range 0A-10A to provide the lowest
EMI at light load. It is still very efficient.
The circuit retains short-circuit protection
even though ROUT is replaced with a short.
The main switch sense resistor RSW limits
the short-circuit current at a higher level
than ROUT, but hiccup mode during shortcircuit limits the power consumption of
the IC, maintaining a low temperature
rise on the components during a short.
When DCM is not needed, ROUT may not
be necessary and removing ROUT slightly
increases circuit efficiency. OVLO is tied
to the output to limit the output voltage
transient during a 10A to 0A transition.
This protects both the output capacitors
and M3 and M4 switches from overvoltage.
VIN
9V TO
36V
0.002Ω
470nF
499k
IVINP
EN/UVLO
51Ω
CIN1
4.7µF
50V
1µF
+
IVINN VIN INTVCC
CCM
76.8k
CIN2
100µF
63V
×2
4.7µF
10V
D1 D2
BST2
0.1µF
BST1
C/10
IVINMON
CLKOUT
ISMON
TG1
BG1
INTVCC
LT3791-1
200k
SHORT
100k
22nF
VC
5.1k
M3
COUT1
4.7µF
50V
×2 +
COUT2
270µF
35V
×2
VOUT
14V
10A
100k
9.31k
BG2
CTRL
SS
10nF
Figure 3. 140W (14V, 10A) CCM buck-boost voltage regulator with
9V–56V input has output OVLO for transient protection.
SNSN
PGND
SYNC
100pF
L1
3.3µH
0.0025Ω
PWM
M5
M2
M4
SNSP
SHORT
VREF
0.1µF
M1
SWI
0.1µF
RT
SGND
147k
200kHz
SW2
TG2
FB
ISP
ISN
OVLO
499k
D1, D2: NXP BAT46WJ
L1: COILCRAFT SER2915H-332L 3.3µH 48A
M1: RENESAS RJK0652DPB 60Vds
M2: RENESAS RJK0651DPB 60Vds
M3, M4: INFINEON BSC0904NSI 30Vds
M5: NXP NX7002AK
COUT2: SUNCON 35HVT270M
CIN2: NIPPON CHEMICON EMZA630ADA101MJAOG
88.7k
October 2012 : LT Journal of Analog Innovation | 23
0.003Ω
VIN
12V TO
58V
499k
51Ω
470nF
499k
IVINP
EN/UVLO
1µF
27.4k
CCM
C/10
BST1
M1
TG1
SWI
SHORT
VREF
M2
BG1
LT3791-1
PWM
0.1µF
Figure 4. Parallel LT3791-1s in a 240W application
L1
10µH
4.7µF
50V
×2
0.015Ω
VOUT
24V
10A
M4
51Ω
M3
+
COUT1
220µF
35V
×2
0.47µF
SNSP
CTRL
0.004Ω
100k
SNSN
PGND
SS
33nF
4.7µF
10V
D1 D2
0.1µF
200k
0.1µF
C1
47µF
80V
BST2
INTVCC1
SHORT
+
INTVCC1
IVINN VIN INTVCC
OVLO
56.2k
4.7µF
100V
BG2
IVINMON
CLKOUT
ISMON
SYNC
SW2
VC
SGND
RT
715k
TG2
ISP
ISN
FB
13.7k
38.3k
147k
200kHz
3.3k
33nF
INTVCC1
+
LTC6240
–
45k
10k
10k
0.003Ω
VIN
499k
470nF
499k
10k
IVINP
EN/UVLO
51Ω
1µF
IVINN VIN INTVCC
27.4k
0.1µF
C/10
BST1
TG1
200k
SHORT
0.1µF
PWM
BG1
LT3791-1
M6
0.1µF
L2
10µH
4.7µF
50V
×2
0.015Ω
M8
M7
51Ω
+
0.22µF
SNSP
0.004Ω
100k
33nF
M5
SWI
SHORT
VREF
C2
47µF
80V
4.7µF
10V
D3 D4
BST2
INTVCC2
45k
+
INTVCC2
CCM
OVLO
56.2k
4.7µF
100V
SNSN
PGND
SS
BG2
CTRL
IVINMON
ISMON
CLKOUT
SYNC
VC
10k
1000pF
2.2k
RT
SW2
TG2
ISP
ISN
FB
SGND
147k
200kHz
22nF
24 | October 2012 : LT Journal of Analog Innovation
715k
13.7k
38.3k
D1–D4: NXP BAT46WJ
L1, L2: COILCRAFT SER2915L-103KL 10µH
M1, M2, M5, M6: RENESAS RJK0651DPB 60Vds
M3, M4, M7, M8: RENESAS RJK0451DPB 40Vds
COUT1, COUT2: SUNCON 35HVT220M ×2
C1, C2: NIPPON CHEMICON EMZA800ADA470MJAOG
COUT2
220µF
35V
×2
PARALLEL CONVERTERS FOR HIGH
POWER USING CLKOUT AND SYNC
The LT3791-1 has a CLKOUT output that
can be used to synchronize other converters to its own clock with a 180°
phase shift. By tying the CLKOUT of one
converter to the SYNC input of another,
the maximum output power is doubled
while the output ripple is reduced.
Figure 4 shows a 24V, 10A regulator
formed by running two LT3791-1s in parallel. By using two parallel circuits, the maximum temperature rise seen on any one
discrete component is only 28°C on the M3
and M7 MOSFETs at the lowest VIN . The top
converter (master) in Figure 4 commands
the current level provided by the bottom
(slave) converter. The ISMON output of the
master indicates how much current the
master is providing, and by connecting
ISMON to the CTRL input of the slave, the
slave is forced to follow the master. A single op amp is needed to provide the simple
200mV level shift needed to match the
CTRL input to the ISMON output levels. The
master converter runs in constant voltage regulation while the slave converter
is running in constant current regulation.
Note that the output voltage of the slave is
set slightly higher (28V) so that the voltage
feedback loop of the slave is not in regulation for it to be able to follow the master.
design features
The LT3791-1 features both continuous conduction mode (CCM) and
discontinuous conduction mode (DCM). CCM provides continuous switching
at light load and inductor current can be either positive or negative. When the
LT3791-1 enters DCM operation at light load, it prevents backward running current
(negative inductor current) and light load power dissipation is minimized.
100W+ 2.5A BUCK-BOOST 36V SLA
BATTERY CHARGER
In some battery charger applications,
once termination voltage is reached and
charge current tails off, a standby or
float voltage regulation level is needed
that is different from the charge voltage.
The C/10 detection level of the LT3791-1
provides this capability. In the circuit
in Figure 3 the C/10 function drops the
battery voltage from charging (44V)
to float (41V) when the battery is near
full charge. When the battery voltage
is then pulled down from an increased
load, the voltage feedback loop returns
the charger to its charge state of 44V.
The LT3791-1 can regulate both constant
voltage and constant current. Large
capacitive loads such as supercapacitors
and batteries require constant current
charging until they are charged up to a
termination voltage, at which point they
require constant voltage regulation. The
LT3791-1 easily satisfies this requirement.
As an example, the buck-boost converter
shown in Figure 5 charges a 36V 12Ah
SLA battery at 44V with 2.5A DC from a
9V-to-58V input. DCM operation prevents
reverse battery current when the output load is overcharged, protecting the
circuit from large negative currents.
Figure 5. SLA battery charger
regardless of the voltage relationship
between them. Furthermore, a microcontroller can be used to create a maximum
power point tracking (MPPT) charger
from a solar panel. The output diagnostics ISMON and IVINMON and current
control pin CTRL make it easy to create a
high power solar panel battery charger.
DCM INCREASES EFFICIENCY AND
PREVENTS REVERSE CURRENT
The LT3791-1 features both continuous
conduction mode (CCM) and discontinuous
conduction mode (DCM). Figure 6 shows
the difference between CCM and DCM. The
mode is selected by simply connecting
the CCM pin to either the INTVCC or C/10
pin. CCM provides continuous switching
The LT3791-1 can be tailored to charge
a range of battery chemistries and
capacities from a variety of input sources
PVIN
9V TO 57V
RIN
0.003Ω
1µF
50Ω
VIN
INTVCC
D1
IVINN
TG1
BG1
OVLO
LT3791-1
19.6k
M1
0.1µF
M4
M2
L1
10µH
M3
SWI
EN/UVLO
RBAT
0.04Ω
SNSN
SHORT
+
PGND
IVINMON
ISMON
CTRL
BG2
SW2
TG2
ISP
CLKOUT
SGND
100k
VREF
PWM
ISN
FB
CCM
SS
C/10
RT
SYNC VC
D1, D2: BAT46WJ
33nF
L1: COILCRAFT SER2915L-103K
M1-M4: RENESAS RJK0651DPB
M5: NXP NX7002AK
CIN2: ×2 NIPPON CHEMI-CON EMZA630ADA101MJA0G
COUT2: ×3 NIPPON CHEMI-CON EMZA630ADA101MJA0G
CIN2
100µF
63V
×2
COUT1
4.7µF
50V
×2
RSENSE
0.004Ω
200k
0.1µF
+
+
COUT2
100µF
63V
×3
SNSP
INTVCC
CHARGE CURRENT CONTROL
CIN1
4.7µF
100V
×2
0.1µF
BST1
IVINP
24.3k
D2
BST2
470nF
332k
4.7µF
3k
84.5k
300kHz
0.1µF
36V
SLA BATTERY
AGM TYPE
41V FLOAT
44V CHARGE
AT 25°C
50Ω
1.00M
INTVCC
10k
0.47µF
2.5A
CHARGE
10k
402k
30.1k
M5
22nF
October 2012 : LT Journal of Analog Innovation | 25
5000
90
1000
CCM RISING THRESHOLD
600
EFFICIENCY (%)
ILOAD (mA)
FOR CCM OPERATION
OVER ALL IOUT
DCM FALLING THRESHOLD
400
INTVCC
LT3791-1
LT3791-1
100k
CCM
CCM
CCCM
OPTIONAL
0
DCM (TG2 FOR M4 STAYS LOW)
18
12
24
30
36
VIN (V)
42
48
at light load and inductor current can
be either positive or negative. Although
zero-load inductor current in CCM is both
positive and negative and more power
is consumed than DCM, the switch node
ringing associated with DCM is eliminated for those that do not want it.
DCM operation at light load, the TG2 driver
for M4 stays low and M4 no longer runs
as a switch, but instead as a catch diode.
This prevents backward running current (negative inductor current) and light
load power dissipation is minimized.
When DCM is selected, the converter
remains in CCM until the load drops below
about 10% of the programmed maximum
output current. When the LT3791-1 enters
The LT3791-1 synchronous buck-boost
controller delivers over 100W at up to
98.5% efficiency to a variety of loads. Its
wide, 4.7V to 60V input range and 0V to
1.2
1
40
0.8
30
INTVCC
IVINN
470nF
0.01
0.1
IOUT (A)
1
TG1
EN/UVLO
OVLO
0.1µF
18.7k
100k
BG1
LT3791-1
200k
0.0125Ω
M2
L1
22µH
M4
M3
COUT1
4.7µF
100V
×2 +
COUT2
330µF
63V
×3
Figure 7. 48V application
VOUT
48V
3A
0.005Ω
SNSN
PGND
BG2
SW2
TG2
ISP
ISN
FB
SGND
PWM
10k
CIN2
100µF
80V
×2
SNSP
SHORT
C/10
CCM
IVINMON
ISMON
CLKOUT
VREF
0.1µF
M1
SWI
INTVCC
100k
+
BST1
0.1µF
CTRL SS SYNC V
C
RT
40k
1µF
22nF
26 | October 2012 : LT Journal of Analog Innovation
105k
250kHz
1000pF
0.4
0.2
10
0
60V output range make it powerful and
versatile, and its built-in short-circuit
capabilities make for robust solutions
in potentially hazardous environments.
CCM and DCM operation make it useful for highest efficiency or lowest noise
operation at light load. Its multiple control
loops make it ideal for regulating constant
voltage, constant current or both. This
feature-rich IC easily fulfills buck-boost
requirements where other topologies fail. n
0.1µF
IVINP
499k
28.7k
4.7µF
10V
D1 D2
BST2
CIN1
4.7µF
100V
0.6
DCM
DCM
CCM
CCM
c. DCM improves efficiency at light loads.
CONCLUSION
VIN
1.4
50
0.004Ω
1µF
51Ω
34.8k
60
1.6
POWER
LOSS
EFFICIENCY
0
0.001
54
b. DCM/CCM transition thresholds remain stable as
the LT3791-1 moves through boost, buck-boost and
buck modes of operation.
499k
70
10
a. DCM vs CCM setup
VIN
18V TO 55V
1.8
20
200
INTVCC
C/10
2
VIN = 24V
VOUT = 24V
80
800
FOR DCM OPERATION
AT IOUT < 10mV/ROUT
100
CCM
POWER LOSS (W)
Figure 6. Overview of continuous conduction
mode (CCM) for low noise and discontinuous
conduction mode (DCM) for light load efficiency
95.3k
2.15k
D1, D2: NXP BAT46WJ
L1: WÜRTH ELECTRONICS 74435572200 22µH 11A
M1, M2, M4: RENESAS RJK0651DPB 60Vds
M3: RENESAS RJK0652DPB 60Vds
COUT2: YAGEO ST 330µF 63V ×3
CIN2: NIPPON CHEMICON EMZA630ADA101MJAOG 100µF 63V ×2
2.49k