LINEAR TECHNOLOGY LT3791 Technical data
60V Buck-Boost Controller Drives High Power LEDs, Charges
Batteries and Regulates Voltage with Up to 98.5% Effi ciency at
100W and Higher –
Design Note 501
Keith Szolusha
Introduction
The LT®3791 is a 4-switch synchronous buck-boost
DC/DC converter that regulates both constant-current
and constant-voltage at up to 98.5% effi ciency with a
single inductor. It can deliver hundreds of watts and
features a 60V input and output rating, making it ideal
for driving high power LED strings and charging high
voltage batteries when both step-up and step-down
conversion is needed. It can also be used as a constantvoltage buck-boost regulator with current limiting and
monitoring for both input and ou tput.
Buck-Boost Controller Drives 100W LED String for
Airplane and Truck Lights
Airplanes and big trucks with 24V batteries need powerful, effi cient and robust headlights and spotlights.
Figure1 shows a 33.3V, 3A (nine Luminus SSR-90 LEDs)
V
IN
15V TO 58V
499k
27.4k
R
499k
43.2k
SHORTLED
OPENLED
IN
0.003Ω
470nF
INTV
200k
0.1μF
C
1μF
50Ω
V
IN
VIN
IVINN
I
VINP
EN/UVLO
OVLO
CC
200k
SHORTLED
OPENLED
PWMPWM DIM
IVINMON
ISMON
CLKOUT
V
REF
10k
CTRLDIM
SGND
10nF
LT3791
INTV
CC
BST2
BST1
TG1
SWI
BG1
SNSP
SNSN
PGND
BG2
SW2
TG2
PWMOUT
RT
VCSYNCSS
2.2k
10nF
FB
ISP
ISN
86.6k
300kHz
M1
M2
D1: BAT46WJ
L1: COOPER HC9-100-R
M1, M2: RENESAS RJK0651DPB
M3-M5: RENESAS RJK0451DPB
Figure 1. 15–58VIN to 33.3V 3A LED Driver with Up to 98.5% Effi ciency
buck-boost LED driver that runs from 15V to 58V input
with up to 98.5% effi ciency.
The 4-switch synchronous topology drives high power
LEDs with minimal switch power loss (and minimal
temperature rise). Unlike other topologies, the LT3791
buck-boost can be shorted from LED+ to both LED– and
GND and, as a feature, can be programmed to latch off
or keep trying to turn back on if the short is removed.
Diagnostic output fl ags report both short-circuit and
open-LED conditions.
The solution in Figure 1 features up to 100:1 PWM dimming at 100Hz for accurate brigh tness adjustment without color shift and analog LED dimming when a PWM
oscillator is not pre sent.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
INTV
CC
D1 D2
0.1μF
M4
0.1μF
L1 10μH
M3
R
SW
0.004Ω
4.7μF
10V
C
IN
2.2μF
100V
w4
1M
34.2k
R
LED
0.033Ω
33.3V
3A
100W
LUMINUS
SST-90
w9
M5
DN501 F01
C
OUT
4.7μF
50V
w5
LED
100
99
BOOST
98
97
96
95
94
EFFICIENCY (%)
93
92
91
90
10
BUCK-BOOST
2015
3025
INPUT VOLTAGE (V)
Figure 2. Effi ciency of
Figure 1
40 45 55
35
BUCK
33.3V
3A LEDs
50
DN501 F02
60
04/12/501
36V, 2.5A SLA Battery Charger
The buck-boost converter shown in Figure 3 charges a
36V 12Ah SLA batter y at 44V with 2.5A DC from a 9V to
58V input.
Figure 1 can be easily turned into a 100W voltage regulator with V
resistor bet ween the SS and V
between 6V and 55V by placing a 100k
OUT
p i n s t o d e f e a t s o f t - s t a r t
REF
reset during a f ault.
Specially integrated C/10 current sensing and battery
voltage detection drops the battery voltage from its
charging voltage (44V) to its fl oat voltage (41V) when
the bat tery is near full charge. The OPENLED fl a g i s u s e d
to change the state of the charger from charge to fl oat.
When the bat tery voltag e drops far enough, volt age feedback returns the charger to its char ge state.
The LT3791 can be tailored to charge a range of battery chemistries and capacities from a variety of input
sources regardless of the voltage relationship between
them. An external microcontroller can be programmed
and used to create a ma ximum power point tracking device to charge the batter y from a solar panel. The output
diagnostics and dimming input pins make this a simple
interf ace for high power solar panel applications design.
120W, 6V to 55V Voltage Regulator
The LT3791 can also be used in high power constantvoltage buck-boost applications. The FB pin doubles
as both the main voltage feedback for the buck-boost,
and the overvol tage protection detec tion and regulation
w h e n u s e d a s a n L E D dr i v e r.
PV
IN
9V TO 58V
57.6k
30.9k
D1, D2: BAT46WJ
L1: WÜRTH 74435571100 10μH
M1-M5: RENESAS RJK0651DPB
R
499k
CHARGE CURRENT
IN
0.003Ω
1μF
50Ω
470nF
CONTROL
200k
INTV
C8
0.1μF
CC
V
IN
IVINN
VINP
I
EN/UVLO
OVLO
SHORTLED
PWMOUT
IVINMON
ISMON
CLKOUT
CTRL
SGND
V
REF
PWM
10nF
LT3791
OPENLED
VCSYNCSS
1k
INTV
4.7nF
BST2
BST1
TG1
SWI
BG1
SNSP
SNSN
PGND
BG2
SW2
TG2
ISP
ISN
RT
CC
FB
86.6k
300kHz
Figure 3. 36V, 2.5A SLA Battery Charger with C/10 Charge Termination
The voltage regulator is short-circuit proof. The
SHORTLED fl ag reports when there is a short circuit on
the output. T he input current monitor can still be used in
constant-voltage regulation to protect any system that
is input-current limited. R
sets the output current
LED
limit for short circuit, but can be removed to provide
a PGOOD fl a g u s i n g OPENLED by shorting ISP to ISN.
Conclusion
The LT3791 synchronous buck-boost controller delivers 100W and higher power at up to 98.5% effi ciency to
a number of different loads. The wide, 4.7V to 60V input
and 0V to 60V output voltage range make it both powerful and versatile, and its short-circuit capability makes
t hi s a r o bu s t c h oi c e f o r m a ny a pp li ca ti on s i n p ot en t ia ll y
hazardous environments. This seemingly limitless IC
can be used in applic ations where a typic al buck or boost
converter cannot because of the crossover of input and
output voltage ranges.
C
IN
2.2μF
100V
M5
DN501 F03
w4
1.00M
30.1k
R
0.04Ω
+
C
OUT
4.7μF
50V
w2
2.5A
S1
CHARGE
36V
SLA BATTERY
AGM TYPE
41V FLOAT
44V CHARGE
AT 25°C
D2D1
0.1μF
M1
0.1μF
L1 10μH
M2
R
SW
0.004Ω
1μF
50Ω
INTV
CC
10k
4.7μF
M4
M3
402k
Data Sheet Download
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
For applications help,
call (408) 432-1900, Ext. 3801
dn501f LT/AP 0412 196K • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2012
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