Linear Technology LT3956 Series Manual

FeaTures
3000:1 True Color PWM
n
Wide Input Voltage Range: 4.5V to 80V
n
Output Voltage Up to 80V
n
Internal 3.3A/84V Switch
n
Constant-Current and Constant-Voltage Regulation
n
250mV High Side Current Sense
n
Drives LEDs in Boost, Buck Mode, Buck-Boost Mode,
n
SEPIC or Flyback Topology
Adjustable Frequency: 100kHz to 1MHz
n
Open LED Protection
n
Programmable Undervoltage Lockout with Hysteresis
n
Constant-Voltage Loop Status Pin
n
PWM Disconnect Switch Driver
n
CTRL Pin Adjusts High Side Current Sense Threshold
n
Low Shutdown Current: <1µA
n
Programmable Soft-Start
n
Available in the 36-Lead (5mm × 6mm) QFN Package
n
applicaTions
High Power LED
n
Battery Charger
n
Accurate Current Limited Voltage Regulator
n
Typical applicaTion
94% Efficient 25W White LED Headlamp Driver
V , 6V TO 60V
IN
(80V TRANSIENT)
2.2µF
332k
V
2 IN
EN/UVLO
100k
V
REF
332k
CTRL
INTV
CC
40.2k
100k
VMODE
PWM
SS
RT
V
C
28.7k
34k
375kHz
47nF 4.7nF
TM Dimming
22µH D1
2.2µF
SW
5
PGND
ISP
LT3956
0.68 370mA
ISN
1M
FB
16.2k
PWMOUT
GND INTV
CC
INTV
CC
4.7µF
Constant-Current,
Constant-Voltage Converter
DescripTion
The LT 3956 is a DC/DC converter designed to operate as
®
a constant-current source and constant-voltage regulator.
It is ideally suited for driving high current LEDs. It features
an internal low side N-channel power MOSFET rated for
84V at 3.3A and driven from an internal regulated 7.15V
supply. The fixed frequency, current-mode architecture
results in stable operation over a wide range of supply
and output voltages. A ground referenced voltage FB pin
serves as the input for several LED protection features,
and also makes it possible for the converter to operate
as a constant-voltage source. A frequency adjust pin
allows the user to program the frequency from 100kHz
to 1MHz to optimize efficiency, performance or external
component size.
The LT3956 senses output current at the high side of the
LED string. High side current sensing is the most flexible
scheme for driving LEDs, allowing boost, buck mode or
buck-boost mode configuration. The PWM input provides
LED dimming ratios of up to 3000:1, and the CTRL input
provides additional analog dimming capability.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners. Protected by U.S. Patents, including 7199560 and 7321203.
25W LED STRING
M1
3956 TA01a
LT3956
80V , 80V
IN
Efficiency vs V
IN
100
96
92
88
84
80
0 20 40 60
V (V)
IN
OUT
80
3956 TA01b
3956f