LINEAR TECHNOLOGY LT3743 Technical data

L DESIGN FEATURES

CTRL_SEL

PWM

SW

INDUCTOR

CURRENT

PWMGH

PWMGL

LED CURRENT

I

CTRL_L

I

CTRL_L

I

CTRL_H

I

CTRL_H

LT3743

V

IN

V

IN

I

REG

V

OUT

FOR I

REG

DETERMINED
BY CTRL_H

V

OUT

FOR I

REG

DETERMINED
BY CTRL_L

HG

CBOOT

PWM

CTRL_SEL

CTRL_L

CTRL_H

LED LOAD

SW

V

CC_INT

LG

GND

SENSE

+

SENSE

_

PWMGH

PWMGL

L R

S

+ +

High Current/High Speed LED Driver
Revolutionizes PWM Dimming

Introduction

Power drivers that can produce regu­lated high current pulses are used
in a number of lighting applications,
ranging from high current LEDs in
DLP projectors to high power laser
diodes. For instance, in high end
video projectors, high power LEDs are
used to produce color illumination.
The RGB LEDs in these projectors
require precise dimming control for
accurate color mixing—in this case,
more control than simple PWM dim­ming can offer. Typically, to achieve
the wide dynamic range required in
color mixing, LED drivers must be
able to rapidly switch between the
two disparate regulated peak current
states, and overlay PWM dimming
without disruption. The LT3743 has
the ability to meet these demanding
accuracy and speed requirements.

The LT3743 is a synchronous
buck DC/DC controller that utilizes
fixed-frequency, average current mode
control to accurately regulate the
inductor current through a sense
resistor in series with the inductor.
The LT3743 regulates the current in
any load with an output voltage range
from 0V to 2V below the input rail with
±6% accuracy.

Precision, broad-range LED current
control is achieved by combining ac­curate analog dimming (high and low
states) with PWM dimming. Analog
dimming is controlled via the CTRL_L,
CTRL_H, and CTRL_T pins; PWM
dimming via the PWM and CTRL_SEL
pins. A rapid transition between the
high and low analog states is made
possible with the LT3743’s unique use
of externally switched load capacitors,
which allows the LT3743 to change
regulated LED current levels within
several microseconds. The switching
frequency may be programmed from
200kHz to 1MHz using an external re­sistor and synchronized to an external
clock from 300kHz to 1MHz.

16

Figure 1. Basic switched-capacitor topology

Switched Output
Capacitor Topology

In traditional current regulators, the
voltage across the load is stored in the
output capacitor. If the load current
is suddenly changed, the voltage in
the output capacitor must charge or
discharge to match the new regulated
current. During the transition, cur­rent in the load is poorly controlled,
resulting in slow load current response
time.

The LT3743 solves this problem
with a unique switched output capaci­tor topology, which enables ultrafast
load current rise and fall times. The
basic idea behind the topology is that

Figure 2. LED current PWM and CTRL_SEL dimming

by Josh Caldwell

the LT3743 acts as a regulated cur­rent source driving into the load. The
voltage drop across the load for a given
current is stored in the first switched
output capacitor. When a different
regulated current state is desired, the
first output capacitor is switched off
and a second capacitor is switched in.
This allows each capacitor to store the
voltage drop for the load corresponding
to the desired regulated current.

Figure 1 shows the basic topology
with the various control pins. The PWM
and CTRL_SEL pins are digital control
pins that determine the state of the
regulated current. The CTRL_H and
CTRL_L pins are analog inputs with a

Linear Technology Magazine • December 2009

DESIGN FEATURES L

PWM DIMMING DUTY CYCLE (%)

0 20 40 60 80 100

EFFICIENCY (%)

0

10

20

30

40

50

60

70

80

90

100

EN/UVLO
PWM
CTRL_SEL

EN/UVLO

PWM

CTRL_SEL

HG

V

IN

CBOOT

V

REF

CTRL_L

CTRL_H

CTRL_T

LT3743

RT
SYNC

SW

LG

GND

VCH

VCL

SENSE

+

SENSE

–

PWMGH

PWMGL

V

CC_INT

100nF

L1

1.0µH

22µF

C1
330µF
×3

2.5mΩ

FB

10nF

34k

4.7nF

34k

D1: LUMINUS PT120
D2: PMEG4002EB
L1: IHLP4040DZER1R0M01
M1: RJK0365DPA
M2: RJK0346DPA
M3, M4: Si7236DP
C1, C2, C3: PTPR330M9L (THREE IN PARALLEL)

D2

4.7nF

82.5k

100k

100k

C3
330µF
×3

D1

40.2k

10k

1µF

2nF

R

NTC

10k

220µF

M1

M2

M3

M4

V

IN

12V

V

OUT

20A MAXIMUM

1µF

C2
330µF
×3

SS

R

HOT

499Ω

10Ω 10Ω

33nF

CTRL_SEL

5V/DIV

PWM

5V/DIV

SW

20V/DIV

I

LED

10A/DIV

20µs/DIVV

IN

= 24V

0A TO 2A TO 20A LED CURRENT STEP

Figure 3. A 24V, 20A LED driver using switched output capacitors

full-scale range of 0 to 1.5V, producing
a regulated voltage of 0mV to 50mV
across the current sense resistor.

Figure 2 shows the timing wave­forms in response to the various states
of the PWM and CTRL_SEL pins. When
PWM is low, all switching is terminated
and both output capacitors are discon­nected from the load.

Although the LT3743 may be
configured with switched output
capacitors, it is easily adapted to any
traditional analog and/or PWM dim­ming scheme.

Switching Cycle
Synchronization

The LT3743 synchronizes all switch­ing edges to the PWM and CTRL_SEL
rising edges. Synchronization gives
system designers the freedom to use
any periodic or non-periodic PWM­dimming pulse width and duty cycle.
This is an essential feature for high
current LED drivers during recovery
from a zero or low current state to a
high current state. By restarting the
clock whenever the CTRL_SEL or PWM
signals go high, the inductor current
begins ramping up immediately with­out having to wait for a rising edge of
the clock. Without synchronization,
the phase relationship of the clock
edge and the PWM edge would be
uncontrolled, possibly resulting in

Linear Technology Magazine • December 2009

visible jitter in the LED light output.
When using an external clock with
the SYNC pin, the switching cycle
resynchronizes to the external clock
within eight switching cycles.

A 24V, 20A LED Driver Using
Switched Output Capacitors
for High End DLP Projectors

High end DLP projectors demand
the highest quality image and color
reproduction. To achieve high color
accuracy, variations in the color of
individual LEDs are corrected by mix­ing in the other two color LEDs. For
example, when the red LED is on at full
current, the blue and green LEDs are
turned on at low current levels so they
can be mixed in to produce accurate
red. This technique requires the ability

Figure 4. Zero to 2A to 20A LED current steps

to rapidly transition between relatively
low (~2A) and high (~20A) LED cur­rents so that PWM dimming edges are
preserved. Figure 3 shows a 24V/20A
LED driver for use specifically with
high end DLP projectors.

The relatively low switching fre­quency of 450kHz allows for a very
small 1.0µH inductor. With 25% ripple
current, the transition times between
the high and low current states is
about two microseconds. The large
1mF output capacitors store the volt­age drop across the LED for the two
different current states and provide
instantaneous current when the
MOSFET dimming switches are turned
on. Use of several low ESR capacitors
in parallel is critical to providing rapid
LED current transitions.

Figure 5. 12V, 20A PWM dimming efficiency
using a green LED

17