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

L DESIGN FEATURES

EN/UVLO
PWM
CTRL_SEL

EN/UVLO

INTV

CC

CTRL_SEL

HG

V

IN

CBOOT

V

REF

CTRL_H

CTRL_L

CTRL_T

LT3743

RT
SYNC

SW

LG

GND

VCH

VCL

SENSE

+

SENSE

–

PWMGH

PWMGL

V

CC_INT

100nF

L1

10µH

22µF

25mΩ

FB

10nF

34k

4.7nF

34k

4.7nF

82.5k

40.2k

D1

10k

2.2nF

8.2µF

M1

M2

M3

V

IN

6V TO 36V

V

OUT

2A MAXIMUM

2.2µF

1µF

SS

CONTROL

INPUT

D1: LUMINUS CBT-40
D2: PMEG4002EB
L1: IHLP4040DZE10R0M01
M1, M2: Si7848BDP
M3: Si2312BDS

D2

SW

10V/DIV

I

L

2A/DIV

CTRL_SEL

5V/DIV

I

LED

1A/DIV

20µs/DIV

EN/UVLO
PWM
CTRL_SEL

EN/UVLO

PWM

V

CC_INT

HG

V

IN

CBOOT

V

REF

CTRL_L

CTRL_H

CTRL_T

LT3743

RT
SYNC

SW

LG

GND

VCH

VCL

SENSE

+

SENSE

–

PWMGL

PWMGH

V

CC_INT

150nF

L1

1.65µH

22µF

2.5mΩ

FB

10nF

51k

4.7nF

82.5k

10Ω 10Ω

60.4k

10k

2.2nF

R

NTC

10k

82µF

M1

D2

M2

M3

D1: LUMINUS PT121
D2: PMEG4002EB
L1: MVR1271C-162ML
M1: RJK0365DPA
M2: RJK0328DPB
M3: SiR496DP
C1: PTPR330M9L (THREE IN PARALLEL)

V

IN

6V TO 30V

V

OUT

20A MAXIMUM

C1
330µF
×3

D1

1µF

SS

R

HOT

499Ω

CONTROL

INPUT

33nF

Figure 6. A 6V to 36V input, 2A LED driver with current limited shunted output

The regulated high and low currents
are set by voltage dividers from the
V

pin to the CTRL_L and CTRL_H

REF

pins. The ±2%, 2V reference at V
is also used to provide the reference
signal the temperature derating circuit
applied at CTRL_T (see “Thermally
Derating the LED Current” below).

To reduce potentially large start-up
currents, the LT3743 uses a unique
soft-start circuit that throttles back the
regulated currents, providing full drive
when the soft-start pin is charged to

Figure 8. A 6V to 30V input, 20A LED driver with switched cathode PWM dimming

18

1.5V. To minimize the transition time
between current levels, the LT3743
employs individual compensation for
each level so that the current control

REF

loop may return to steady-state opera­tion as quickly as possible. Figure 4
shows the LED current step from 0A
to 2A to 20A.

High Efficiency Over a Wide
Range of PWM Duty Cycles

Power dissipation is a critical design
parameter in portable DLP projectors.

Figure 7. 0A to 2A current limited shunted
output PWM dimming

Unlike many shunt-type high current
LED drivers currently available, the
LT3743 has excellent efficiency over
a wide range of PWM duty cycles. By
delivering power only to the load in­stead of either shunting power away or
charging the output capacitor, most of
the energy lost in common traditional
PWM-dimmed drivers is conserved.
Figure 5 shows the efficiency with VIN
= 12V, driving a green LED between
0A and 20A over the entire duty cycle
range.

Linear Technology Magazine • December 2009

DESIGN FEATURES L

PWM

5V/DIV

I

LED

10A/DIV

SW

10V/DIV

10µs/DIV

Shutdown and
Precision Enable

When delivering high load currents,
the amount of supply undervoltage
lock-out (UVLO) hysteresis required
for proper operation is highly depen­dent on board layout. For maximum
flexibility, the LT3743 incorporates
a precision enable threshold with a

5.5µA current source flowing into the
pin when the EN/UVLO pin is lower
than 1.55V. Using a voltage divider
from the input supply to ground any
amount of hysteresis may be added
to the system. To conserve power in
portable applications, the LT3743
is completely disabled and supply
current drops below 1µA when the
EN/UVLO pin is lower than 0.5V.

Thermally Derating
the LED Current

Proper thermal management is vital
with any high current load to protect
expensive high current LEDs and
prevent system-wide damage. The
LT3743 uses the CTRL_T pin to reduce
the effective regulated current in the
load for both the high and low control
currents. Whenever CTRL_T is lower
than the control voltage on the CTRL_L
or CTRL_H pins, the regulated current
is reduced. The temperature derating
is programmed using a temperature
dependent resistor divider from the
V

pin to ground.

REF

Output Voltage Protection

Voltage protection is important to
prevent damaging expensive projector
LEDs. The LT3743 utilizes the FB pin
to provide a regulated voltage point for
the output. To simplify system design,
the LT3743 uses an internal 1V ref­erence, softly reducing the regulated
current when the FB voltage reaches
900mV.

Powerful Gate Drivers

To provide adequate drive and reduce
switching losses in high current
power MOSFETs, the LT3743 uses very
strong switching MOSFET drivers. The
on-resistance of the LG and HG PMOS
pull-up drivers is typically 2.5Ω. The
LG and HG NMOS pull-down drivers
on-resistance is typically less than

1.3Ω. With on-resistance this low, two
high current MOSFETs may be used
in parallel for applications exceeding
20A. Most currently available LED
drivers do not provide adequate gate
drive for dimming MOSFETs and as a
result need an additional external gate
driver. The LT3743 integrates this into
the PWMGL and PWMGH drivers and
has a 2Ω typical NMOS pull-down and
a 3.7Ω typical PMOS pull-up to drive
any 5V dimming MOSFET.

Traditional PWM Dimming

The LT3743 adapts to any traditional
PWM dimming method. Shunted out­put dimming used by competing LED
drivers wastes energy and has poor
efficiency for LED duty cycles below

The LT3743 produces

ultrafast high current

LED rise times while

providing accurate current

regulation. Its ability to

support multiple current

states meets the demands of

high performance theater-

quality DLP projectors by

allowing LED colors to be

easily mixed. In addition to

speed, the LT3743’s switched

capacitor topology reduces

board size by allowing the

use of a compact, low value

inductor. Additional features

include switching cycle

synchronization, overvoltage

protection, high efficiency

and easy adaptability for
varied application needs.

approximately 50%. Since the LT3743
has two levels of current regulation,
the regulated current can to drop to
zero when the shunt is engaged. This
provides excellent efficiency even for
low LED duty cycles.

Figure 6 shows a 2A LED driver con­figured with a current-limited shunted
output. Note that the CTRL_L pin is
tied to ground, PWMGL is used to drive

Figure 9. 0A to 20A switched cathode PWM
dimming

the shunting MOSFET, and CTRL_SEL
is used for dimming. With CTRL_L tied
to ground, when the CTRL_SEL pin
is low, the shunt is engaged and the
current in the inductor is regulated
at 0A. When CTRL_SEL is high, the
shunting MOSFET is turned off, and
the regulated current is determined by
the voltage at the CTRL_H pin. Figure
7 shows the current-limited shunted
PWM dimming with a 12V input.

In addition to the shunt, the LT3743
is readily configured to driving the
dimming MOSFET in series with the
cathode of the LED. When multiple
current states are not required, this is
the preferred method of PWM dimming.
Figure 8 illustrates a 6V to 30V, 20A
LED driver with switched cathode PWM
dimming. Figure 9 shows switched
cathode, PWM dimming with a 0A to
20A current step and a dimming ratio
of 100:1.

Conclusion

The LT3743 produces ultrafast high
current LED rise times while provid­ing accurate current regulation. Its
ability to support multiple current
states meets the demands of high
performance theater-quality DLP
projectors by allowing LED colors to
be easily mixed. In addition to speed,
the LT3743’s switched capacitor topol­ogy reduces board size by allowing the
use of a compact, low value inductor.
Additional features include switching
cycle synchronization, overvoltage
protection, high efficiency and easy
adaptability for varied application
needs.

L

Linear Technology Magazine • December 2009

19