Can’t Find the Right Synchronous Boost LED Driver? Use a Synchronous Buck Converter Instead: Boost Mode Topology Drives 25V, 3A LEDs from 12V

Can’t Find the Right Synchronous Boost LED Driver?
Use a Synchronous Buck Converter Instead:
Boost Mode Topology Drives 25V, 3A LEDs from 12V
Keith Szolusha
Synchronous buck converter drivers are commonly used when the current required for
high power LEDs, such as 10A–40A projector LEDs, would overstress the components
in a nonsynchronous converter. Synchronous rectification limits power losses and thermal
rise due to high current in the converter switches. Synchronous rectification can offer the
same benefits in high power step-up (boost) LED drivers—even with 1A to 3A LEDs. In
contrast to a buck converter, the peak switch current of a boost can be much higher than
the LED current, especially when output power is high and the input voltage is low.
For instance, take the LT3744 40V synchronous buck LED driver, which is designed
to drive high current LEDs for projectors.
It features a versatile, floating VEE output
that allows it to be used in both high
current buck applications and positive-tonegative (buck-boost) topologies where
There are a number of situations where a
synchronous boost LED driver is not available for a particular application. For some
of these cases, a synchronous buck LED
driver IC can be used, but instead of operating as a step-down converter, it operates
as a step-up, or boost mode* LED driver.
Figure 1. Boost mode 9V–16V input to 25V, 3A LED driver with 98% efficiency
VIN
9V TO 16V
VIN
SGND
200k
TG
EN/UVLO
EN/UVLO
1µF
25V
M1
220nF
D1
INTVCC
CTRLT
VREF
2V
2.2nF
100k
10k
2V
0V
VEE
VFNEG
ISP
ISN
L1
4.7µH
10µF
5mΩ
VEE
VC2
VC3
20k
VEE
680pF
VEE
102k
VEE
22 | April 2015 : LT Journal of Analog Innovation
LT3744 BOOST MODE LED DRIVER
The LT3744 synchronous boost mode LED
driver shown in Figure 1 regulates a 3A,
25V (75W) LED string from an automotive
input (9V–16V) at 98% efficiency. Even at
this power level, the maximum component temperature rise is 45ºC with a 12V
input, as shown in Figure 2. The IC enables
easy implementation of both 10:1 analog
and 100:1 PWM dimming at 120Hz with
ground-referred input signals, even though
neither the LED string nor the PWM dimming MOSFET are connected to GND.
Although the 5mΩ sense resistor sets a 10A
peak switch current in this application, the
solution can be altered to operate with a
6V input and a 15A peak switch current;
with an appropriately valued inductor
and a lowered undervoltage lockout.
1M
43.2k
VEE
RT
4×
10µF
50V
M2
PWM1
FB
PWM2
PWM3
SYNC PWM_OUT2
PWM_OUT3
PWM_OUT1
SS
LED_ISP
VC1
10nF
VEE
BG
FAULT
CTRL1
CTRL2
CTRL3
30.1k
VEE
LT3744 SW
25V
3A LED
2×
10µF
25V
BOOST
30.9k
the anode of the high current LED is connected to ground to satisfy heat-sinking
requirements. It is the floating VEE feature
that allows us to effectively use this part,
originally designed for buck applications,
as a synchronous boost mode LED driver.
M3
20mΩ
LED_ISN
M1, M2: INFINEON BSC026N04LS
M3: INFINEON BSC010N04LS
D1: PMEG4002EB
D2: PMEG4010
L1: WÜRTH 7443551470 4.7µH
VEE
D2
The negative VEE rail on the LT3744
can reach −21V. The LT3744 simplifies
design by handling the level-shifting of
ground referenced input control signals.
design ideas
Synchronous rectification is a feature commonly called
on to limit power losses in high power LED drivers. As
shown, a synchronous buck LED driver can be used as a
boost mode converter, capitalizing on the wide availability
of this feature in high performance buck regulator ICs.
A simple ground referenced PWM input
signal is level shifted to PWMOUT, so
no additional level shifting circuitry is
required to control the PWM MOSFET.
Likewise, the LED current-setting sense
resistor can be tied directly to the negative VEE rail, all the way down to −21V.
LEVEL-SHIFTED GND AND V EE
The level-shifting, positive-to-negative
conversion, feature of the LT3744
LED driver is designed to support high
current grounded-anode LED drivers.
Nevertheless, the same feature can be
used for boost mode applications where
the LED string is connected between VIN
and a negative VEE potential. Because the
Figure 3. Boost mode current
ripple, duty cycle and voltage
stress are the same as those of
traditional boost regulators.
VIN = 12V
VLED = 23.45V
ILED = 3A
NO FORCED AIR
Figure 2. Boost mode LED
thermal scan shows cool
operation.
LED sense resistor and PWM dimming
MOSFET are both located at the bottom of the LED string, the level-shifted
no more complicated than a traditional
S2
L1
L1
VOUT
VIN
CIN
PWMOUT signal from the ground-referred
PWM input yields a topology that looks
S1
VIN = LED+
VIN
COUT
+
CIN
COUT
S2
S1
VOUT = VLED
–
VEE (NEGATIVE) = LED–
CONTINUOUS MODE
CURRENT
VOLTAGE
VOUT
S1
VIN
0
VOUT
D1
L1
0
VIN
0
VIN – VOUT
VOUT
COUT
VIN
CONTINUOUS MODE
IIN
0
VOUT
S1
0
IOUT
0
D1
IOUT
0
L1
0
IOUT
0
CURRENT
VOLTAGE
IOUT
0
VIN
0
VOUT
VIN
0
IOUT
0
0
VIN
0
VIN – VOUT
VOUT
COUT
IIN
IOUT
0
0
IOUT
0
0
IOUT
0
April 2015 : LT Journal of Analog Innovation | 23
The LT3744 synchronous buck LED driver can be used as a high efficiency
9V–16V input, 25V LED 3A boost mode LED driver, yielding 98% efficiency for
a 75W converter. The unique ability of this IC to level shift control signals as
needed from SGND to –VEE makes it possible to produce a floating boost mode
topology with no more components than needed in a traditional boost.
boost PWM dimming setup. The input side
looks like a straightforward LED driver,
with CTRL analog dimming, SYNC input
and enable inputs all referred to signal
GND, no matter where negative VEE lies.
The LT3744’s VEE can go all the way down
to −21V. Open LED overvoltage protection is set at about 26.5V for the 25V
VLED application. With some open LED
overshoot in mind, this limits the VIN
minimum to about 6V for a 25V VLED
boost mode application before negative VEE goes beyond the −21V limit. To
operate at the minimum 6V input, the
solution in Figure 1 requires a lower
undervoltage lockout and a sense resistor and inductor that can accommodate
15A peak switch current limit. With lower
VLED strings (at any current level), the
minimum input voltage can be dropped
to 4V VIN with some simple adjustments.
MORE ABOUT BOOST MODE
CONCLUSION
A boost mode converter has many of
the same properties of traditional boost
regulators. As shown in Figure 3, with
the exception of the unusual topological
hookup and the main control of high side
switch S1 instead of a low side switch,
this boost mode converter features the
same duty cycle, ripple current and voltage stress that a tradition boost converter
would. If synchronous rectification is not
required, a nonsynchronous buck regulator can be used as a boost mode driver,
with S2 replaced by a typical catch diode
D1, as in a traditional boost regulator.
Synchronous rectification is a feature commonly called on to limit power losses in
high power LED drivers. As shown here,
a synchronous buck LED driver can be
used as boost mode converter, capitalizing
on the wide availability of this feature
in high performance buck regulator ICs.
Specifically, the LT3744 synchronous buck
LED driver can be used as a high efficiency 9V–16V input, 25V LED 3A boost
mode LED driver, yielding 98% efficiency
for a 75W converter. The unique ability
of this IC to level shift control signals as
needed from SGND to –VEE makes it possible to produce a floating boost mode
topology with no more components
than needed in a traditional boost. n
The gain-phase Bode plot in Figure 4
shows that even the control loop of boost
mode converter acts like a traditional
boost regulator. With a 10kHz crossover
frequency, 60° of phase margin, and
−15dB of gain margin, the LED driver
shown in Figure 1 is stable and reliable.
60
180
50
150
40
120
30
90
20
60
10
30
0
0
–10
–30
–20
–60
–30
–90
–40
–120
–50
–150
–60
24 | April 2015 : LT Journal of Analog Innovation
1
10
FREQUENCY (kHz)
–180
100
PHASE (DEGREES)
GAIN (dB)
Figure 4. Boost mode control loop gain and phase
shows typical bandwidth.
Notes:
* Boost mode is a patent pending technology.
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