LINEAR TECHNOLOGY LT3782A Technical data

DESIGN IDEAS L

0.1µF

22pF

10Ω

30.9k

51.1k

220pF

6.8nF

15k

10Ω

V

CC1

V

EE1

BGATE1 BG1

BG2

GBIAS1

GBIAS2

BGATE2

V

EE2

GND

RUN

FB

V

C

GBIAS

SGATE1

SYNC

DELAY

DCL

SENSE1

+

SENSE1

+

SENSE1

–

SLOPE

R

SET

SENSE2

–

SENSE2

+

SS

SGATE2

SGATE1

SGATE2

SENSE1

–

SENSE2

+

SENSE2

–

SENSE2

+

LT3782A

SENSE2

–

475k
1%

53.6k
1%

4.7µF

L2

8.3µH

L1

8.3µH

2.2nF

2.2nF

1µF

PD3S160

DFLS160

1µF

0.01µF

825k

316k

Q6

Q1–Q6 = HAT2266

Q5

BG2BG2

0.008Ω0.008Ω

TG

TS

BST

GND

IN

V

CC

LTC4440-5

SGATE2

5V

SENSE1

–

SENSE1

+

Q3Q2

BG1BG1

0.008Ω0.008Ω

1µF

PD3S160

DFLS160

10µF
s4

680µF

V

OUT

24V AT 8A

1µF

Q1

TG

TS

BST

GND

IN

V

CC

LTC4440-5

SGATE1

5V

10µF
s4

V

IN

10V TO 14V

Q4

+

680µF

+

60.4k

4.7µF

High Power 2-Phase Synchronous
Boost Replaces Hot Diodes with
Cool FETs—No Heat Sinks Required

by Narayan Raja, Tuan Nguyen and Theo Phillips

Introduction

For low power designs, non-synchro­nous boost converters offer a simple
solution. However, as power levels in­crease, the heat dissipated in the boost
diode becomes a significant design
problem. In such cases, a synchro-

nous boost converter, with the diode
replaced with a lower forward voltage
drop switch, significantly improves ef­ficiency and relieves many issues with
thermal layout. Although the topology
is more complicated, Linear Technol-

ogy offers controller ICs that simplify
the design of high power synchronous
boost applications. The LT3782A boost
controller, for instance, includes pre­drive outputs for external synchronous
switch drivers. It also integrates strong

Figure 1. Compact high power boost application efficiently produces a 24V/8A output from a 10V–15V input.

Linear Technology Magazine • January 2009

3535

L DESIGN IDEAS

POWER LOSS (W)

EFFICIENCY (%)

LOAD CURRENT (A)

71

100

10

1

2

3 4

5 6

97

96

94

95

93

92

91

90

VIN = 12V

VIN = 12V

VIN = 24V

VIN = 24V

EFFICIENCY

POWER LOSS

Figure 2. Layout of the circuit in Figure 1. Note that no heat sinks are needed,
even at the high power levels produced by this relatively compact circuit.

bottom switch drivers for high gate
charge high voltage MOSFETs and
uses a constant frequency, peak cur­rent mode architecture to produce
high output voltages from 6V to 40V
inputs. Its 2-phase architecture keeps
external components small and low
profile.

Synchronous Operation

At high current levels, a boost diode
dissipates a significant amount of
power, while a synchronous switch
can burn far less. It all comes down
to the forward voltage drop. The power
dissipated in the boost diode is IIN • VD,
while the power dissipated by the
synchronous switch is I
(or I

IN

• V

). A typical sub-10mΩ

DS(ON)

MOSFET running 10A dissipates
1W, while the 0.5V drop of a typical

2

IN

• R

DS(ON)

.

Schottky diode burns a whopping
5W. Because the forward drop of a
synchronous MOSFET is proportional
to the current flowing through it, FETs
can be paralleled to share current and
drastically reduce power dissipation.
On the other hand, paralleling boost
diodes does little to reduce power dis­sipation as the forward drop through
the diodes holds fairly constant. The
non-synchronous boost diode topology
is more than just inefficient relative
to a synchronous solution—the extra
heat generated in a boost diode must
go somewhere, necessitating a larger
package footprint and heat sinking. At
high power levels, a non-synchronous
boost application becomes larger in
size and higher in cost over a syn­chronous solution.

Figure 3. Efficiency and power loss of the
circuit in Figure 1 compared to the efficiency
of the circuit when the synchronous FETs are
replaced with non-synchronous boost diodes.

Multiphase Operation
Reduces Application Size

There are a number of good reasons
to choose a multiphase/multi-channel
DC/DC converter over an equivalent
single-phase solution, including
reduced EMI and improved thermal
performance, but the biggest advan­tage can be a significant reduction in
application size. Although a 2-phase
solution requires more components,
two inductors and two MOSFETs in­stead of one, it offers a net reduction
in space and cost. This is because the
inductors and MOSFETs are more
than proportionally smaller than those
required in the single-phase solution.
Moreover, because the switching sig­nals are mutually anti-phase, their
output ripples tend to cancel each

continued on page 39

COOL FETs

a. Thermal image of the board in Figure 2
built up with synchronous FETs

Figure 4. The board in Figure 2 runs fairly cool (a), but when the synchronous FETs are replaced with boost diodes, the entire board heats up
considerably with the diodes running significantly hotter than the FETs (b). (V

36

36

= 12V, I

IN

HOT DIODES HEAT UP
THE WHOLE BOARD

b. Thermal image of the board in Figure 2
built up with boost diodes

= 6A, two minutes after power up.)

LOAD

Linear Technology Magazine • January 2009

V

IN

LT3756

GND

SHDN/UVLO

INTV

CC

499k

110k

2.49k

130k

1M

140k

0.25Ω

10V–50V

2.2µF
50V
s2

V

IN

9V TO 36V
(6V UVLO)

L1

22µH

M1: VISHAY SILICONIX Si7454DP
D1: DIODES INC. PDS3100
L1: SUMIDA CDRH127-220

4.7k

LED

–

LED

+

0.1µF

4.7µF

CTRL

V

C

5.1k

10k

100k

4700pF

OPENLED

SS

FB

ISN

ISP

PWMOUT

PWM

V

REF

SENSE

R

T

GATE

C

OUT

2.2µF
100V
s2

M1

D1

28.7k
400kHz

0.025Ω

V

IN

3906

I

LED

400mA

EFFICIENCY (%)

VIN (V)

V

LED

= 10V

V

LED

= 50V

3010

100

0

15 20 25

10

20

30

40

50

60

70

80

90

Figure 4. A buck-boost mode LED driver with wide-ranging V

current; the peak inductor current
is also equal to the peak switching
current—higher than either a buck
mode or boost topology LED driver
with similar specs due to the nature
of the hookup. The 4A peak switch
current and inductor rating reflects
the worst-case 9V input to 50V LED
string at 400mA.

Below 9V input, the CTRL analog

dimming input pin is used to scale back

LT3782A, continued from page 36

other out, thus reducing the total
output ripple by 50%, which in turn
reduces output capacitance require­ments. The input current ripple is also
halved, which reduces the required
input capacitance and reduces EMI.
Finally, the power dissipated as heat is
spread out over two phases, reducing
the size of heat sinks or eliminating
them altogether.

24V at 8A from
a 10V–15V Input

Figure 1 shows a high power boost
application that efficiently produces a
24V/8A output from a 10V–15V input.
The LTC4440 high side driver is used

Linear Technology Magazine • January 2009

and V

IN

LED

the LED current to keep the inductor
current under control if the battery
voltage drops too low. The LEDs turn
off below 6V input due to undervoltage
lockout and will not turn back on until
the input rises above 7V, to prevent
flickering. In buck-boost mode, the
output voltage is the sum of the input
voltage and the LED string voltage. The
output capacitor, the catch diode, and

small) strings of high power LEDs.
It can be used in boost, buck-boost
mode, buck mode, SEPIC and flyback
topologies. Its high voltage rating, op­timized LED driver architecture, high
performance PWM dimming, host of
protection features and accurate high
side current sensing make the LT3756
a single-IC choice for a variety of high
voltage input and high power lighting
systems.

to level shift the SGATE signals and
drive the synchronous MOSFETs. The
250kHz switching frequency optimizes
efficiency and component size/board
area. Figure 2 shows the layout. Proper
routing and filtering of the sense pins,
placement of the power components
and isolation using ground and sup­ply planes ensure an almost jitter free
operation, even at 50% duty cycle.

Figure 3 shows the efficiency of the
circuit in Figure 1 with synchronous
MOSFETs (measured to 8A) and the
efficiency of an equivalent non-syn­chronous circuit using boost diodes
(measured to 6A). The 1% improvement
in peak efficiency may not seem signifi­cant, but take a look at the difference

in heat dissipation shown in Figure 4,
which shows thermal images of both
circuits under equivalent operating
conditions. The thermal advantages
of using synchronous switches are
clear.

Conclusion

The 2-phase synchronous boost
topology possible with the LT3782A
offers several advantages over a non­synchronous or a single-phase boost
topology. Its combination of high ef­ficiency, small footprint, heat sink-free
thermal characteristics and low in­put/output capacitance requirements
make it an easy fit in automotive and
industrial applications.

DESIGN IDEAS L

Figure 5. Efficiency for the buck-boost
mode converter in Figure 4

the power MOSFET can see voltages
as high as 90V for this design.

Conclusion

The 100V LT3756 controller is osten­sibly a high power LED driver, but its
architecture is so versatile, it can be
used in any number of high voltage
input applications. Of course, it has
all the features required for large (and

L

L

3939