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