LINEAR TECHNOLOGY LTC3788 Technical data

2-Phase, Dual Output Synchronous Boost Converter Solves
Thermal Problems in Harsh Environments –

Goran Perica

Introduction

Boost converters are regularly used in automotive and
indust ri al ap pl ic ations to p ro du ce hi gh er ou tp ut vo lt ages
from lower input voltages. A simple boost converter us­ing a Schottky boost diode (Figure 1) is often suffi cient
for low current applications. However, in high current or
space-constrained applications, the power dissipated
by the boost diode can be a problem especially in high
ambient temperature environments. Heat sinks and fans
may be needed to keep the circuit cool, resulting in high
cost and complexity.

To s o l v e t h i s p r o b l e m , t h e S c h o t t k y o u t p u t r e c t i fi e r c a n b e
replaced by a synchronous MOSFET rectifi er (Figure 2).
If MOSFETs with very low R

are used, the power

DS(ON)

dissipation can be reduced to the point where no heat
sinks or active cooling is required, thus reducing costs
and saving space.

Advantages of Synchronous Rectifi cation

Consider the power dissipation o f the single output circuit
in Figure 1. The output diode D1 carries 6.7A of RMS
current to produce 3A of output current from a 5V input.
At this current level, diode D1’s voltage drop is 0.57V,
resulting in 1.6W of power lost as heat. Dissipating 1.6W
in an 85°C (or higher) automotive operating environment
is not trivial. To keep the circuit cool, heat sinks, cooling
fans and multilayer printed circuit boards must be used.
This, of course, adds complexity, cost and size to an
ostensibly simple boost converter.

INPUT*

5V TO 36V

4.7nF

115k

54.9k

1

RUN

12k

2

I

TH

80.6k

4

FREQ

5

MODE/SYNC

12.4k

3

V

FB

FOLLOWS VIN FOR VIN > 24.6V

*V

OUT

LTC1871

229k

SENSE

INTV

GND

V

IN

G

CC

Figure 1. Although This Simple Circuit is Capable of 3A of
Output Current, Beware of Power Dissipation in the Output
Diode D1

22μF

3.3μH

s2

9

10

7

RJK0452

8

6

4.7μF

D1

MBRB2545

22μF

X7R

s4

L1: PULSE PA1494.362
ALL CERAMIC CAPACITORS ARE X7R, TDK

OUTPUT
24V

+

3A

150μF

DN481 F01

A far better solution (featured in a dual output confi gura­tion) is shown in Figure 2, where a synchronous power
MOSFET rectifi er replaces the output diode. Under the
same conditions, th e voltage drop across output synchro ­nous MOSFET Q2 is only 42mV or 7.4% of the voltage
drop in the diode D1. The resulting power dissipation of
115mW in Q2 is relatively trivial. Another advantage of
using a MOSFET as t he output rectifi er is the elimina tion of
leakage current, about 10mA in the case of the MBR2545
diode—an additional 240mW of power dissipation in the
application of Figure 1.

Dual Output Automotive Boost Converter

Figure 2 illustrates a typical automotive boost application
with a 5V to 36V input voltage range. Here, the converter
produces a 12V output for generic automotive loads
such as entertainment systems, and a 24V output for
circuits such as high power audio amplifi ers. The two
outputs are c ompletely independent and can be con trolled
separately.

Because the circuit in Figure 2 is a boost converter, the
output voltage can be regulated only for input voltages
that are lower than the output voltage. The output volt­age regulation versus input voltage is shown in Figure 3.
When the input voltage is higher than the preset output
voltage, synchronous MOSFETs Q2 and Q4 are turned
continuously ON and boost MOSFETs are not switching.
This feature allow s the converter to be used in applica tions
that require boosting only during load transients such as
cold-cranking of a car engine. In this case, the LTC
circuit’s input voltage could be as low as 2.5V.

The effi ciency of this converter (Figure 4) is high enough
that it can be built entirely with surface mount compo­nents, requiring no he at sinks. A multilayer PCB with large
c o p p e r a r e a m a y b e s u f fi cient to dissipat e the small amount
of heat resulting from the MOSFETs’ DC resistance, even
at high ambient temperatures.

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective
owners.

Design Note 481

®

3788

08/10/481

15nF

16.9k

V

OUT1

232k

10k

1nF

12.1k

+V

IN

38.3k

12.1k

32313029282726

+

1

SENSE1

2

FREQ

3

PHASMD

4

CHKOUT

5

PLLIN/MODE

6

SGND

7

RUN1

8

RUN2

GND

33910111213141516

10Ω

SENSE1

–

–

SENSE2

1nF

FB1

V

ITH1

SENSE2+V

100pF

0.1μF

SS1

LTC3788

FB2

ITH2

9.09k

LIM

I

100pF

SS2

PGOOD1

PGOOD2

25

SW1

BOOST1

BG1

V

BIAS

PGND

EXTV

INTV

BG2

BOOST

SW2

15nF

TG1

24

23

22

21

20

CC

19

CC

18

17

TG2

12.1k 110k

Figure 2. The LTC3788 Converter is Over 95% Effi cient Even Under Worst-Case Conditions.
When VIN > V

36
34
32
30
28
26

24V OUTPUT

24
22
20
18

OUTPUT VOLTAGE (V)

16
14
12
10

11 13 15

579

17 19 21

INPUT VOLTAGE (V)

, Effi ciency Approaches 100% as Shown in Figure 4

OUT(SET)

12V OUTPUT

23 25 27

29 31 33

35

DN481 F03

Figure 3. The Output Voltage Follows the Input Voltage
when V

IN

> V

OUT(SET)

+V

IN

PA1494.362

0.1μF

BAS140W

V

OUT2

0.1μF

4.7μF

BAS140W

0.1μF

V

OUT2

0.004Ω

0.003Ω

22μF
s2

RJK0452

PA1294.132

RJK0452

99

97

95

93

EFFICIENCY (%)

91

89

87

Q2

RJK0452

22μF

Q1

X7R

s4

Q4

RJK0452

Q3

22μF

0

3

5

4

2

1

+

X7R

s4

7

68
I

(A)

OUT

V

OUT1

24V
3A

150μF

+

DN481 F02

12VIN, 12V
12VIN, 24V
5VIN, 12V
5VIN, 24V

10

9

V

OUT2

12V
8A

150μF

11 12 13

OUT

OUT
OUT
OUT

DN481 F04

Figure 4. The Converter in Figure 2 Peaks at 95%
Effi ciency when Operating from a 5V Input

14

If higher output currents are required, or if lower output
ripple voltage is desired, the two LTC3788 channels can
be combined for a single current-shared output. Sim­ply connect the two outputs and short the respective
FB, ITH, SS and RUN pins. Because the two channels
operate out of phase, output ripple currents are greatly
reduced—nearly canceling out at 50% duty cycle. Thus,
smaller output capacitors can be used with lower output
ripple currents and voltages.

Data Sheet Download

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Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

●

[phone]

FAX: [phone] ● www.linear.com

Conclusion

The LTC3788 dual synchronous boost controller is a
versatile and effi cient solution for demanding automotive
and industrial applications. By minimizing power losses
in the output rectifi er, this converter can be designed
in a very small footprint and operate safely at elevated
ambient temperatures.

For applications help,

call [phone], Ext. 3788

dn481f LT/AP 0810 226K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2010