LINEAR TECHNOLOGY LT3782 Technical data

High Efficiency 2-Phase Boost Converter Minimizes Input and
Output Current Ripple –

Design Note 371

Goran Perica

Introduction

Many automotive and industrial applications require higher
voltages than is available on the input power supply rail.
A simple DC/DC boost converter suffices when the power
levels are in the 10W to 50W range, but if higher power
levels are required, the limitations of a straightforward
boost converter become quickly apparent. Boost con­verters convert a low input voltage to a higher output
voltage by processing the input current with a boost
inductor, power switch, output diode and output capaci­tor. As the output power level increases, the currents in
these components increase as well. Switching currents

INPUT

10V TO 23V

13.3k

4.7nF

80.6k

12.4k

75k

12k

1

RUN

2

I

TH

4

FREQ

5

MODE/SYNC

3

V

FB

LTC1871-7

229k

GATE

SENSE

INTV

GND

V

IN

CC

Figure 1a. Single-Phase Boost Converter: Can be Used
to Convert 12V Input to 24V, 10A Output

22μF

L1

×2

4.7μH

HAT2165
×2

0.002Ω

MBRB2545

22μF

×6

9

7

10

8

4.7μF

6

L1: COOPER HC3-4R7
ALL CERAMIC CAPACITORS ARE X7R, TDK

also increase proportional to the output-to-input voltage
conversion ratio, so if the input voltage is low, the
switching currents can overwhelm a simple boost con­verter and generate unacceptable EMI.

For example, consider Figure 1, a 12V input to 24V, 10A
output switching converter operating at 300kHz. The
currents processed by the converter in Figure 1 are shown
in the first row of Table 1. The relatively high current levels
in the switcher are reflected in high input and output ripple
currents, which results in increased EMI.

, LTC, LT and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.

OUTPUT
24V

+

220μF

10A

DN371 F01

V

OUT

100mV/DIV

1μs/DIV

DN371 F01b

Figure 1b. Single-Phase Boost
Converter Output Voltage Ripple

Table 1. Dual-Phase Boost Converter Has Lower Input and Output Ripple Currents and Voltages Than Single-Phase Boost
Converter

MOSFET OUTPUT OUTPUT OUTPUT OUTPUT

INPUT RMS INPUT RIPPLE RMS DRAIN DIODE CAPACITOR CAPACITOR VOLTAGE

CURRENT CURRENT CURRENT RMS CURRENT RMS CURRENT FREQUENCY RIPPLE

SINGLE-PHASE

BOOST 21.1A 4.2A

CONVERTER

DUAL-PHASE

BOOST 20.7A 0.17A

CONVERTER

09/05/371

P-P

P-P

15.4A 14.4A 10.5A 300kHz 212mV

2 × 7.4A 2 × 7.2A 1.9A 600kHz 65mV

The circuit shown in Figure 2 performs the same DC/DC
conversion, but with greatly reduced input and output
ripple, significantly reducing EMI, and at a higher effec­tive switching frequency, which allows the use of two
22μF output capacitors versus six 22μF output capacitors
required in Figure 1.

The trick is the 2-phase boost topology, which interleaves
two 180° out-of-phase output channels to mutually can­cel out input and output ripple current—the results are
shown in the second row of Table 1. Each phase operates
at 50% duty cycle and the rectified output currents from
each phase flow directly to the load—namely the low
inductor ripple current—so only a small amount of output
current (shown in Table 1) is handled by the output
capacitors.

The centerpiece of the design in Figure 2 is the LT

®

3782
2-phase current mode PWM controller. Current mode
operation ensures balanced current sharing between the
two power converters resulting in even power dissipation
between the power stages.

The efficiency of the dual-phase converter, shown in
Figure 3, is high enough that it can be built entirely with
surface mount components—no need for heat sinks. In a

240W boost supply application, the power dissipation of

12.9W is relatively easy to manage in a well laid out, large
multilayer PCB with some forced airflow.

Conclusion

The simple LT3782 dual-phase switching boost converter
improves on single-phase alternatives by allowing high
power output with lower ripple currents, reduced heat
dissipation and a more compact design.

96

95

94

93

EFECIENCY (%)

92

91

90

0

468

2

OUTPUT CURRENT (A)

Figure 3. 12V Input to 24V Output Dual-Phase
Boost Converter Efficiency

10 12

DN371 F03

UPS840

HAT2165

220μF

HAT2165

UPS840

22μF

×2

+

59k

4.7nF

4.7nF

100pF

825k

274k

62k

15k

RUN

R

SET

DELAY

DCL

SLOPE

SS

GND

V

EE1

V

EE2

V

C

LT3782

BGATE1

SEN1P

SEN1N

V

GBIAS

GBIAS1

GBIAS2

SEN2N

SEN2P

BGATE2

INPUT

10V TO 23V

10nF

CC

FB

2.2μF

10nF

220k

24.9k

L1, L2: PULSE PB2020-153
ALL CERAMIC CAPACITORS ARE X7R TDK

L1

10Ω

0.004Ω

22μF

0.004Ω

10Ω

L2

Figure 2a. Dual-Phase Boost Converter Reduces EMI and Ripple
Currents with a Minimum Input and Output Filtering

Data Sheet Download

http://www.linear.com

OUTPUT
24V
10A

V

OUT

100mV/DIV

Figure 2b. Dual-Phase Boost
Converter Output Voltage Ripple

DN371 F02

For applications help,

call [phone], Ext. 2593

1μs/DIV

DN371 F02b

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone] ● www.linear.com

dn371f LT/TP 0905 305K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2005