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LTC1871-7

18717fc

APPLICATIONS INFORMATION

Application Circuits

A basic LTC1871-7 application circuit is shown in Figure 9.

External component selection is driven by the characteris-

tics of the load and the input supply. The ﬁrst topology to

be analyzed will be the boost converter, followed by SEPIC

(single-ended primary inductance converter).

Boost Converter: Duty Cycle Considerations

For a boost converter operating in a continuous conduction

mode (CCM), the duty cycle of the main switch is:

VO + VD –VIN

VO + VD

where VD is the forward voltage of the boost diode. For

converters where the input voltage is close to the output

voltage, the duty cycle is low and for converters that develop

a high output voltage from a low voltage input supply,

the duty cycle is high. The maximum output voltage for a

boost converter operating in CCM is:

VO(MAX) =

VIN(MIN)

1–DMAX

(

) –VD

The maximum duty cycle capability of the LTC1871-7 is

typically 92%. This allows the user to obtain high output

voltages from low input supply voltages.

Boost Converter: The Peak and Average Input Currents

The control circuit in the LTC1871-7 is measuring the input

current typically using a sense resistor in the MOSFET

source, so the output current needs to be reﬂected back

to the input in order to dimension the power MOSFET

properly. Based on the fact that, ideally, the output power

is equal to the input power, the maximum average input

current is:

IIN(MAX) =

IO(MAX)

1–DMAX

The peak input current is:

IIN(PEAK) = 1+ 2 •

IO(MAX)

1–DMAX

The maximum duty cycle, DMAX, should be calculated at

minimum VIN.

Figure 9. A High Efﬁciency 42V, 1.5A Automotive Boost Converter

RUN

ITH

FREQ

MODE/SYNC

SENSE

VIN

INTVCC

GATE

GND

LTC1871-7

f = 250kHz

100k

12.4k

412k

CVCC

4.7μF

X5R

RSENSE

0.005Ω

CIN2

10μF

50V

X5R

×2

6.8μH

24k

CC1

2.2nF

CC2

100pF

COUT1

68μF

100V

×2

VIN

8V TO 28V

VOUT

42V

1.5A

GND

18717 F09

CIN1*

560μF

50V

COUT2

10μF

50V

X5R

×2

CIN1:

SANYO 50MV560AXL (*RECOMMENDED FOR LAB EVALUATION

FOR SUPPLY LEAD LENGTHS GREATER THAN A FEW INCHES)

CIN2:

TDK C5750X5R1H106M

COUT1: SANYO 100CV68FS

COUT2: TDK C5750X5R1H106M

D1: DIODES INC B360B

L1: COOPER DR127-6R8

M1: SILICONIX/VISHAY Si7370DP

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