LTC1700_ 查看數據表（PDF） - Linear Technology

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LTC1700_

High Input Voltage, Current Mode Boost, Flyback and SEPIC Controller

Linear Technology 

LTC1871-7

APPLICATIO S I FOR ATIO

Application Circuits

A basic LTC1871-7 application circuit is shown in

Figure 9. External component selection is driven by the

characteristics 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 conduc-

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

D

=





VO

+ VD

VO +

– VIN

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) – VD

1– DMAX

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 prop-

erly. 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+

χ

2 

•

IO(MAX)

1– DMAX

The maximum duty cycle, DMAX, should be calculated at

minimum VIN.

RC

24k

CC1

2.2nF

CC2

100pF

R3

1M

f = 250kHz

1

RUN

2

ITH

10

SENSE

9

VIN

LTC1871-7

3

FB

4

FREQ

8

INTVCC

7

GATE

5

MODE/SYNC

RT

100k

6

GND

1%

CIN2

10µF

50V

X5R

×2

+

L1

6.8µH

CIN1*

560µF

50V

D1

CVCC

4.7µF

X5R

M1

RSENSE

0.005Ω

1W

COUT2

10µF

+

50V

X5R

×2

R1

R2

12.4k 412k

1% 1%

18717 F09

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

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

VIN

8V TO 28V

VOUT

42V

1.5A

COUT1

68µF

100V

×2

GND

18717f

13

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