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LTC1871

1871fe

BLOCK DIAGRAM

1.230V

85mV

50k

START-UP

CONTROL

BURST

COMPARATOR

LOGIC

PWM LATCH

CURRENT

COMPARATOR

0.30V

1.230V

5.2V

2.00V

1.230V

SLOPE

1.230V

ILOOP

ITH

MODE/SYNC

FREQ

INTVCC

LDO

V-TO-I

OSC

V-TO-I

SLOPE

COMPENSATION

BIAS AND

START-UP

CONTROL

VIN

BIAS

VREF

IOSC

RLOOP

–

SENSE

GND

1871 BD

GATE

INTVCC

GND

VIN

1.248V

RUN

0.6V

Main Control Loop

The LTC1871 is a constant frequency, current mode con-

troller for DC/DC boost, SEPIC and ﬂyback converter ap-

plications. The LTC1871 is distinguished from conventional

current mode controllers because the current control loop

can be closed by sensing the voltage drop across the power

MOSFET switch instead of across a discrete sense resistor,

as shown in Figure 2. This sensing technique improves

efﬁciency, increases power density, and reduces the cost

of the overall solution.

For circuit operation, please refer to the Block Diagram of

the IC and Figure 1. In normal operation, the power MOSFET

is turned on when the oscillator sets the PWM latch and

is turned off when the current comparator C1 resets the

latch. The divided-down output voltage is compared to an

internal 1.230V reference by the error ampliﬁer EA, which

outputs an error signal at the ITH pin. The voltage on the

ITH pin sets the current comparator C1 input threshold.

When the load current increases, a fall in the FB voltage

relative to the reference voltage causes the ITH pin to rise,

which causes the current comparator C1 to trip at a higher

peak inductor current value. The average inductor current

will therefore rise until it equals the load current, thereby

maintaining output regulation.

OPERATION

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