MCP1640_13 datasheet(12/32 Pages) MICROCHIP

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MCP1640/B/C/D

DS22234B-page 12

 2011 Microchip Technology Inc.

4.2

Functional Description

The MCP1640/B/C/D is a compact, high-efficiency,

fixed

frequency,

step-up

DC-DC

converter

that

provides an easy-to-use power supply solution for

applications powered by either one-cell, two-cell, or

three-cell alkaline, NiCd, or NiMH, or one-cell Li-Ion or

Li-Polymer batteries.

Figure 4-1 depicts the functional block diagram of the

MCP1640/B/C/D.

FIGURE 4-1:

MCP1640/B/C/D Block Diagram.

4.2.1

LOW-VOLTAGE START-UP

The MCP1640/B/C/D is capable of starting from a low

input voltage. Start-up voltage is typically 0.65V for a

3.3V output and 1 mA resistive load.

When enabled, the internal start-up logic turns the

rectifying P-Channel switch on until the output

capacitor is charged to a value close to the input

voltage. The rectifying switch is current-limited during

this time. After charging the output capacitor to the

input voltage, the device starts switching. If the input

voltage is below 1.6V, the device runs open-loop with a

fixed duty cycle of 70% until the output reaches 1.6V.

During this time, the boost switch current is limited to

50% of its nominal value. Once the output voltage

reaches 1.6V, normal closed-loop PWM operation is

initiated.

The MCP1640/B/C/D charges an internal capacitor

with a very weak current source. The voltage on this

capacitor, in turn, slowly ramps the current limit of the

boost switch to its nominal value. The soft-start

capacitor is completely discharged in the event of a

commanded shutdown or a thermal shutdown.

There is no undervoltage lockout feature for the

MCP1640/B/C/D. The device will start up at the lowest

possible voltage and run down to the lowest possible

voltage. For typical battery applications, this may result

in “motor-boating” (emitting a low-frequency tone) for

deeply discharged batteries.

GATE DRIVE

AND

SHUTDOWN

CONTROL

LOGIC

VIN

VOUT

GND

ISENSE

IZERO

ILIMIT

.3V

SOFT-START

DIRECTION

CONTROL

OSCILLATOR

SLOPE

COMP.

PWM/PFM

LOGIC

1.21V

INTERNAL

BIAS

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