MCP1640 datasheet(12/32 Pages) MICROCHIP

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MCP1640/B/C/D

DS22234A-page 12

 2010 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” 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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