MCP1623 datasheet(11/26 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS41420A-page 11

MCP1623/24

4.2

Functional Description

The MCP1623/24 is a compact, high-efficiency, fixed

frequency, step-up DC-DC converter that provides an

easy-to-use

power

supply

solution

for

PIC

microcontroller 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

MCP1623/24.

4.2.1

LOW-VOLTAGE START-UP

The MCP1623/24 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 MCP1623/24 charges an internal capacitor with a

very weak current source. The voltage on this capaci-

tor, 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

MCP1623/24. 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.

FIGURE 4-1:

MCP1623/24 Block Diagram.

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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