MCP1640CT-IMC datasheet(12/32 Pages) MICROCHIP

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MCP1640/B/C/D

DS20002234D-page 12

 2010-2015 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 single-cell, two-cell, or

three-cell alkaline, NiCd, NiMH, and single-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 to

approximately 100 mA during this time. This will affect

the start-up under higher load currents, and the device

may not start to the nominal value. 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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