Datasheet

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
V
IN
EN
V
OUT
GND
I
SENSE
I
ZERO
I
LIMIT
.3V
0V
SOFT-START
DIRECTION
CONTROL
OSCILLATOR
SLOPE
COMP.

PWM/PFM
LOGIC
1.21V
INTERNAL
BIAS
SW
FB
EA