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