Datasheet

Installation and Operation
 2010-2011 Microchip Technology Inc. DS51922B-page 15
Periodically, the microcontroller will come out from Sleep mode for a short period of
time to search if its power supply (V
DD
), which comes from the MCP1640 V
OUT
pin,
reaches 2.3V threshold. This is accomplished by the internal analog comparator of the
microcontroller.
This compares a fraction of V
OUT
(from R4) with the 0.6V internal voltage reference.
To avoid losing power on the passive components and reduce the complexity of the
schematic, the application also uses the feedback network (R2-R3-R4) of the
MCP1640 as the input to the PIC MCU comparator.
The PIC12F617 microcontroller works down to 2.0V V
DD
. If a 2.3V threshold is
detected, the GP2 pin of the microcontroller will set the EN pin of the MCP1640 high
for a short time, to pump up the output capacitor to a 3.3V regulated voltage. After
that, EN is pulled down again and the MCP1640 enters into the True Output
Disconnect Shutdown option. V
OUT
is sustained by the output capacitor (C2) for
approximately 2 seconds until it reaches the 2.3V threshold.
FIGURE 2-4: Waveforms while MCP1640 is Shutdown (OUT is off).
Generally, the EN drive signal frequency depends on the MCP1640 devices’ output
capacitor value and the PIC12F617 MCUs Sleep current. The typical time to charge
the output capacitor to 3.3V is 750 ns, with a load less than 10 µA.
The EN signal is inverted using the transistor Q1 to ensure the start of the application
when a battery is inserted.