Datasheet

 2014 Microchip Technology Inc. DS20005308B-page 15
MCP16331
4.2.4 HIGH SIDE DRIVE
The MCP16331 features an integrated high-side
N-Channel MOSFET for high-efficiency step-down
power conversion. An N-Channel MOSFET is used for
its low resistance and size (instead of a P-Channel
MOSFET). A gate drive voltage above the input is
necessary to turn on the high side N-Channel. The high
side drive voltage should be between 3.0V and 5.5V.
The N-Channel source is connected to the inductor and
Schottky diode, or switch node. When the switch is off,
the boost cap voltage is replenished, typically from the
output voltage for 3V to 5V output applications. A
boost-blocking diode is used to prevent current flow
from the boost cap back into the output during the
internal switch-on time.
Prior to startup, the boost cap has no stored charge to
drive the switch. An internal regulator is used to
“precharge” the boost cap. Once precharged, the
switch is turned on and the inductor current flows.
When the switch turns off, the inductor current free-
wheels through the Schottky diode, providing a path to
recharge the boost cap. Worst-case conditions for
recharge occur when the switch turns on for a very
short duty cycle at light load, limiting the inductor
current ramp. In this case, there is a small amount of
time for the boost capacitor to recharge. For high input
voltages there is enough precharge current to replace
the boost cap charge. For input voltages above 5.5V
typical, the MCP16331 device will regulate the output
voltage with no load. After starting, the MCP16331 will
regulate the output voltage until the input voltage
decreases below 4V. See
Figure 2-23 for device range
of operation over input voltage, output voltage and
load.
4.2.5 ALTERNATIVE BOOST BIAS
For 3.0V to 5.0V output voltage applications, the boost
supply is typically the output voltage. For applications
with V
OUT
< 3.0V or V
OUT
> 5.0V, an alternative boost
supply can be used.
Alternative boost supplies can be from the input, input
derived, output derived or an auxiliary system voltage.
For low-voltage output applications with unregulated
input voltage, a shunt regulator derived from the input
can be used to derive the boost supply. For
applications with high-output voltage or regulated high-
input voltage, a series regulator can be used to derive
the boost supply. In case the boost is biased from an
external source while in shutdown, the device will draw
slightly higher current.