Microchip Technology ADM00427 Ficha De Dados
MCP16323
DS22284A-page 18
© 2011 Microchip Technology Inc.
turns off and the low-side switch turns on. The error
amplifier output slews up or down to increase or
decrease the inductor peak current feeding into the
output LC filter. If the regulated output voltage is lower
than its target, the inverting error amplifier output rises.
This results in an increase in the inductor current to
correct for errors in the output voltage. The fixed
frequency duty cycle is terminated when the sensed
inductor peak current, summed with the internal slope
compensation, exceeds the output voltage of the error
amplifier. The PWM latch is set by turning off the high-
side internal switch and preventing it from turning on
until the beginning of the next cycle.
amplifier output slews up or down to increase or
decrease the inductor peak current feeding into the
output LC filter. If the regulated output voltage is lower
than its target, the inverting error amplifier output rises.
This results in an increase in the inductor current to
correct for errors in the output voltage. The fixed
frequency duty cycle is terminated when the sensed
inductor peak current, summed with the internal slope
compensation, exceeds the output voltage of the error
amplifier. The PWM latch is set by turning off the high-
side internal switch and preventing it from turning on
until the beginning of the next cycle.
4.2.4
HIGH-SIDE DRIVE
The MCP16323 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). The N-Channel MOSFET gate must be
driven above its source to fully turn on the device,
resulting in a gate-drive voltage above the input to turn
on the high-side N-Channel. The high-side N-Channel
source is connected to the inductor and boost cap or
switch node. When the high-side switch is off and the
low-side is on, the inductor current flows through the
low-side switch, providing a path to recharge the boost
cap from the boost voltage source. An internal 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 “pre-charge” the boost cap. Once pre-
charged, the switch is turned on and the inductor
current flows. When the high-side switch turns off and
the low-side turns on, current freewheels through the
inductor and low-side switch, providing a path to
recharge the boost cap. When the duty cycle
approaches its maximum value, there is very little time
for the boost cap to be recharged due to the short
amount time that the low-side switch is on. Therefore,
when the maximum duty cycle approaches, the switch
node is forced off for 240 ns every 8 cycles to ensure
that the boost cap gets replenished.
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). The N-Channel MOSFET gate must be
driven above its source to fully turn on the device,
resulting in a gate-drive voltage above the input to turn
on the high-side N-Channel. The high-side N-Channel
source is connected to the inductor and boost cap or
switch node. When the high-side switch is off and the
low-side is on, the inductor current flows through the
low-side switch, providing a path to recharge the boost
cap from the boost voltage source. An internal 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 “pre-charge” the boost cap. Once pre-
charged, the switch is turned on and the inductor
current flows. When the high-side switch turns off and
the low-side turns on, current freewheels through the
inductor and low-side switch, providing a path to
recharge the boost cap. When the duty cycle
approaches its maximum value, there is very little time
for the boost cap to be recharged due to the short
amount time that the low-side switch is on. Therefore,
when the maximum duty cycle approaches, the switch
node is forced off for 240 ns every 8 cycles to ensure
that the boost cap gets replenished.