MCP16331 datasheet(15/40 Pages) MICROCHIP

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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 VOUT < 3.0V or VOUT > 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.

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