MCP16301HT-ESLASHCH datasheet(13/38 Pages) MICROCHIP

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DS20005004E-page 15

MCP16301/H

4.2.2

PEAK CURRENT MODE CONTROL

The MCP16301/H devices integrate a Peak Current

Mode Control architecture, resulting in superior AC

regulation while minimizing the number of voltage loop

compensation

components

and

their

size,

for

integration. Peak Current Mode Control takes a small

portion of the inductor current, replicates it, and

compares this replicated current sense signal to the

output of the integrated error voltage. In practice, the

inductor current and the internal switch current are

equal during the switch-on time. By adding this peak

current sense to the system control, the step-down

power train system is reduced from a 2nd order to a 1st

order. This reduces the system complexity and

increases its dynamic performance.

For Pulse-Width Modulation (PWM) duty cycles that

exceed 50%, the control system can become bimodal

where a wide pulse followed by a short pulse repeats

instead of the desired fixed pulse width. To prevent this

mode of operation, an internal compensating ramp is

added to the current information, as shown in

Figure 4-1.

4.2.3

PULSE-WIDTH MODULATION

(PWM)

The internal oscillator periodically starts the switching

period, which, for MCP16301, occurs every 2 µs (or

with a frequency of 500 kHz). With the integrated

switch turned on, the inductor current ramps up until

the sum of the current sense and slope compensation

ramp exceeds the integrated error amplifier output. 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 the 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 reset by

turning off the internal switch and prevents it from

turning on until the beginning of the next cycle. An

overtemperature

signal,

boost

capacitor

undervoltage, can also reset the PWM latch to

asynchronously terminate the cycle.

4.2.4

HIGH-SIDE DRIVE

The MCP16301/H devices feature an integrated

high-side N-Channel MOSFET for high-efficiency

step-down power conversion; an N-Channel MOSFET

is preferred 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

transistor. Therefore, 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 inductor current flows

through the Schottky diode, providing a path to

recharge the boost capacitor from the boost voltage

source: typically, the output voltage for 3.0V to 5.0V

output applications. A boost-blocking diode is used to

prevent current flow from the boost capacitor back into

the output during the internal switch-on time. Prior to

start-up, 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 capacitor.

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 pre-

charge current to replenish the boost capacitor charge.

For input voltages above 5.5V typical, the MCP16301/

H devices will regulate the output voltage with no load.

After starting, the MCP16301/H devices will regulate

the output voltage until the input voltage decreases

below 4V. See Figure 2-17 for device range of opera-

tion 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 the output voltage lower than 3V or higher than 5V,

an alternative boost supply can be used.

Alternative boost supplies can be directly used from the

input, input derived, output derived or an auxiliary sys-

tem voltage.

For low voltage output applications with unregulated

input voltage, a shunt regulator derived from the input

can be used to obtain 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.

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