MSS7341 datasheet(13/38 Pages) MICROCHIP

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DS25004A-page 13

MCP16301

4.2.2

PEAK CURRENT MODE CONTROL

The MCP16301 integrates a Peak Current Mode

Control architecture, resulting in superior AC regulation

while

minimizing

the

number

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 with 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

summed into the current shown in Figure 4-1.

4.2.3

PULSE-WIDTH MODULATION

(PWM)

The internal oscillator periodically starts the switching

period, which in MCP16301’s case occurs every 2 µs

or 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 inte-

grated 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 reg-

ulated output voltage is lower than its target, the invert-

ing 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 internal switch and

preventing it from turning on until the beginning of the

next cycle. An overtemperature signal, or boost cap

undervoltage, can also reset the PWM latch to asyn-

chronously terminate the cycle.

4.2.4

HIGH SIDE DRIVE

The MCP16301 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 transistor. 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 cur-

rent flows through the Schottky diode, providing a path

to recharge the boost cap from the boost voltage

source, typically the output voltage for 3.0V to 5.0V out-

put applications. A boost-blocking diode is used to pre-

vent 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 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

pre-charge current to replace the boost cap charge. For

input voltages above 5.5V typical, the MCP16301

device will regulate the output voltage with no load.

After starting, the MCP16301 will regulate the output

voltage until the input voltage decreases below 4V. See

Figure 2-16 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 3.0V < 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.

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