MCP16311SLASH2 datasheet(16/40 Pages) MICROCHIP

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MCP16311/2

DS20005255C-page 16

 2013-2019 Microchip Technology Inc.

4.2.2

PEAK CURRENT MODE CONTROL

The MCP16311/2 integrates a peak current mode

control architecture, resulting in superior AC regulation

while minimizing the number and size of voltage loop

compensation components 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 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 can be approximated by a

first order system rather than a second order system.

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 sense signal.

4.2.3

PULSE-WIDTH MODULATION

The internal oscillator periodically starts the switching

period, which in the MCP16311/2’s case occurs every

2 µs or 500 kHz. With the high-side integrated

N-Channel MOSFET turned on, the inductor current

ramps up until the sum of the current sense and slope

compensation ramp exceeds the integrated error

amplifier output. Once this occurs, the high-side switch

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.

The MCP16312 devices will operate in PWM-only

mode even during periods of light load operation. By

operating in PWM-only mode, the output ripple remains

low and the frequency is constant (Figure 2-28).

Operating in fixed PWM mode results in lower

efficiency during light-load operation (when compared

to PFM mode (MCP16311)).

When working close to the boundary conduction

threshold, a jitter on the SW node may occur, reflecting

in the output voltage. Although the low-frequency

output component is very small, it may be desirable to

completely eliminate this component. To achieve this,

an RC Snubber between the SW node and GND is

used.

Typical values for the snubber are: 680 pF and 430

.

Using such a snubber completely eliminates the jitter

on the SW node, but slightly decreases the overall

efficiency of the converter.

4.2.4

PFM MODE OPERATION

The MCP16311 devices are capable of automatic

operation in normal PWM or PFM mode to maintain

high efficiency at all loads. In PFM mode, the output

ripple has a variable frequency component that

changes with the input voltage and output current. With

no load, the quiescent current drawn from the output is

very low.

There are two comparators that decide when device

starts switching in PFM mode. One of the comparators

is monitoring the output voltage and has a reference of

810 mV with 10 mV hysteresis. If the load current is

low, the output rises and triggers the comparator, which

will put the logic control of the drivers and other block

circuitry (including the internal regulator VCC) in Sleep

mode to minimize the power consumption during the

switching cycle’s off period. When the output voltage

drops below its nominal value, PFM operation pulses

one or several times to bring the output back into

regulation (Figure 2-26). The second comparator fixes

the minimum duty cycle for PFM mode. Minimum duty

cycle in PFM mode depends on the sensed peak

current and input voltage. As a result, the PFM-to-PWM

mode threshold depends on load current and value of

the input voltage (Figure 2-17). If the output load

current rises above the upper threshold, the

MCP16311 transitions smoothly into PWM mode.

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