MCP16331 datasheet(14/40 Pages) MICROCHIP

14 / 40 page

MCP16331

DS20005308B-page 14

 2014 Microchip Technology Inc.

FIGURE 4-2:

Step-Down Converter.

4.2.2

PEAK CURRENT MODE CONTROL

The MCP16331 integrates 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 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 4-2.

4.2.3

PULSE-WIDTH MODULATION

(PWM)

The internal oscillator periodically starts the switching

period, which in MCP16331’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

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

error amplifier output rises. This results in an increase

in the inductor current to correct for error 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 terminate the cycle.

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 out-

put component is very small, it may be desirable to

completely eliminate this component. To achieve this,

different methods can be applied to reduce or com-

pletely eliminate this component. In addition to a very

good layout, a capacitor in parallel with the top feed-

back resistor or an RC snubber between the SW node

and GND can be added.

Typical values for the snubber are 680 pF and 430

,

while the capacitor in parallel with the top feedback

resistor can use values from 10 pF to 47 pF. Using

such a snubber eliminates the ringing on the SW node,

but decreases the overall efficiency of the converter.

Schottky

Diode

COUT

VOUT

VIN

off

IOUT

VOUT

VIN

off

IOUT

VIN

Continuous Inductor Current Mode

Discontinuous Inductor Current Mode

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