MCP16331 datasheet(15/48 Pages) MICROCHIP

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DS20005308D-page 15

MCP16331

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

exceeds 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

cycle, which for MCP16331, 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

error amplifier output rises. This results in an increase of

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 capacitor undervoltage

can also reset the PWM latch, to asynchronously

terminate the cycle.

When working close to the boundary conduction

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

it into the output voltage. Although the low-frequency

output component is very small, it may be desirable to

completely eliminate this component. To achieve this,

different methods can be applied to reduce or

completely eliminate this component. In addition to a

very good layout, a capacitor connected in parallel with

the top feedback 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 connected in parallel with the top

feedback resistor can have values from 10 pF to 47 pF.

Utilizing such a snubber eliminates the ringing on the

SW node, but decreases the overall efficiency of the

converter.

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 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 switch.

The high-side drive voltage should be between 3.0V

and 5.5V. The N-Channel MOSFET source is

connected to the inductor and Schottky diode, or switch

node.

When the switch is off, the boost capacitor 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 capacitor back

into the output during the internal switch-on time.

Prior to start-up, the boost capacitor has no stored

charge to drive the switch, therefore an internal regulator

is used to “precharge” the boost capacitor. Once

precharged, the switch is turned on and the inductor

current starts to flow. When the switch turns off, the

inductor current freewheels 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

Schottky

Diode

COUT

VOUT

VIN

–

off

IOUT

VOUT

VIN

off

IOUT

VIN

Continuous Inductor Current Mode

Discontinuous Inductor Current Mode

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