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DS22285A-page 15

MCP16321/2

4.0

DETAILED DESCRIPTION

4.1

Device Overview

The MCP16321/2 is a high-input voltage, step-down

regulator, capable of supplying 1A/2A to a regulated

output voltage from 0.9V to 5V. Internally, the 1 MHz

oscillator provides a fixed frequency, while the Peak

Current Mode Control architecture varies the duty cycle

for output voltage regulation. An internal floating driver

is used to turn the high-side integrated N-Channel

MOSFET on and off. The power for this driver is

derived from an external boost capacitor whose energy

is replenished when the low-side N-Channel MOSFET

is turned on. When the maximum duty cycle

approaches 100%, the boost capacitor is replenished

for 240 ns after every 8 cycles.

4.1.1

INTERNAL REFERENCE VOLTAGE

VREF

For the adjustable version, an integrated precise 0.9V

reference combined with an external resistor divider

sets the desired converter output voltage. The resistor

divider can vary without affecting the control system

gain. High-value resistors consume less current, but

are more susceptible to noise. For the fixed version, an

integrated precise voltage reference is set to the

desired VOUT value and is directly connected to VOUT.

4.1.2

INTERNAL COMPENSATION

All control system components necessary for stable

operation over the entire device operating range are

integrated, including the error amplifier and inductor

current slope compensation.

4.1.3

EXTERNAL COMPONENTS

External components consist of:

• Input capacitor

• Output filter (inductor and capacitor)

• Boost capacitor

• Resistor divider (adjustable version only)

The selection of the external inductor, output capacitor,

input capacitor and boost capacitor is dependent upon

the output voltage and the maximum output current.

4.1.4

ENABLE INPUT

The enable input (EN) is used to disable the device. If

disabled, the device consumes a minimal current from

the input. Once enabled, the internal soft start controls

the output voltage rate of rise, preventing high-inrush

current and output voltage overshoot. The EN is

internally pulled up or enabled; to disable the converter,

it must be pulled low.

4.1.5

SOFT START

The internal reference voltage rate of rise is controlled

during startup, minimizing the output voltage overshoot

and the inrush current.

4.1.6

OUTPUT OVER VOLTAGE

PROTECTION

If the output of the regulator exceeds 103% of the

regulation voltage, the SW outputs will tri-state to

protect the device from damage. This check occurs at

the start of each switching cycle.

4.1.7

INPUT UNDER VOLTAGE LOCKOUT

An integrated Under Voltage Lockout (UVLO) prevents

the converter from starting until the input voltage is high

enough for normal operation. The converter will

typically start at 5.75V (typical) and operate down to

5.25V (typical). Hysteresis of 500 mV (typical) is added

to prevent starting and stopping during startup, as a

result of loading the input voltage source.

4.1.8

MINIMUM DUTY CYCLE

A minimum duty cycle of 70 ns typical prevents the

device from constant switching for high step-down

voltage ratios. Duty cycles less than this minimum will

initiate pulse skipping to maintain output voltage

regulation, resulting in higher output voltage ripple.

Duty cycle for continuous inductor current operation is

approximated by VOUT/VIN. For a 1 MHz switching

frequency or 1 µs period, this results in a 7% duty cycle

minimum. Maximum VIN for continuous switching can

be approximated dividing VOUT by the minimum duty

cycle or 7%. For example, the maximum input voltage

for continuous switching for a 1.5V output is equal to

approximately 21V.

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