MCP1603-ADJISLASHOS datasheet(12/26 Pages) MICROCHIP

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MCP1603

DS22042A-page 12

4.0

DETAILED DESCRIPTION

4.1

Device Overview

The MCP1603 is a synchronous buck regulator that

operates in a Pulse Frequency Modulation (PFM)

mode or a Pulse Width Modulation (PWM) mode to

maximize system efficiency over the entire operating

current range. Capable of operating from a 2.7V to

5.5V input voltage source, the MCP1603 can deliver

500 mA of continuous output current.

When using the MCP1603, the PCB area required for

a complete step-down converter is minimized since

both the main P-Channel MOSFET and the synchro-

nous N-Channel MOSFET are integrated. Also while in

PWM mode, the device switches at a constant

frequency of 2.0 MHz (typ) which allow for small filter-

ing components. Both fixed and adjustable output

voltage options are available. The fixed voltage options

(1.2V, 1.5V 1.8V, 2.5V, 3.3V) do not require an external

voltage divider which further reduces the required

circuit board footprint. The adjustable output voltage

options allow for more flexibility in the design, but

require an external voltage divider.

Additionally the device features undervoltage lockout

(UVLO),

overtemperature

shutdown,

overcurrent

protection, and enable/disable control.

4.2

Synchronous Buck Regulator

The MCP1603 has two distinct modes of operation that

allow the device to maintain a high level of efficiency

throughout the entire operating current and voltage

range. The device automatically switched between

PWM mode and PFM mode depending upon the output

load requirements.

4.2.1

FIXED FREQUENCY, PWM MODE

During heavy load conditions, the MCP1603 operates

at a high, fixed switching frequency of 2.0 MHz (typical)

using current mode control. This minimizes output rip-

ple (10 - 15 mV typically) and noise while maintaining

high efficiency (88% typical with VIN = 3.6V,

VOUT = 1.8V, IOUT = 300 mA).

During normal PWM operation, the beginning of a

switching cycle occurs when the internal P-Channel

MOSFET is turned on. The ramping inductor current is

sensed and tied to one input of the internal high-speed

comparator. The other input to the high-speed compar-

ator is the error amplifier output. This is the difference

between the internal 0.8V reference and the divided-

down output voltage. When the sensed current

becomes equal to the amplified error signal, the high-

speed comparator switches states and the P-Channel

MOSFET is turned off. The N-Channel MOSFET is

turned on until the internal oscillator sets an internal RS

latch initiating the beginning of another switching cycle.

PFM-to-PWM mode transition is initiated for any of the

following conditions:

• Continuous device switching

• Output voltage has dropped out of regulation

4.2.2

LIGHT LOAD, PFM MODE

During light load conditions, the MCP1603 operates in

a PFM mode. When the MCP1603 enters this mode, it

begins to skip pulses to minimize unnecessary quies-

cent current draw by reducing the number of switching

cycles per second. The typical quiescent current draw

for this device is 45 µA.

PWM-to-PFM mode transition is initiated for any of the

following conditions:

• Discontinuous inductor current is sensed for a set

duration

• Inductor peak current falls below the transition

threshold limit

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