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MCP1601

500 mA Synchronous BUCK Regulator

Microchip Technology 

MCP1601

4.0 DEVICE OPERATION

The MCP1601 is a synchronous DC/DC converter with

integrated switches. Developed to provide high effi-

ciency across a wide line and load range, the

MCP1601 integrates the three modes of operation

described below. In addition to three operating modes,

the MCP1601 also integrates many features that mini-

mize external circuitry, saving board space and cost.

With two external resistors used to set the output volt-

age, the MCP1601 output is adjustable from 0.9V to

VIN.

4.1 Operating Modes

The MCP1601 has three distinct modes of operation,

with each one optimized for a specific operating condi-

tion commonly encountered in handheld portable

power applications.

4.1.1

FEEDFORWARD VOLTAGE PULSE

WIDTH MODULATION (PWM) MODE

The Pulse Width Modulation (PWM) mode of operation

is desired when operating from typical to maximum out-

put currents with the proper head room voltage at the

input. This mode of operation optimizes efficiency and

noise by switching at a fixed frequency. Typical output

ripple voltage is less than 10 mV when using a 10 µH

inductor and 10 µF ceramic capacitor. The internal

operating frequency of the MCP1601 is 750 kHz, nom-

inal. The duty cycle, or “ON” time, of the high-side, inte-

grated, P-Channel MOSFET is determined by the

continuous mode BUCK transfer function. For the con-

tinuous inductor current case, the duty cycle can be

approximated by VOUT/VIN. The integrated high-side

BUCK P-Channel switch will conduct for the “on” time.

At the end of the “on” time, the high-side P-Channel

switch is turned off and the integrated, low-side, N-

Channel synchronous switch is turned on to freewheel

the inductor current. The PWM mode architecture

employed in the MCP1601 is a feedforward voltage

mode control and feeds the input voltage into the PWM

oscillator ramp. This information is used to quickly

change the operating duty cycle in the event of a sud-

den input voltage change. The effects on the output

voltage are minimized. To force the MCP1601 into

PWM mode, the SYNC/PWM pin should be tied to a

logic high. The forced PWM mode should be used for

applications that require the fastest transient response

from light load to heavy load or applications that require

a single switching frequency independent of load.

An external oscillator between 850 kHz and 1 MHz can

be connected to the SYNC/PWM pin for synchroniza-

tion to an external clock source. The MCP1601 will

always operate in the PWM mode when synchronized

to an external oscillator.

4.1.2

PULSE FREQUENCY MODULATION

(PFM) MODE

The MCP1601 is also capable of operating in a pulse

frequency modulation mode. This mode of operation is

desired for applications that have very long periods of

inactivity and the output current requirement placed on

the MCP1601 is very low. By entering the PFM mode of

operation, the switching frequency becomes mainly a

function of load current and will decrease as the load

current decreases. By switching slower, the energy

used turning “on” and “off” the high-side P-Channel and

low-side N-Channel is reduced, making the PFM mode

more efficient with light output load currents. When

load activity is encountered, the MCP1601 will auto-

matically switch from the PFM mode to the fixed fre-

quency PWM mode by sensing the increase in load

current. The auto PWM/PFM mode is selected by plac-

ing a logic low at the SYNC/PWM input pin. If an exter-

nal clock is used to synchronize the MCP1601

switching frequency, the PFM mode is automatically

disabled.

To enter the PFM mode of operation, the SYNC/PWM

pin must be held to a logic low level and the peak induc-

tor current, sensed internal to the MCP1601, is below

the internal PFM threshold for more than 1024 clock

cycles. If both of these conditions are met, the

MCP1601 will enter the PFM mode. While in the PFM

mode, the MCP1601 will disable the low-side N-Chan-

nel switch to optimize efficiency at low operating cur-

rents. A cycle will begin by turning on the high-side

P-Channel switch and will end when the output voltage

exceeds a predetermined voltage set point. If the peak

inductor current exceeds the internal PFM mode cur-

rent threshold prior to the output voltage exceeding the

voltage set point, the load current has increased and

the MCP1601 will automatically switch to PWM opera-

tion. The typical hysteresis on the PFM comparator is

6 mV. The typical output ripple voltage is below 40 mV

when using a 10 µH inductor and 10 µF ceramic output

capacitor when VIN = 4.2V. For proper PFM mode oper-

ation, the value of the external inductor and the exter-

nal capacitor should be the same. For example, when

using a 10 µH inductor, a 10 µF capacitor should be

used. When using a 22 µH inductor, a 22 µF capacitor

should be used.

4.1.3 LOW DROP OUT (LDO) MODE

When the input voltage to the MCP1601 is decreasing

and approaches the set output voltage level, the duty

cycle increases to a maximum of 90% (typically). To

continue to regulate the output to as high a voltage as

possible, the MCP1601 enters the low drop out mode

of operation. In this mode, the high-side P-Channel

MOSFET acts like a saturated LDO. This mode allows

the operation of the load circuitry down to the minimum

input supply that is typical in battery-powered

applications.

DS21762B-page 10

 2003-2013 Microchip Technology Inc.

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