MCP1623 datasheet(10/26 Pages) MICROCHIP

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MCP1623/24

DS41420A-page 10

 2010 Microchip Technology Inc.

4.0

DETAILED DESCRIPTION

4.1

Device Option Overview

The MCP1623/24 family of devices is capable of low

start-up voltage and delivers high efficiency over a wide

load range for single cell, two cell, three cell alkaline,

NiMH, NiCd and single cell Li-Ion battery inputs. A high

level of integration lowers total system cost, eases

implementation and reduces board area. The devices

feature low start-up voltage, adjustable output voltage,

PWM/PFM mode operation, low IQ, integrated synchro-

nous switch, internal compensation, low noise anti-ring

control, inrush current limit and soft start. There is one

feature option for the MCP1623/24 family: PWM/PFM

mode or PWM mode only.

4.1.1

PWM/PFM MODE OPTION

The MCP1624 devices use an automatic switchover

from PWM to PFM mode for light load conditions to

maximize efficiency over a wide range of output current.

During PFM mode, higher peak current is used to pump

the output up to the threshold limit. While operating in

PFM or PWM mode, the P-Channel switch is used as a

synchronous rectifier, turning off when the inductor

current reaches 0 mA to maximize efficiency. In PFM

mode, a comparator is used to terminate switching when

the output voltage reaches the upper threshold limit.

Once switching has terminated, the output voltage will

decay or coast down. During this period, very low IQ is

consumed from the device and input source, which

keeps power efficiency high at light load. The

disadvantages of PWM/PFM mode are higher output

ripple voltage and variable PFM mode frequency. The

PFM mode frequency is a function of input voltage,

output voltage and load. While in PFM mode, the boost

converter pumps the output up at a switching frequency

of 500 kHz.

4.1.2

PWM MODE ONLY OPTION

The MCP1623 devices disable PFM mode switching,

and operate only in PWM mode over the entire load

range. During periods of light load operation, the

MCP1623 continues to operate at a constant 500 kHz

switching frequency, keeping the output ripple voltage

lower than PFM mode. During PWM-only mode, the

MCP1623 P-Channel switch acts as a synchronous

rectifier by turning off to prevent reverse current flow

from the output cap back to the input in order to keep

efficiency high. For noise immunity, the N-Channel

MOSFET current sense is blanked for approximately

100 ns. With a typical minimum duty cycle of 100 ns,

the MCP1623 continues to switch at a constant

frequency under light load conditions. Figure 2-12

represents the input voltage versus load current for the

pulse-skipping threshold in PWM-only mode. At lighter

loads, the MCP1623 device begins to skip pulses.

TABLE 4-1:

PART NUMBER SELECTION

Part Number

PWM/PFM

PWM

MCP1624

MCP1623

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