MCP1640_13 datasheet(11/32 Pages) MICROCHIP

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 2011 Microchip Technology Inc.

DS22234B-page 11

MCP1640/B/C/D

4.0

DETAILED DESCRIPTION

4.1

Device Option Overview

The MCP1640/B/C/D 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 startup voltage, adjustable out-

put voltage, PWM/PFM mode operation, low IQ, inte-

grated synchronous switch, internal compensation, low

noise anti-ring control, inrush current limit, and soft

start.

There are two feature options for the MCP1640/B/C/D

family:

• PWM/PFM mode or PWM-Only mode

• “True Output Disconnect” mode or Input Bypass

mode

4.1.1

PWM/PFM MODE OPTION

The MCP1640C 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 out-

put 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-ONLY MODE OPTION

The

MCP1640B/D

devices

disable

PFM

mode

switching, and operate only in PWM mode over the

entire load range. During periods of light load opera-

tion, the MCP1640B/D continues to operate at a con-

stant 500 kHz switching frequency, keeping the output

ripple voltage lower than PFM mode.

During PWM-Only mode, the MCP1640B/D 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 typ-

ical minimum duty cycle of 100 ns, the MCP1640B/D

continues to switch at a constant frequency under light

load conditions. Figure 2-12 represents the input volt-

age versus load current for the pulse skipping threshold

in PWM-Only mode. At lighter loads, the MCP1640B/D

devices begin to skip pulses.

4.1.3

TRUE OUTPUT DISCONNECT

MODE OPTION

The MCP1640B devices incorporate a true output

disconnect feature. With the EN pin pulled low, the

output of the MCP1640B is isolated or disconnected

from the input by turning off the integrated P-Channel

switch and removing the switch bulk diode connection.

This removes the DC path that is typical in boost con-

verters, which allows the output to be disconnected

from the input. During this mode, less than 1 µA of cur-

rent is consumed from the input (battery). True output

disconnect does not discharge the output, the output

voltage is held up by the external COUT capacitance.

4.1.4

INPUT BYPASS MODE OPTION

The MCP1640C/D devices incorporate the Input

Bypass shutdown option. With the EN input pulled low,

the output is connected to the input using the internal

P-Channel MOSFET. In this mode, the current draw

from the input (battery) is less than 1 µA with no load.

The Input Bypass mode is used when the input voltage

range is high enough for the load to operate in Sleep or

low IQ mode. When a higher regulated output voltage is

necessary to operate the application, the EN input is

pulled high, enabling the boost converter.

TABLE 4-1:

PART NUMBER SELECTION

Part

Number

PWM/

PFM

PWM-

Only

True

Disconnect

Input

Bypass

MCP1640

MCP1640B

MCP1640C

MCP1640D

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