MCP1640T-IMC datasheet(15/32 Pages) MICROCHIP

Part #	MCP1640T-IMC

Description	0.65V Start-up Synchronous Boost Regulator with True Output Disconnect or Input/Output Bypass Option

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 2010 Microchip Technology Inc.

DS22234A-page 15

MCP1640/B/C/D

5.0

APPLICATION INFORMATION

5.1

Typical Applications

The MCP1640/B/C/D synchronous boost regulator

operates over a wide input voltage and output voltage

range. The power efficiency is high for several decades

of load range. Output current capability increases with

input voltage and decreases with increasing output

voltage. The maximum output current is based on the

N-Channel peak current limit. Typical characterization

curves in this data sheet are presented to display the

typical output current capability.

5.2

Adjustable Output Voltage

Calculations

To calculate the resistor divider values for the

MCP1640/B/C/D, the following equation can be used.

Where RTOP is connected to VOUT, RBOT is connected

to GND and both are connected to the FB input pin.

EQUATION 5-1:

Example A:

VOUT = 3.3V

VFB = 1.21V

RBOT = 309 k

RTOP = 533.7 k (Standard Value = 536 k)

Example B:

VOUT = 5.0V

VFB = 1.21V

RBOT = 309 k

RTOP = 967.9 k (Standard Value = 976 k)

There are some potential issues with higher value

resistors.

For

small

surface

mount

resistors,

environment contamination can create leakage paths

that significantly change the resistor divider that effect

the output voltage. The FB input leakage current can

also impact the divider and change the output voltage

tolerance.

5.3

Input Capacitor Selection

The boost input current is smoothed by the boost

inductor reducing the amount of filtering necessary at

the input. Some capacitance is recommended to

provide decoupling from the source. Low ESR X5R or

X7R are well suited since they have a low temperature

coefficient and small size. For most applications,

4.7 µF of capacitance is sufficient at the input. For high

power applications that have high source impedance or

long leads, connecting the battery to the input 10 µF of

capacitance

is

recommended.

Additional

input

capacitance can be added to provide a stable input

voltage.

Table 5-1 contains the recommended range for the

input capacitor value.

5.4

Output Capacitor Selection

The output capacitor helps provide a stable output

voltage during sudden load transients and reduces the

output voltage ripple. As with the input capacitor, X5R

and X7R ceramic capacitors are well suited for this

application.

The MCP1640/B/C/D is internally compensated so

output capacitance range is limited. See Table 5-1 for

the recommended output capacitor range.

While the N-Channel switch is on, the output current is

supplied by the output capacitor COUT. The amount of

output capacitance and equivalent series resistance

will have a significant effect on the output ripple

voltage. While COUT provides load current, a voltage

drop also appears across its internal ESR that results

in ripple voltage.

EQUATION 5-2:

Where dV represents the ripple voltage and dt

represents the ON time of the N-Channel switch (D * 1/

FSW).

Table 5-1 contains the recommended range for the

input and output capacitor value.

R

TOP

R

BOT

V

OUT

V

FB

-------------

1

–







=

TABLE 5-1:

CAPACITOR VALUE RANGE

CIN

COUT

Min

4.7 µF

10 µF

Max

none

100 µF

I

OUT

C

OUT

dV

dt

-------







=

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