© 2007 Microchip Technology Inc.

DS21704B-page 13

MCP73827

source, 100 m

Ω, 1% sense resistor, and a maximum

sink voltage of 1.6V is:

must be low enough as to not impede the performance

of the charging system. The maximum allowable

The Fairchild NDS8434 and International Rectifier

IRF7404 both satisfy these requirements.

6.1.1.3

EXTERNAL CAPACITORS

The MCP73827 is stable with or without a battery load.

In order to maintain good AC stability in the constant

voltage mode, a minimum capacitance of 10 µF is rec-

itance provides compensation when there is no battery

load. In addition, the battery and interconnections

appear inductive at high frequencies. These elements

are in the control feedback loop during constant voltage

mode. Therefore, the bypass capacitance may be nec-

essary to compensate for the inductive nature of the

battery pack.

Virtually any good quality output filter capacitor can be

used, independent of the capacitor’s minimum ESR

(Effective Series Resistance) value. The actual value of

the capacitor and its associated ESR depends on the

external pass transistor. A 10 µF tantalum or aluminum

electrolytic capacitor at the output is usually sufficient

to ensure stability for up to a 1 A output current.

6.1.1.4

REVERSE BLOCKING PROTECTION

The

optional

reverse

blocking

protection

diode

depicted in Figure 6-1 provides protection from a

faulted or shorted input or from a reversed polarity input

source. Without the protection diode, a faulted or

shorted input would discharge the battery pack through

the body diode of the external pass transistor.

If a reverse protection diode is incorporated in the

design, it should be chosen to handle the peak fast

charge current continuously at the maximum ambient

temperature. In addition, the reverse leakage current of

the diode should be kept as small as possible.

6.1.1.5

SHUTDOWN INTERFACE

In the stand-alone configuration, the shutdown pin is

generally tied to the input voltage. The MCP73827 will

automatically enter a low power mode when the input

voltage is less than the output voltage reducing the bat-

tery drain current to 8 µA, typically.

By connecting the shutdown pin as depicted in

Figure 6-1, the battery drain current may be further

reduced. In this application, the battery drain current

becomes a function of the reverse leakage current of

the reverse protection diode.

6.1.1.6

CHARGE STATUS INTERFACE

The charge status indicator, MODE, can be utilized to

illuminate an LED when the MCP73827 is in the con-

trolled current phase. When the MCP73827 transitions

to constant voltage mode, the MODE pin will transition

to a high impedance state. A current limit resistor

should be used in series with the LED to establish a

nominal LED bias current of 10 mA. The maximum

allowable sink current of the MODE pin is 30 mA.

6.2

PCB Layout Issues

For optimum voltage regulation, place the battery pack

pins. It is recommended to minimize voltage drops

along the high current carrying PCB traces.

If the PCB layout is used as a heatsink, adding many

vias around the external pass transistor can help con-

duct more heat to the back-plane of the PCB, thus

reducing the maximum junction temperature.

V

GS

1.6V

4.5V

758mA

99m

Ω

×

–

()

–

2.8

–

V

==

R

DSON

V

INMIN

I

PEAK

R

SEN SE

×

–

V

BATMAX

–

I

OUT

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

=

R

DSON

4.5V

758mA

99m

Ω

×

–

4.242V

–

758mA

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

242m

Ω

==