 2002 Microchip Technology Inc.

DS21706A-page 13

MCP73828

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 MCP73828 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 MCP73828 will

automatically enter a low power mode when the input

voltage is less than the output voltage reducing the bat-

tery drain current to 10 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

CELL TEMPERATURE MONITOR

As discussed in Section 5.1.1, the MCP73828 can

monitor a temperature range for –0.5°C to 44.2°C. This

temperature range can be expanded or shifted by plac-

ing fixed value resistors in series/parallel combinations

with the thermistor (see Figure 6-1). Given that the

nominal output current of the THERM pin is 25 µA, the

resistor values must satisfy the following equations:

Where:

at the upper temperature of interest

at the lower temperature of interest.

For example, by utilizing a 931

Ω resistor in series with

the typical NTC thermistor described previously, the

monitored temperature window will shift to 0°C to

+50°C, typically. Again, with the same thermistor, a

1k

Ω series resistor and a 140 kΩ parallel resistor will

produce a monitored window of -5°C to +50°C, typi-

cally.

1.6V

4.5V 758mA 99m

Ω

×

–

()

–

2.8

–

V

==

×

–

–

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

=

4.5V 758mA 99m

Ω

×

–

4.242V

–

758mA

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

242m

Ω

==

–

×

–

+

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

+

4520

Ω typ

()

=

–

×

–

+

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

+

33560

Ω typ

()

=