MCP73844 datasheet(17/26 Pages) MICROCHIP

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DS21823D-page 17

MCP73841/2/3/4

Worst-case VGS with a 5V, ±10% input voltage source

and a maximum sink voltage of 1.0V is:

At this worst-case (VGS) the RDSON of the MOSFET

must be low enough as to not impede the performance

of the charging system. The maximum allowable

RDSON at the worst-case VGS is:

The Fairchild NDS8434 and International Rectifier

IRF7404 both satisfy these requirements.

6.1.1.3

EXTERNAL CAPACITORS

The MCP7384X are stable with or without a battery

load. In order to maintain good AC stability in the

Constant-Voltage mode, a minimum capacitance of

4.7 µF is recommended to bypass the VBAT pin to VSS.

This capacitance provides compensation when there is

battery

load.

Additionally,

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 necessary 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

forward transconductance (gm) and capacitance of the

external pass transistor. A 4.7 µF tantalum or aluminum

electrolytic capacitor at the output is usually sufficient

to ensure stability for up to a 1A 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 into the

design, it should be chosen to handle the fast charge

current

continuously

the

maximum

ambient

temperature. In addition, the reverse-leakage current

of the diode should be kept as small as possible.

6.1.1.5

ENABLE INTERFACE

In the stand-alone configuration, the enable pin is

generally tied to the input voltage. The MCP7384X

automatically enters a Low-power mode when voltage

on the VDD input falls below the undervoltage lockout

voltage (VSTOP), reducing the battery drain current to

0.4 µA, typically.

6.1.1.6

CHARGE STATUS INTERFACE

A status output provides information on the state of

charge. The current-limited, open-drain output can be

used to illuminate an external LED. Refer to Table 5-1

for a summary of the state of the status output during a

charge cycle.

6.2

PCB Layout Issues

For optimum voltage regulation, place the battery pack

as close as possible to the device’s VBAT and VSS pins.

This 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

conduct more heat to the back plane of the PCB, thus

reducing the maximum junction temperature.

1.0V

4.5V

120mV

–



–

3.38V

–

DSON

DDMIN

FC SMAX

–

BATMAX

–

REGMAX

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

DSON

4.5V

120 115

mV

–

4.221V

–

551 581

mA

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

288m



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