MCP73831-2ACISLASHMC datasheet(18/28 Pages) MICROCHIP

18 / 28 page

MCP73831/2

DS20001984G-page 18

 2005-2014 Microchip Technology Inc.

6.1.1.2

Input Overvoltage Protection (IOVP)

Input overvoltage protection must be used when the

input power source is hot-pluggable. This includes USB

cables and Wall-type power supplies. The cabling of

these supplies acts as an inductor. When the supplies

are connected/disconnected from the system, large

voltage transients are created which may damage the

system circuitry. These transients should be snubbed

out. A transzorb connected from the V+ input supply

connector to the 0V ground reference will snub the

transients.

6.1.1.3

Thermal Considerations

The worst-case power dissipation in the battery

charger occurs when the input voltage is at the

maximum and the device has transitioned from the

Preconditioning mode to the Constant-Current mode.

In this case, the power dissipation is:

Power dissipation with a 5V, ±10% input voltage source

is:

This power dissipation with the battery charger in the

SOT-23-5 package will cause thermal regulation to be

entered as depicted in Figure 6-3. Alternatively, the

2mm x 3mm DFN package could be utilized to reduce

charge cycle times.

6.1.1.4

External Capacitors

The MCP73831/2 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

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 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

Effective Series Resistance (ESR) value. The actual

value of the capacitor (and its associated ESR)

depends on the output load current. A 4.7 µF ceramic,

tantalum or aluminum electrolytic capacitor at the

output is usually sufficient to ensure stability for output

currents up to a 500 mA.

6.1.1.5

Reverse-Blocking Protection

The MCP73831/2 provide protection from a faulted or

shorted input. Without the protection, a faulted or

shorted input would discharge the battery pack through

the body diode of the internal pass transistor.

6.1.1.6

Charge Inhibit

The current regulation set input pin (PROG) can be

used to terminate a charge at any time during the

charge cycle, as well as to initiate a charge cycle or

initiate a recharge cycle.

Placing a programming resistor from the PROG input to

VSS enables the device. Allowing the PROG input to

float or by applying a logic-high input signal, disables

the device and terminates a charge cycle. When

disabled, the device’s supply current is reduced to

25 µA, typically.

6.1.1.7

Charge Status Interface

A status output provides information on the state of

charge. The output can be used to illuminate external

LEDs or interface to a host microcontroller. 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 heat sink, adding many

vias in the heat sink pad can help conduct more heat to

the PCB backplane, thus reducing the maximum

junction temperature. Figure 6-4 and Figure 6-5 depict

a typical layout with PCB heatsinking.

FIGURE 6-4:

Typical Layout (Top).

FIGURE 6-5:

Typical Layout (Bottom).

Powe rDissipation

DDMAX

PTHMIN

–

 I

REGMAX



Where:

VDDMAX = the maximum input voltage

IREGMAX = the maximum fast charge current

VPTHMIN = the minimum transition threshold

voltage

Po werDissipation

5.5 V

2.7V

–

 550mA



1.54W

COUT

LED

RPROG

CIN

MCP73831

RLED

VBAT

VDD

VSS

VBAT

VSS

VDD

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