MCP73837AMIMF datasheet(21/30 Pages) MICROCHIP

21 / 30 page

DS22071A-page 21

MCP73837/8

EQUATION 6-1:

For example, power dissipation with a 5V, ±10% input

voltage source and 500 mA, ±10% fast charge current

is:

EXAMPLE 6-1:

This power dissipation with the battery charger in the

MSOP-10 package will cause thermal regulation to be

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

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

the charge cycle times.

6.1.1.3

External Capacitors

The MCP73837/8 is stable with or without a battery

load. In order to maintain good AC stability in the

Constant Voltage mode, a minimum capacitance of

1 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 1 µF ceramic,

tantalum, or aluminum electrolytic capacitor at the

output is usually sufficient to ensure stability for output

currents up to 500 mA.

6.1.1.4

Reverse-Blocking Protection

The MCP73837/8 provides 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.

6.1.1.5

Charge Inhibit

The current regulation set input pin (PROG1/2) 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 PROG1 input

to VSS or driving PROG2 to logic High or Low enables

the device. Allowing either the PROG1 or PROG2 input

float disables the device and terminates a charge cycle.

When disabled, the device’s supply current is reduced

to 75 µA, typically.

6.1.1.6

Temperature Monitoring

The charge temperature window can be set by placing

fixed value resistors in series-parallel with a thermistor.

The resistance values of RT1 and RT2 can be calculated

with the following equations in order to set the

temperature window of interest.

For NTC thermistors:

EQUATION 6-2:

For example, by utilizing a 10 k

Ω at 25°C NTC

thermistor with a sensitivity index,

β, of 3892, the

charge temperature range can be set to 0°C - 50°C by

placing a 1.54 k

Ω resistor in series (RT1), and a

69.8 k

Ω resistor in parallel (RT2) with the thermistor.

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

Figure 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,

recommended to minimize voltage drops along the

high current-carrying PCB traces.

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

vias in the heatsink pad can help conduct more heat to

the backplane of the PCB, thus reducing the maximum

junction temperature.

PowerDissipation

DDMAX

PTHMIN

–

() I

REGMAX

Where:

VDDMAX = the maximum input voltage

IREGMAX = the maximum fast charge current

VPTHMIN = the minimum transition threshold

voltage

PowerDissipation

5.5V

2.7V

–

() 550mA

1.54W

24k

COLD

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

HOT

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

Where:

RT1

the fixed series resistance

RT2

the fixed parallel resistance

RCOLD

the thermistor resistance at the

lower temperature of interest

RHOT

the thermistor resistance at the

upper temperature of interest

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