MCP73837-AMISLASHMF datasheet(22/36 Pages) MICROCHIP

22 / 36 page

MCP73837/8

DS20002071C-page 22

 2007-2015 Microchip Technology Inc.

6.1.1.3

Thermal Considerations

The worst-case power dissipation in the battery char-

ger 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:

EQUATION 6-1:

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

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

current is calculated in the following example:

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.

6.1.1.4

External Capacitors

The MCP73837/8 is stable with or without a battery

load. In order to maintain good AC stability in the Con-

stant 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 interconnec-

tions appear inductive at high frequencies. These ele-

ments 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 Effec-

tive 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 alumi-

num electrolytic capacitor at the output is usually suffi-

cient to ensure stability for output currents up to

500 mA.

6.1.1.5

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.

6.1.1.6

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 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 PROG1/2 input to float

disables the device and terminates a charge cycle.

When disabled, the device’s supply current is reduced

to 75 µA, typically.

6.1.1.7

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, see Equation 6-2.

EQUATION 6-2:

For example, by utilizing a 10 k

 at +25°C NTC

thermistor with a sensitivity index,

, of 3892, the

charge temperature range can be set to 0°C – +50°C

by placing a 1.54 k

 resistor in series (RT1), and a

69.8 k

 resistor in parallel (RT2) with the thermistor.

6.1.1.8

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 or Figure 4-1 for information on the state of

the status output during a charge cycle.

PowerDissipation

DDMAX

PTHMIN

–

 I

REGMAX



Where:

VDDMAX = the maximum input voltage

IREGMAX = the maximum fast charge current

VPTHMIN = the minimum transition threshold

voltage

Po werDissipa tion

5.5V

2.7V

–

 550mA



1.54 W

24k



COLD



COLD

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



HOT



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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