MCP73833-AMISLASHMF datasheet(20/32 Pages) MICROCHIP

20 / 32 page

MCP73833/4

DS22005B-page 20

6.1.1.2

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:

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

is:

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

DFN-10 (3 mm x 3 mm) package could be utilized to

reduce charge cycle times.

6.1.1.3

External Capacitors

The MCP73833/4 is 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.

6.1.1.4

Reverse-Blocking Protection

The MCP73833/4 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 (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

100 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:

For example, by utilizing a 10 k

Ω at 25C NTC

thermistor with a sensitivity index,

β, of 3892, the

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

placing a 1.54 k

Ω resistor in series (RT1), and a

69.8 k

Ω resistor in parallel (RT2) with the thermistor as

depicted in Figure 6-1.

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.

PowerDissipation

VDDMAX VPTHMIN

–

() IREGMAX

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

RT1

RT2 RCOLD

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

RT1

RT2 RHOT

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

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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