AAT3682

DATA SHEET

Li-Ion/Polymer Linear Battery Charger

14

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that the voltage at TS is within a voltage window bound-

for specifying resistors:

R

5 ∙ R

3 · (R

R

5 ∙ R

(2 ∙ R

R

5 ∙ R

3 · (R

R

5 ∙ R

(2 ∙ R

tance. See thermistor specifications for additional infor-

mation. To ensure there is no dependence on the input

supply changes, connect divider between VCC and VSS.

Disabling the temperature-monitoring function is

the TS pin.

Capacitor Selection

Input Capacitor

In general, it is good design practice to place a decou-

pling capacitor between the VCC and VSS pins. An input

capacitor in the range of 1μF to 10μF is recommended.

If the source supply is unregulated, it may be necessary

to increase the capacitance to keep the input voltage

above the under-voltage lockout threshold. If the

AAT3682 is to be used in a system with an external

power supply source, such as a typical AC-to-DC wall

be used. A larger input capacitor in this application will

minimize switching or power bounce effects when the

power supply is “hot plugged” in.

Output Capacitor

The AAT3682 does not need an output capacitor for sta-

bility of the device itself. However, a capacitor connected

between BAT and VSS will control the output voltage

when the AAT3682 is powered up when no battery is

connected. The AAT3682 can become unstable if a high

impedance load is placed across the BAT pin to VSS.

Such a case is possible with aging li-ion/poly battery

cells. As cells age through repeated charge and dis-

charge cycles, the internal impedance can rise over time.

A 10μF or larger output capacitor will compensate for the

adverse effects of a high impedance load and assure

device stability over all operating conditions.

Power Dissipation

The voltage drop across the VP and BAT pins multiplied

times the charge current is used to determine the inter-

nal power dissipation. The maximum power dissipation

occurs when the input voltage is at a maximum and the

battery voltage is at the minimum preconditioning volt-

age threshold. This power is then multiplied times the

package theta to determine the maximum junction tem-

perature. The worst case power junction temperature is

calculated as follows:

P

⋅ I

CHG(MAX)

= (5.5V - 0.1V - 0.2V - 3.04V)

⋅ 550mA

= 1.2W

This equation can be used to determine the maximum

input voltage given the maximum junction and ambient

temperature and desired charge current.

T

V

+ V

=

+ 3.1V + 0.2V + 0.1V

= 5.3V

θ ⋅ I

CHG(MAX)

120

°C - 70°C

50

°C/W ⋅ 500mA

Operation Under No-Load

Under no-load conditions (i.e., when the AAT3682 is

powered with no battery connected between the BAT pin

and VSS), the output capacitor is charged up very quick-

ly by the trickle charge control circuit to the BAT pin until

point, the AAT3682 will drop into sleep mode. The output

capacitor will discharge slowly by the capacitor’s own

internal leakage until the voltage seen at the BAT pin