AAT3680 Datasheet, PDF(14/18 Page) Advanced Analogic Technologies – Lithium-Ion Linear Battery Charge Controller

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AAT3680 Datasheet, PDF (14/18 Pages) Advanced Analogic Technologies – Lithium-Ion Linear Battery Charge Controller

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with a P-channel MOSFET. If there is no other pro-

tection in the system, a shorted input could dis-

charge the battery through the body diode of the

pass MOSFET. If a reverse-blocking diode is

added to the system, a device should be chosen

which can withstand the maximum constant cur-

rent charge current at the maximum system ambi-

ent temperature.

Diode Selection

Typically, a Schottky diode is used in reverse current

blocking applications with the AAT3680. Other

lower cost rectifier type diodes may also be used if

sufficient input power supply headroom is available.

The blocking diode selection should based on mer-

its of the device forward voltage (VF), current rat-

ing, and input supply level versus the maximum

battery charge voltage and cost.

First, determine the minimum diode forward voltage

drop requirement. Refer to the following equation:

VIN(MIN) = VBAT(MAX) + VF(TRAN) + VF(DIODE)

Where:

VIN(MIN) = Minimum input supply level

VBAT(MAX) = Maximum battery charge

required

voltage

VF(TRAN) = Pass transistor forward voltage drop

VF(DIODE) = Blocking diode forward voltage

Based on the maximum constant current charge

level set for the system, the next step is to deter-

mine the minimum current rating and power han-

dling capacity for the blocking diode. The constant-

current charge level itself will dictate what the mini-

mum current rating must be for a given blocking

diode. The minimum power handling capacity must

be calculated based on the constant current ampli-

tude and the diode forward voltage (VF):

PD(MIN) =

ICC

AAT3680

Lithium-Ion/Polymer

Linear Battery Charge Controller

Where:

PD(MIN) = Minimum power rating for a diode selection

VF = Diode forward voltage

ICC = Constant current charge level for the

system

Schottky Diodes

Schottky diodes are selected for this application

because they have a low forward voltage drop, typ-

ically between 0.3V and 0.4V. A lower VF permits

a lower voltage drop at the constant current charge

level set by the system; less power will be dissi-

pated in this element of the circuit. Schottky

diodes allow for lower power dissipation, smaller

component package sizes, and greater circuit lay-

out densities.

Rectifier Diodes

Any general-purpose rectifier diode can be used

with the AAT3680 application circuit in place of a

higher cost Schottky diode. The design trade-off is

that a rectifier diode has a high forward voltage

drop. VF for a typical silicon rectifier diode is in the

range of 0.7V. A higher VF will place an input sup-

ply voltage requirement for the battery charger sys-

tem. This will also require a higher power rated

diode since the voltage drop at the constant current

charge amplitude will be greater. Refer to the pre-

viously stated equations to calculate the minimum

VIN and diode PD for a given application.

PCB Layout

For the best results, it is recommended to physical-

ly place the battery pack as closely as possible to

the AAT3680's BAT pin. To minimize voltage drops

in the PCB, keep the high current carrying traces

adequately wide. For maximum power dissipation

in the pass transistor, it is critical to provide enough

copper to spread the heat. Refer to the AAT3680

demo board PCB layout in Figures 8, 9, and 10.

3680.2006.03.1.6

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