AAT3682(2005) 데이터 시트보기 (PDF) - Advanced Analogic Technologies

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AAT3682
(Rev.:2005)

Lithium-Ion/Polymer Linear Battery Charger

Advanced Analogic Technologies 

AAT3682

Lithium-Ion/Polymer Linear Battery Charger

Diode Selection

Typically, a Schottky diode is used in reverse cur-

rent blocking applications with the AAT3682. Other

lower cost rectifier type diodes may also be used to

save cost if sufficient input power supply head

room is available.

The blocking diode selection should based on merits

of the device forward voltage (VF), current rating,

input supply level versus the maximum battery

charge voltage, and cost.

First, one must determine the appropriate minimum

diode forward voltage drop. 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 voltage required

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 determine

the minimum current rating and power handling

capacity for the blocking diode. The constant current

charge level itself will dictate what the minimum cur-

rent rating must be for a given blocking diode. The

minimum power handling capacity must be calculat-

ed based on the constant current amplitude and the

diode forward voltage (VF):

PD(MIN) =

VF

ICC

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, typi-

cally 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 dissipated

in this element of the circuit. A Schottky diode allows

for lower power dissipation, smaller component

package sizes, and greater circuit layout densities.

Rectifier Diodes

Any general purpose rectifier diode can be used with

the AAT3682 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 supply voltage

requirement for the battery charger system. This will

also require a higher power rated diode since the

voltage drop at the constant current charge ampli-

tude will be greater. Refer to the previously stated

equations to calculate the minimum VIN and diode

PD for a given application.

16

3682.2005.02.1.1

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