AAT3697 datasheet(13/22 Pages) ANALOGICTECH

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AAT3697

2A Lithium-Ion/Polymer Battery Charger

3697.2007.02.1.1

Protection Circuitry

Programmable Watchdog Timer

The AAT3697 contains a watchdog timing circuit to

shut down charging functions in the event of a defec-

tive battery cell not accepting a charge over a preset

period of time. Typically, a 0.1μF ceramic capacitor

is connected between the CT pin and ground. When

a 0.1μF ceramic capacitor is used, the device will

time out a shutdown condition if the trickle charge

mode exceeds 25 minutes and a combined trickle

charge plus fast charge mode of 3 hours. When the

device transitions to the constant voltage mode, the

timing counter is reset and will time out after an addi-

tional 3 hours if the charge current does not drop to

the charge termination level.

Table 2: Summary for a 0.1μF Ceramic

Capacitor Used for the Timing Capacitor.

The CT pin is driven by a constant current source

and will provide a linear response to increases in the

timing capacitor value. Thus, if the timing capacitor

were to be doubled from the nominal 0.1μF value,

the time-out periods would be doubled.

If the programmable watchdog timer function is not

needed, it can be disabled by terminating the CT

pin to ground. The CT pin should not be left float-

ing or un-terminated, as this will cause errors in the

internal timing control circuit.

The constant current provided to charge the timing

capacitor is very small, and this pin is susceptible

to noise and changes in capacitance value.

Therefore, the timing capacitor should be physical-

ly located on the printed circuit board layout as

close as possible to the CT pin. Since the accura-

cy of the internal timer is dominated by the capaci-

tance value, a 10% tolerance or better ceramic

capacitor is recommended.

Ceramic capacitor

materials, such as X7R and X5R types, are a good

choice for this application.

Over-Voltage Protection

An over-voltage event is defined as a condition

where the voltage on the BAT pin exceeds the

maximum battery charge voltage and is set by the

over-voltage protection threshold (VOVP). If an

over-voltage condition occurs, the AAT3697 charge

control will shut down the device until the voltage

on the BAT pin drops below VOVP. The AAT3697

will resume normal charging operation after the

over-voltage condition is removed. During an over-

voltage event, the STAT LEDs will report a system

fault, and the actual fault condition can be read via

the DATA pin signal.

Over-Temperature Shutdown

The AAT3697 has a thermal protection control cir-

cuit which will shut down charging functions should

the internal die temperature exceed the preset

thermal limit threshold.

Battery Temperature Fault Monitoring

In the event of a battery over-temperature condi-

tion, the charge control will turn off the internal pass

device and report a battery temperature fault on the

DATA pin function. The STAT LEDs will also dis-

play a system fault. After the system recovers from

a temperature fault, the device will resume charg-

ing operation.

The AAT3697 checks battery temperature before

starting the charge cycle, as well as during all

stages of charging. This is accomplished by mon-

itoring the voltage at the TS pin. Either a negative

temperature coefficient thermistor (NTC) or posi-

tive temperature coefficient thermistor (PTC) can

be used because the AAT3697 checks to see that

the voltage at TS is within a voltage window bound-

ed by VTS1 and VTS2. Please see the equations

below for specifying resistors:

RT1 and RT2 for use with NTC Thermistor

Eq. 2:

⋅ RTH ⋅ RTC

⋅ (RTC - RTH)

⋅ RTH ⋅ RTC

⋅ RTC) - (7 ⋅ RTH)

T1 =

T2 =

Mode

Time

Trickle Charge (TC) Time Out

25 minutes

Trickle Charge (TC) +

3 hours

Fast Charge (CC) Time Out

Constant Voltage (VC) Mode

3 hours

Time Out

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