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LTC4009

LTC4009-1/LTC4009-2

4009fb

PWM Watchdog Timer

As input and output conditions vary, the LTC4009 may need

to utilize PWM duty cycles approaching 100%. In this case,

operating frequency may be reduced well below 550kHz.

An internal watchdog timer observes the activity on the

TGATE pin. If TGATE is on for more than 40μs, the watchdog

activates and forces the bottom NFET on (top NFET off)

for about 100ns. This avoids a potential source of audible

noise when using ceramic input or output capacitors and

prevents the boost supply capacitor for the top gate driver

from discharging. In low drop out operation, the actual

charge current may not be able to reach the programmed

full-scale value due to the watchdog function.

Overvoltage Protection

The LTC4009 also contains overvoltage detection that

prevents transient battery voltage overshoots of more than

about 6% above the programmed output voltage. When

battery overvoltage is detected, both external MOSFETs are

turned off until the overvoltage condition clears, at which

time a new soft start sequence begins. This is useful for

properly charging battery packs that use an internal switch

to disconnect themselves for performing functions such

as calibration or pulse mode charging.

Reverse Charge Current Protection (Anti-Boost)

Because the LTC4009 always attempts to operate synchro-

nously in full continuous mode (to avoid audible noise from

ceramic capacitors), reverse average charge current can

occur during some invalid operating conditions. To avoid

boosting a lightly loaded system supply during reverse

operation, the LTC4009 monitors the voltage on CLP to

determine if it rises 25mV above DCIN during charge.

However, under heavier system loads, CLP may not boost

above DCIN, even though reverse average current is ﬂow-

ing. In this case a second circuit monitors indication of

reverse average current on PROG.

If the designer intends to replace the input diode with a

MOSFET for improved efﬁciency, using the ACP signal of

the LTC4009 to control the MOSFET is not recommended.

In this case, the LTC4012 is strongly suggested, because

it includes ideal diode control of the MOSFET, instead of

driving it as a simple switch. This solution is the most ef-

fective at detecting boost conditions and quickly shutting

down the IC. If for some reason the LTC4012 solution is

not acceptable, and a MOSFET with external control is

used to replace the input diode, and there are conditions

involving very low reverse current under no system load

with an AC adapter that cannot sink current, it may still

be possible to boost the DCIN input supply. To cover this

case, the LTC4009 monitors the resistor divider attached

to the DCDIV pin and sets an input overvoltage fault if that

voltage exceeds 1.825V.

If any of these circuits detects boost operation, The LTC4009

turns off both external MOSFETs until the reverse current

condition clears. Once DCIN-CLP > 25mV, a new soft-start

sequence begins.

OPERATION

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