LTC4009
LTC4009-1/LTC4009-2
14
4009fd
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
