15
4012fa
LTC4012/
LTC4012-1/LTC4012-2
The ITH output of the error amplifier is a scaled control
voltage for one input of the PWM comparator CC. ITH
sets a peak inductor current threshold, sensed by R1, to
maintain the desired average current through R
SENSE
. The
current comparator output does this by switching the state
of the RS latch at the appropriate time.
At the beginning of each oscillator cycle, the PWM
clock sets the RS latch and turns on the external top-
side NFET (bottom-side synchronous NFET off) to
refresh the current carried by the external inductor L1.
The inductor current and voltage across R
SENSE
begin
to rise linearly. CA buffers this instantaneous voltage
rise and applies it to CC with gain supplied by R1.
When the voltage across R1 exceeds the peak level set by
the ITH output of EA, the top FET turns off and the bottom
FET turns on. The inductor current then ramps down lin-
early until the next rising PWM clock edge. This closes the
loop and sources the correct inductor current to maintain
the desired parameter (charge current, battery voltage,
or input current). To produce a near constant frequency,
the PWM oscillator implements the equation:
t
CLP BAT
CLP kHz
OFF
=
–
• 550
Repetitive, closed-loop waveforms for stable PWM opera-
tion appear in Figure 2.
PWM Watchdog Timer
As input and output conditions vary, the LTC4012 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 LTC4012 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 LTC4012 always attempts to operate syn-
chronously in full continuous mode (to avoid audible
noise from ceramic capacitors), reverse average charge
current can occur during some invalid operating condi-
tions. INFET PowerPath control avoids boosting a lightly
loaded system supply during reverse operation. However,
under heavier system loads, CLP may not boost above
DCIN, even though reverse average current is flowing. In
this case a second circuit monitors indication of reverse
average current on PROG.
If either of these circuits detects boost operation, The
LTC4012 turns off both external MOSFETs until the reverse
current condition clears. At that point, a new soft-start
sequence begins.
operation
