15
40123fb
LTC4012-3
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 topside 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 cur-
rent then ramps down linearly 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 LTC4012-3 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 LTC4012-3 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 LTC4012-3 always attempts to operate
synchronously 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
LTC4012-3 turns off both external MOSFETs until the
reverse current condition clears. At that point, a new
soft-start sequence begins.
operaTion
