LTC4012-1 datasheet(15/28 Pages) LINER

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4012f

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 RSENSE. 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 RSENSE 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 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:

CLP BAT

CLP

kHz

OFF =

–

• 550

Repetitive, closed-loop waveforms for stable PWM

operation 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

synchronously in full continuous mode (to avoid audible

noise from ceramic capacitors), reverse average charge

current can occur during some invalid operating conditions.

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

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