UCC1895

UCC2895

UCC3895

SLUS157L − DECEMBER 1999 − REVISED APRIL 2008

9

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DETAILED PIN DESCRIPTION

Adaptive Delay Set (ADS)

This function sets the ratio between the maximum and minimum programmed output-delay dead time. When

the ADS pin is directly connected to the CS pin, no delay modulation occurs. The maximum delay modulation

occurs when ADS is grounded. In this case, delay time is four times longer when CS = 0 than when CS = 2.0 V

(the peak-current threshold), ADS changes the output voltage on the delay pins DELAB and DELCD by the

following formula:

V

V

) 0.5 V

to CS. DELAB and DELCD are clamped to a minimum of 0.5 V.

Current Sense (CS)

The inverting input of the current-sense comparator and the non-inverting input of the overcurrent comparator

and the ADS amplifier. The current sense signal is used for cycle-by-cycle current limiting in peak current mode

control, and for overcurrent protection in all cases with a secondary threshold for output shutdown. An output

disable initiated by an overcurrent fault also results in a restart cycle, called soft stop, with full soft start.

Oscillator Timing Capacitor (CT)

voltage of 2.35 V. The approximate oscillator period is calculated by the following formula:

t

5

R

T

C

T

48

) 120 ns

The increased fall time increases the SYNC pulse width, hence limiting the maximum phase shift

between OUTA, OUTB and OUTC, OUTD outputs, which limits the maximum duty cycle of the

converter. (Refer to Figure 1)

Delay Programming Between Complementary Outputs (DELAB, DELCD)

DELAB programs the dead time between switching of OUTA and OUTB, and DELCD programs the dead time

between OUTC and OUTD. This delay is introduced between complementary outputs in the same leg of the

external bridge. The UCC2895N allows the user to select the delay, in which the resonant switching of the external

power stages takes place. Separate delays are provided for the two half-bridges to accommodate differences in

resonant-capacitor charging currents. The delay in each stage is set according to the following formula:

t

(25

R

DEL

V

DEL

) 25 ns

maximum. Choose the delay resistors so that this maximum is not exceeded. Programmable output delay is

defeated by tying DELAB and/or DELCD to REF. For an optimum performance keep stray capacitance on these

pins at less than 10 pF.

(1)

(2)

(3)