Datasheet

( )
12
DEL
DELAY
DEL
25 10 R
t 25 ns
V
-
´ ´
= +
T T
OSC
5 R C
t 120 ns
48
´ ´
= +
( )
DEL CS ADS
V 0.75 V V 0.5 V
é ù
= ´ - +
ë û
UCC1895, UCC2895, UCC3895
SLUS157P –DECEMBER 1999–REVISED JUNE 2013 www.ti.com
DETAILED PIN DESCRIPTION
ADS (Adaptive Delay Set)
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 V (the peak-
current threshold), ADS changes the output voltage on the delay pins DELAB and DELCD by Equation 1.
where
• V
CS
and V
ADS
are in volts (1)
ADS must be limited to between 0 V and 2.5 V and must be less-than or equal-to CS. DELAB and DELCD are
clamped to a minimum of 0.5 V.
CS (Current Sense)
The CS input connects to 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.
CT (Oscillator Timing Capacitor)
The UCC3895 oscillator charges CT via a programmed current. The waveform on C
T
is a sawtooth, with a peak
voltage of 2.35 V. The approximate oscillator period is calculated by Equation 2.
where
• C
T
is in Farads
• R
T
is in Ohms
• t
OSC
is in seconds
• C
T
can range from 100 to 880 pF. (2)
NOTE
A large C
T
and a small R
T
combination results in extended fall times on the C
T
waveform.
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 (see to Figure 3).
DELAB and DELCD (Delay Programming Between Complementary Outputs)
DELAB programs the dead time between switching of OUTA and OUTB. 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 Equation 3.
where
• V
DEL
is in volts
• R
DEL
is in Ohms
• t
DELAY
is in seconds (3)
10 Submit Documentation Feedback Copyright © 1999–2013, Texas Instruments Incorporated
Product Folder Links: UCC1895 UCC2895 UCC3895