Datasheet

AB
ABSET
A
5 R
T ns 5ns
0.15 V CS K 1.46
æ ö
´
= +
ç ÷
+ ´ ´
è ø
CD
CDSET
A
5 R
T ns 5ns
0.15 V CS K 1.46
æ ö
´
= +
ç ÷
+ ´ ´
è ø
A
A
A AHI
R
K
R R
=
+
UCC28950
SLUSA16B –MARCH 2010– REVISED OCTOBER 2011 www.ti.com
The delay time T
ABSET
is defined by the following Equation 3.
(3)
The same equation is used to define the delay time T
CDSET
in another leg except R
AB
is replaced by R
CD
.
(4)
In these equations R
AB
and R
CD
are in kΩ and CS, the voltage at pin CS, is in volts and K
A
is a numerical
coefficient in the range from 0 to 1. The delay time T
ABSET
and T
CDSET
are in ns. These equations are empirical
and they are approximated from measured data. Thus, there is no unit agreement in the equations. As an
example, assume R
AB
= 15 kΩ, CS = 1 V and K
A
= 0.5. Then the T
ABSET
is going to be 90.25 ns. In both
Equation 3 and Equation 4, K
A
is the same and is defined as:
(5)
K
A
sets how the delay is sensitive to CS voltage variation. If K
A
= 0 (ADEL shorted to GND), the delay is fixed. If
K
A
= 1 (ADEL is tied to CS), the delay is maximum at CS = 0.2 V and gradually decreases when CS goes up to
1.8 V. The ratio between the maximum and minimum delay can be up to 6:1.
It is recommended to start by setting K
A
= 0 and set T
ABSET
and T
CDSET
relatively large using equations or plots in
the data sheet to avoid hard switching or even shoot through current. The delay between outputs A, B and C, D
set by resistors R
AB
and R
CS
accordingly. Program the optimal delays at light load first. Then by changing K
A
set
the optimal delay for the outputs A, B at maximum current. K
A
for outputs C, D is the same as for A,D. Usually
outputs C, D always have ZVS if sufficient delay is provided.
NOTE
The allowed resistor range on DELAB and DELCD, R
AB
and R
CD
are 13 kΩ to 90 kΩ.
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