Datasheet

9
1110
12
DELCD
DELAB
ADS
CS
R
DELAB
R
DELCD
UCC3895
( )
DEL CS ADS
V 0.75 V V 0.5 V
é ù
= ´ - +
ë û
( )
12
DEL
DELAY
DEL
25 10 R
t 25 NS
V
-
´ ´
= +
UCC1895, UCC2895, UCC3895
SLUS157P –DECEMBER 1999–REVISED JUNE 2013 www.ti.com
APPLICATION INFORMATION
Programming DELAB, DELCD and the Adaptive Delay Set
The UCC2895N allows the user to set the delay between switch commands within each leg of the full-bridge
power circuit according to Equation 5.
(5)
From Equation 5 VDEL is determined in conjunction with the desire to use (or not) the ADS feature from
Equation 6.
(6)
Figure 12 illustrates the resistors needed to program the delay periods and the ADS function.
Figure 12. Programming Adaptive Delay Set
The ADS allows the user to vary the delay times between switch commands within each of the two legs of the
converter. The delay-time modulation is implemented by connecting ADS (pin 11) to CS, GND, or a resistive
divider from CS through ADS to GND to set V
ADS
as shown in Figure 12. From Equation 6 for V
DEL
, if ADS is tied
to GND then V
DEL
rises in direct proportion to V
CS
, causing a decrease in t
DELAY
as the load increases. In this
condition, the maximum value of V
DEL
is 2 V.
If ADS is connected to a resistive divider between CS and GND, the term (V
CS
– V
ADS
) becomes smaller,
reducing the level of V
DEL
. This reduction decreases the amount of delay modulation. In the limit of ADS tied to
CS, V
DEL
= 0.5 V and no delay modulation occurs. Figure 13 graphically shows the delay time versus load for
varying adaptive delay set feature voltages (V
ADS
).
In the case of maximum delay modulation (ADS = GND), when the circuit goes from light load to heavy load, the
variation of V
DEL
is from 0.5 to 2 V. This change causes the delay times to vary by a 4:1 ratio as the load is
changed.
The ability to program an adaptive delay is a desirable feature because the optimum delay time is a function of
the current flowing in the primary winding of the transformer, and changes by a factor of 10:1 or more as circuit
loading changes. Reference 5 (see References) describes the many interrelated factors for choosing the
optimum delay times for the most efficient power conversion, and illustrates an external circuit to enable ADS
using the UC3879. Implementing this adaptive feature is simplified in the UCC3895 controller, giving the user the
ability to tailor the delay times to suit a particular application with a minimum of external parts.
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Product Folder Links: UCC1895 UCC2895 UCC3895