Datasheet

9
1110
12
DELCD
DELAB
ADS
CS
R
DELAB
R
DELCD
UCC2895
UCC2895-Q1
www.ti.com SLUS783C –MAY 2008–REVISED AUGUST 2012
APPLICATION INFORMATION
Programming DELAB, DELCD, and ADS
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:
t
DELAY
= [(25 × 10
–12
× R
DEL
)/V
DEL
] + 25 ns (5)
From Equation 5, V
DEL
is determined in conjunction with the desire to use (or not use) the ADS feature from
Equation 6:
V
DEL
= [0.75 × (V
CS
– V
ADS
)] + 0.5 V (6)
Figure 4 illustrates the resistors needed to program the delay periods and the ADS function.
Figure 4. Programming Adaptive Delay Set
The ADS feature 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 1. 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 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 5 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 V to 2 V. This 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 can change by a factor of 10:1 or more as
circuit loading changes. Reference [5] describes the many interrelated factors for choosing the optimum delay
times for the most-efficient power conversion, and illustrates an external circuit to enable adaptive delay set
using the UC3879. Implementing this adaptive feature is simplified in the UCC2895-Q1 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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