UCC2895-EP datasheet(9/20 Pages) TI1 | BiCMOS ADVANCED PHASE-SHIFT PWM CONTROLLER

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DELAY

(25

10*12)

DEL

) 25 ns

DEL

+ [0.75

CS * VADS)] ) 0.5 V

DELCD

DELAB

ADS

DELCD

DELAB

UCC2895−EP

UCC2895-EP

www.ti.com

SCBS809F – DECEMBER 2005 – REVISED OCTOBER 2009

APPLICATION INFORMATION

Programming DELAB, DELCD, and Adaptive Delay Set (ADS)

The UCC2895-EP allows the user to set the delay between switch commands within each leg of the full-bridge

power circuit, according to the formula from the data sheet:

For this equation, VDEL is determined in conjunction with the desire to utilize (or not utilize) the ADS feature from:

Figure 3 shows the resistors needed to program the delay periods and the ADS function.

Figure 3. Resistors Needed in Programming

The ADS allows the user to vary the delay times between switch commands within each of the converter’s two

legs. The delay-time modulation is implemented by connecting ADS (pin 11) to CS, GND, or a resistive divider

from CS to GND to set VADS. From the previous equation for VDEL, if ADS is tied to GND, VDEL rises in direct

proportion to VCS, causing a decrease in tDELAY as the load increases. In this condition, the maximum value of

VDEL is 2 V. If ADS is connected to a resistive divider between CS and GND, the term (VCS – VDS) becomes

smaller, reducing the level of VDEL. This decreases the amount of delay modulation. In the limit of ADS tied to

CS, VDEL = 0.5 V and no delay modulation occurs. In the case with maximum delay modulation (ADS = GND)

when the circuit goes from light load to heavy load, the variation of VDEL 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 [1] delves into 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 UC2879.

Implementing this adaptive feature is simplified in the UCC2895-EP controller, giving the user the ability to tailor

the delay times to suit a particular application, with a minimum of external parts.

[1] L. Balogh, "Design Review: 100W, 400 kHz, DC/DC Converter With Current Doubler Synchronous

Rectification Achieves 92% Efficiency," Unitrode Power Supply Design Seminar Manual, Unitrode Corporation,

1996, Topic 2.

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