UCC28C40, UCC28C41, UCC28C42, UCC28C43, UCC28C44, UCC28C45

UCC38C40, UCC38C41, UCC38C42, UCC38C43, UCC38C44, UCC38C45

SLUS458C − AUGUST 2001 − REVISED SEPTEMBER 2003

7

www.ti.com

APPLICATION INFORMATION

± 1% initial reference voltage

The BiCMOS internal reference of 2.5 V has an enhanced design and utilizes production trim to allow initial

accuracy of

±1% at room temperature and ± 2% over the full temperature range. This can be used to eliminate

an external reference in applications that do not require the extreme accuracy afforded by the additional device.

This is very useful for nonisolated dc-to-dc applications where the control device is referenced to the same

common as the output. It is also applicable in offline designs that regulate on the primary side of the isolation

boundary by looking at a primary bias winding, or perhaps from a winding on the output inductor of a

buck-derived circuit.

reduced discharge current variation

The original UC3842 oscillator did not have trimmed discharged current, and the parameter was not specified

on the datasheet. Since many customers attempted to use the discharge current to set a crude deadtime limit

the UC3842A family was released with a trimmed discharge current specified at 25

°C. The

UCC28C4x/UCC38C4x series now offers even tighter control of this parameter, with approximately

±3%

accuracy at 25

°C, and less than 10% variation over temperature using the UCC28C4x devices. This level of

accuracy can enable a meaningful limit to be programmed, a feature not currently seen in competitive BiCMOS

devices. The improved oscillator and reference also contribute to decreased variation in the peak to peak

variation in the oscillator waveform, which is often used as the basis for slope compensation for the complete

power system.

soft-start

The following diagram provides a typical soft-start circuit for use with the UCC38C42. The values of R and C

should be selected to bring the COMP pin up at a controlled rate, limiting the peak current supplied by the power

the PNP transistor from circuit considerations.

The optional diode in parallel with the resistor forces a soft-start each time the PWM goes through UVLO and

of supply or brown-out, and no soft-start is enabled upon reapplication of VIN.

Figure 1

UDG−01072

VREF

UCC38C42

8

1

COMP

5

GND