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Reduced Discharge Current Variation

Soft-Start

UCC28C42

8

1

COMP

5

GND

UDG-01072

VREF

Oscillator Synchronization

UCC28C4x-EP

BiCMOS LOW-POWER CURRENT-MODE PWM CONTROLLERS

SGLS352B – DECEMBER 2006 – REVISED MAY 2007

APPLICATION INFORMATION (continued)

The original UC2842 oscillator did not have trimmed discharged current, and the parameter was not specified on

the data sheet. Since many customers attempted to use the discharge current to set a crude dead-time limit, the

UC2842A family was released with a trimmed discharge current specified at 25

°C. The UCC28C4x 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.

Figure 1 provides a typical soft-start circuit for use with the UCC28C42. 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 stage.

After the soft-start interval is complete, the capacitor continues to charge to VREF, effectively removing 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

the reference (VREF) goes low. Without the diode, the capacitor otherwise remains charged during a brief loss of

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

Figure 1.

The UCC28C4x oscillator has the same synchronization characteristics as the original bipolar devices. Thus, the

information in the application report U-100A, UC2842/3/4/5 Provides Low-Cost Current-Mode Control (SLUA143)

still applies. The application report describes how a small resistor from the timing capacitor to ground can offer

an insertion point for synchronization to an external clock (see Figure 2 and Figure 3). Figure 2 shows how the

UCC28C42 can be synchronized to an external clock source. This allows precise control of frequency and dead

time with a digital pulse train.

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