UC1846-SP

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SLUS871C – JANUARY 2009 – REVISED OCTOBER 2015

Feature Description (continued)

7.3.2 Oscillator

Figure 8 highlights the oscillator circuit. Connecting a resistor RT from pin 9 to ground establishes a current,

which is mirrored to pin 8 and charges the capacitor connected from pin 8 to ground. Maximum on-time

corresponds to the maximum charging time of the timing capacitor. Oscillator frequency can be determined by

Equation 5.

Off-time corresponds to capacitor discharge time establishes the converter dead time between the pulses

according to Equation 4. Internal 8-mA current sink discharges the CT pin capacitor.

7.3.3 Slope Compensation

For duty cycle above 50% slope compensating can be implemented by using a buffer (that is, 2N2222) and

connecting base to timing capacitor pin 8 , collector to VREF (5 V), a resistor in series with emitter connected to

(pin 4) CS+ of differential current sense amplifier. Injecting a downslope proportional to the sawtooth into current

sense amplifier.

As with any bipolar PWM IC, outputs should be protected from negatively biasing the substrate. This is typically

done by using Schottky diodes from ground to each output. Failure to do this could cause spurious interruption

and restart of the oscillator, dropping of output pulses and a significant increase in propagation delays.

7.3.4 Error Amplifier

UC1846-SP incorporates an error amplifier with typical open loop gain of 100 db and gain bandwidth of 1.5 MHz.

With Source and sink capability of 10 mA and 0.5 mA respectively.

Error amplifier sources up to 0.5 mA.

Figure 4. Error Amplifier Output Configuration

7.3.5 Current Sense Amplifier

UC1846-SP incorporates a differential current sense amplifier which can eliminate ground loop problems and

increase noise immunity. An R-C snubber can also be implemented thus helping in blanking the peak current

spike when the switch is turned on. The input of the current sense amplifier is slew rate limited allowing lower

values of filter capacitors to be used to eliminate leading edge noise.

A small RC filter may be required in some applications to reduce switch transients. Differential input allows remote,

noise-free sensing.

Figure 5. Current-Sense Amplifier Connections

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