UC1846-SP_15 Datasheet, PDF (11/29 Pages) Texas Instruments – Current-Mode PWM Controller

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UC1846-SP
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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