7.3.1 Current Sense Amplifier

The current sense amplifier may be used in a variety of ways to sense peak switch current for comparison with an error voltage. Referring to Functional Block Diagram, maximum swing on the inverting input of the PWM comparator is limited to approximately 3.5 V by the internal regulated supply. Accordingly, for a fixed gain of 3, maximum differential voltages must be kept below 1.2 V at the current sense inputs.

Figure 3. Current Sense Amplifier Connection

7.3.2 Oscillator

By implementing the oscillator using all NPN transistors, the UCx846/7 achieves excellent temperature stability and waveform clarity at frequencies in excess of 1 MHz.

Referring to Figure 4, an external resistor R_T is used to generate a constant current into a capacitor C_T to produce a linear sawtooth waveform. Oscillator frequency may be approximated by selecting R_T and C_T such that:

Equation 1.

Figure 4. Oscillator Circuit

7.4.1 Current Limit

One of the most attractive features of a current-mode converter is the ability to limit peak-switch currents on a pulse-by-pulse basis by simply limiting the error voltage to a maximum value.

7.4.2 Shutdown

The shutdown circuit was designed to provide a fast acting general purpose shutdown port for use in implementing both protection circuitry and remote shutdown functions. The circuit may be divided into an input section consisting of a comparator with a 350-mV temperature compensated offset, and an output section consisting of a three transistor latch. Shutdown is accomplished by applying a signal greater than 350 mV to pin 16, causing the output latch to fire, and setting the PWM latch to provide an immediate signal to the outputs.