KA3524

5

Error Amplifier

This circuit is a simple differential-input, transconductance amplifier. The output is the compensation terminal pin 9, which is

and can easily be reduced from a nominal of 10,000 by an external shunt resistance from pin 9 to ground, as shown in Figure

8,In addition to DC gain control, the compensation terminal is also the place for AC phase compensation. The frequency

response curves of Figure 5 show the uncompensated amplifier with a single pole at approximately 200Hz and a unity gain

cross-over frequency at 5MHz.typically, most output filter designs will introduce one or more addition poles at a significantly

higher power frequency. Therefore, the best stabilizing network is a series R-C combination between pin 9 and ground which

introduces a zero to cancel one of the output filter poles. A good starting point is 50K

Ω plus 0.001 micro farad. One final point

on the compensation terminal is that this is also a convenient place to insert any programming signal which is to override the

error amplifier. Internal shutdown and current limit circuits are connected here, but any other circuit which can sink 200uA

can pull this point to ground, thus shutting off both outputs. While feedback is normally applied around the entire regulator, the

error amplifier can be used with conventional operational amplifier feedback and is stable in either the inverting or non-

inverting mode. Regardless of the connections, however, input common-mode limits must be observed or output sign inver-

sions may happen. For conventional regulator applications, the 5 volt reference voltage must be divided down as shown in Fig-

ure 3. The error amplifier may also be used in fixed duty cycle applications by using the unity gain configuration shown in the

open loop test circuit.

Current Limiting

The current limiting circuitry of the KA3524 is shown in Figure 4.

Although this circuit provides a relatively small threshold with a negligible temperature coefficient, there are some limitations

to its use, the most important of which is the

±1 volt common mode range which requires sensing in the ground line. Another

300Hz.Since the gain of this circuit is relatively low, there is a transition region as the current limit amplifier takes over pulse

width control from the error amplifier. For testing purposes, the threshold is defined as the input voltage to get 25% duty cycle

with the error amplifier signaling maximum duty cycle. In addition to constant current limiting, pins 4 and 5 may also be used

in transformer-coupled circuits to sense primary current and shorten an output pulse, should transformer saturation occur.

Another application is to ground pin 5 and use pin 4 as an additional shutdown terminal: i.e., the output will be off with pin 4

open and on when it is grounded. Finally, fold back current limiting can be provided with the network of Figure 5. This cir-

Figure 3. Error Amplifier Biasing Circuits

8I

2K

T

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