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AN458 APPLICATION NOTE

The operation frequency is equal to the clock frequency, that is determined by the resistor Rt, connected be-

tween pin. 17 and ground:

In fig.3 are shown the voltage and current waveforms associated to this mode of operation.

Figure 3. Fixed Frequency Operation

Referring again to the block diagram in fig. 1, and with a given inductor L, we suppose to have a load current

that reduces the Dead Time toff" to zero. At this point we suppose that the clock send a set pulse to the latch

lowing the law stated in Eq. 1.

If the peak current is below the Current Limitation threshold, it is again the Error Amplifier that turns off the power

stage and the inductor will discharge following the eq. 2.

If the system is not able to discharge completely the inductor during the maximum toff time allowed by the fixed

frequency operating mode an internal comparator, (which compares the voltage on the free-wheeling diode

cathode with a precise internal reference Vr = -100mV) will mantain the power stage off until the inductor will be

completely discharged.

completely discharged and the diode D ceases to conduct (see fig.4).

f

()

0.033

R

()

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