AN557 datasheet(35/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

Resistors value for standard output voltages.

Rx corresponds to R9 for L4974A and L4972A Ry

corresponds to R10 for L4970A, L4977A and

L4975A

Rx corresponds to R7 for L4974A and L4972A Ry

corresponds to R8 for L4970A, L4977A and

L4975A

The suggested switching frequency, and used in

the dynamic tests, is 200KHz for the Multiwatt®

package (MW) and 100KHz for the powerdip plas-

tic package (PDIP). The maximum switching fre-

quency allowed is 500KHz.

For the types in plastic package (Powerdip), the

lower switching frequency suggested is only de-

pended by the minor dissipating power of a plastic

package versus a "power package" because it is

well known that switching losses are directly pro-

portional to the commutation frequency. Higher

switching frequencies are possible if limited output

current is required and the operating ambient tem-

perature are lower than 70°C. Infact the oscillator

of the devices assembled in dual in line is com-

pletely equivalent to Multiwatt® package.

The most important external components which

need a little more attention (because a properly di-

mensioning affects on the performance of the ap-

plication) are the input and output capacitors, the

freewheeling diode and the coil.

Figure 45. Oscillator waveform and sync. pulse

for Vi = 35V

Figure 46. Oscillator waveform and sync. pulse

for Vi = 15V

Figure 47. Oscilloscope photograph showing

the short circuit output voltage and

current waveforms.

INPUT OUTPUT CAPACITORS

The output voltage ripple

∆Vo, essentially de-

pends on the current ripple in the coil and the ESR

of the output capacitor at the switching frequency.

The capacitor that present a low ESR are capable

of supporting higher current ripples.

Today, the majority of the constructors of elec-troli-

thic capacitors offer in their data book also a wide

range of "low ESR" types generally suggested for

switching power supply application. In our case

EKR and EKE series (ROE) has been preferred.

Such a series capacitors are designed for applica-

tions at high frequency, 200KHz, and built to have

a low ESR in order of supporting high current rip-

ple.

In order to minimize the effects caused by the ESR

of the capacitors on the output voltage ripple 3 ca-

pacitors of 220uF/40V (for high output current ap-

plication) are connected in parallel.

(V)

Ω)

4.7

6.2

4.7

9.1

4.7

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