AN937

Typical application

Doc ID 5655 Rev 15

17/29

3

Typical application

Figure 22 shows the typical application circuit, where the input supply voltage, Vcc, can

range from 8 to 55 V operating, and the output voltage adjustable from 3.3 V to 40 V.

The selected components, and in particular input and output capacitors, are able to sustain

the device voltage ratings, and the corresponding RMS currents.

3.1

Electrical specification

●

Input voltage range: 8 V - 55 V

●

Output voltage: 5.1 V ±3% (Line, load and thermal)

●

Output ripple: 51 mV

●

Output current range: 1 mA - 1.5 A

●

Max output ripple current: 30% Iomax

●

Max output current: 1.5 A

●

Switching frequency: 200 kHz

Figure 21.

Application circuit

3.2

Input capacitor

The input capacitor has to be able to support the max input operating voltage of the device

and the max rms input current. The input current is squared and the quality of these

capacitors has to be very high to minimize its power dissipation generated by the internal

ESR, improving the system reliability. Moreover, input capacitors are also affecting the

system efficiency.

The max Irms current flowing through the input capacitors is:

100V

INH

INH

C11

100n

C11

100n

R5

R5

C9

4.7n

C9

4.7n

C10

150uF

C10

150uF

GND

GND

OSC

3

OUT

4

VFB

8

SS/INH

2

U1

L4971 DIP 8

U1

L4971 DIP 8

C8

C8

L1

120uH (CC MSS1260-124KLD)

L1

120uH (CC MSS1260-124KLD)

C6

220n

C6

220n

63V

C2

220n

C2

220n

R2

2.7k

R2

2.7k

C4

22n

C4

22n

C3

2.2n

C3

2.2n

GND

GND

R3

4.99k

R3

4.99k

R1

12k

R1

12k

C5

82p

C5

82p

R4

15k

R4

15k

VOUT

VOUT

VIN

VIN

C7

100n

C7

100n

C1

180u 63v

C1

180u 63v