13/21

AN1061 APPLICATION NOTE

4.2 INPUT CAPACITOR

The input capacitors have to be able to support the max input operating voltage of the device

and the max rms input current.

Figure 21. Application Circuit

The input current is squared and the quality of these capacitors has to be very high to minimise

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:

where h is the expected system efficiency, D is the duty cycle and Io the output dc current. This

function reaches the maximum value at D = 0.5 and the equivalent rms current is equal to Io/2.

The following diagram is the graphical representation of the above equation, with an estimated

efficiency of 85% at different output currents.

The maximum and minimum duty cycles are:

This formula is not taking into account the power mos Rdson, considering negligible the inher-

ent voltage drop, respect input and output voltages.

At full load, 2A and D = 0.5% the rms capacitor current to be sustained is of 1A.

The selected EKE 220

µF/63V Roderstain is able to support this current.

D98IN915

5

2

8

4

1

L4978

220

µF

63V

330

µF

Vin=8V to 55V

20K

2.7nF

9.1K

22nF

3

7

L1

126

µH

(77120)

6

D1

STPS3L60U

100nF

220nF

100nF

C1=220

µF/63V EKE

C2=2.7nF

C5=100nF

C6=100nF

C7=220nF/63V

C8=330

µF/35V CG Sanyo

L1=126

µH KoolMu 77120 - 55 Turns - 0.5mm

R1=20K

R2=9.1K

D1=STPS3L60U

R3(K

Ω)

R4(K

Ω)

3.3

5.1

12

15

18

24

0

2.7

12

16

20

30

4.7

4.7

4.7

4.7

4.7

L4978

I

rms

I

O

D

2D

2

⋅

η

---------------

–

D

2

η

2

-------

+

⋅

=

D

max

V

O

V

f

+

V

inmin

V

f

+

----------------------------

0.66

==

D

max

V

O

V

f

+

V

inmax

V

f

+

------------------------------

0.1

==