Selecting components for your applications

AN2728

8/20

You must choose the resistor divider according to the following equation:

Equation 1

Figure 9 shows the feedback voltage versus temperature.

We suggest using a resistor with values in the range of 10 k

Ω to 100 kΩ. Lower values are

suitable as well but increase current consumption.

For output voltages close to the feedback voltage, we suggest adding a very small capacitor

2.2

Input capacitor

The input capacitor must be able to support the maximum input operating voltage and the

maximum RMS input current.

Since step-down converters draw current from the input in pulses, the input current is

squared and the height of each pulse is equal to the output current. The input capacitor has

to absorb switching current that can be as high as the load current divided by two (worst

case, with duty cycle of 50 %). For this reason, the quality of these capacitors has to be very

high to minimize its power dissipation generated by the internal ESR, thus improving the

system reliability and efficiency. The critical parameter is usually the RMS current rating,

which must be higher than the RMS input current. The maximum RMS input current (flowing

through the input capacitor) is:

Equation 2

where

current. This function reaches its maximum value at D = 0.5 and the equivalent RMS current

+

=

2

1

FB

OUT

R

R

1

V

V

[]

+

=

2

1

FB

OUT

R

R

1

V

V

[]

Figure 9.

Feedback voltage vs. temperature

550

570

590

610

630

650

-75

-50

-25

0

25

50

75

100

125

150

175

T [°C]

V

550

570

590

610

630

650

-75

-50

-25

0

25

50

75

100

125

150

175

T [°C]

V

η

D

η

D

2

D

I

I

2

2

OUT

RMS

+

×

-

×

=

η

D

η

D

2

D

I

I

2

2

OUT

RMS

+

×

-

×

=