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Revision 1.12

10 - 20

AS1312

Datasheet - D e t a i l e d D e s c r i p t i o n

8.1.4

Input Capacitor Selection

The input capacitor supports the triangular current during the on-time of the power switch, and maintains a broadly constant input voltage during

this time. The capacitance value is obtained from choosing a ripple voltage during the on-time of the power switch. Additionally, ripple voltage is

generated by the equivalent series resistance (ESR) of the capacitor. For worst case, use maximum peak current values from the datasheet.

(EQ 15)

Nearest preferred would be 10µF.

(EQ 16)

Typically, the ripple due to ESR is not dominant. ESR for the recommended capacitors (Murata GMR), ESR = 5m

Ω to 10mΩ. For the AS1312,

maximum peak current is 400mA. Ripple due to ESR is 2.0mV to 4.0mV.

Ripple at the input propagates through the common supply connections, and if too high in value can cause problems elsewhere in the system.

The input capacitance is an important component to get right.

8.1.5

Output Capacitor Selection

The output capacitor supports the triangular current during the off-time of the power switch (inductor discharge period), and also the load current

during the wait time (Region D in Figure 10) and on-time (Region A in Figure 10) of the power switch.

(EQ 17)

Note:

There is also a ripple component due to the equivalent series resistance (ESR) of the capacitor.

8.2 Summary

Inductor Selection:

Select Max on-time = 0.5µs to 3µs for AS1312. Use (EQ 3) to calculate inductor value.

Use (EQ 5) to determine off-time.

Use (EQ 6) to check that power delivery matches load requirements assume 70% conversion efficiency.

C

IN

I

V

RIPPLE

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

=

ESR

PK

ESR

RIPPLE

PK

R

I

V

=

_

_

NOM

OUT

WAIT

ON

LOAD

OUT

V

T

T

I

C

_

)

99

.

0

1

(

)

(

−

+

=