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AS1339

Datasheet - A p p l i c a t i o n I n f o r m a t i o n

Capacitor Selection for LDO’s

Input Capacitor

The capacitor for the LDO Input should have at least a value of the sum of the output capacitors of LDO1 and LDO2.

With a larger input capacitance and lower ESR a better noise rejection and line transient response can be achieved.

Output Capacitor

For the LDO outputs the capacitor value depends on the needed load current. For a stable operation with rated maxi-

mum load currents a minimum output capacitor of 1µF is recommended. At light loads of 10mA or less a 0.1µF capac-

itor is sufficient. With larger output capacitance a reduced output noise, improved load-transient response, better

stability and power-supply rejection can be achieved.

Inductor Selection

For most applications the value of the external inductor should be in the range of 1.5µH to 4.7µH as the inductor value

has a direct effect on the ripple current. The selected inductor must be rated for its DC resistance and saturation cur-

rent. The inductor ripple current (

In Equation (EQ 3) the maximum inductor current in PWM mode under static load conditions is calculated. The satura-

tion current of the inductor should be rated higher than the maximum inductor current as calculated with Equation (EQ

4). This is recommended because the inductor current will rise above the calculated value during heavy load tran-

sients.

The inductor current ripple

Figure 51. Ripple Current Diagram

Table 7. Recommended Capacitors for the LDO’s

Name

Part Number

C

Voltage

Type

Size

Manufacturer

C0402C104K4RAC

100nF

16V

X7R

0402

KEMET

www.kemet.com

GRM155R61A105KE15

1µF

10V

X5R

0402

Murata

www.murata.com

t

dI

dt

1 / f

(EQ 1)

dI

dt

-----

V

IN

V

OUT

–

L

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

=

(EQ 2)

t

ON

DutyCycle

1

f

---

×

DutyCycle

V

OUT

V

IN

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

=

=