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AS1335

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

External Component Selection

Inductor Selection

For most applications the value of the external inductor should be in the range of 2.2µH 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 2) 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

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

sients.

f = Switching Frequency (1.5 MHz typical)

L = Inductor Value

The recommended starting point for setting ripple current is

The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current

to prevent core saturation. Thus, a 1.8A rated inductor should be sufficient for most applications (1.5A + 300mA).

Note: For highest efficiency, a low DC-resistance inductor is recommended.

Accepting larger values of ripple current allows the use of low inductance values, but results in higher output voltage

ripple, greater core losses, and lower output current capability.

The total losses of the coil have a strong impact on the efficiency of the DC/DC conversion and consist of both the

losses in the DC resistance and the following frequency-dependent components:

1. The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies).

2. Additional losses in the conductor from the skin effect (current displacement at high frequencies).

3. Magnetic field losses of the neighboring windings (proximity effect).

4. Radiation losses

.

Output Capacitor Selection

The advanced fast-response voltage mode control scheme of the AS1335 allows the use of tiny ceramic capacitors.

Because of their lowest output voltage ripple low ESR ceramic capacitors are recommended. X7R or X5R dielectric

output capacitor are recommended.

At high load currents, the device operates in PWM mode and the RMS ripple current is calculated as:

While operating in PWM mode the overall output voltage ripple is the sum of the voltage spike caused by the output

capacitor ESR plus the voltage ripple caused by charging and discharging the output capacitor:

(EQ 2)

ΔI

L

V

OUT

1

V

OUT

V

IN

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

–

Lf

×

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

×

=

(EQ 3)

I

LMAX

I

OUTMAX

ΔI

L

2

--------

+

=

(EQ 4)

I

RMSC

OUT

V

OUT

1

V

OUT

V

IN

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

–

Lf

×

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

1

23

×

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

×

×

=