AS1321 datasheet(8/12 Pages) AMSCO

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Data Sheet

Inductor Selection

8 Application Information

Inductor Selection

The control circuitry of the AS1321 permits a wide range of inductor values to be selected – from 4.7 to 47µH; 10µH is

ideal for most applications.

The intended application should dictate the value of L. The trade-off between required PCB surface area and desired

output ripple are the determining factors: smaller values for L require less PCB space, larger values of L reduce output

ripple. If the value of L is large enough to prevent IMAX from being reached before tON expires, the AS1321 output

power will be reduced.

For maximum output current calculate the value for L as:

(VBATT(MAX) (1µs))/0.7A < L < (VBATT(MIN)(7µs))/0.7A

(EQ 2)

IOUT(MAX) = (0.7A/2)(VBATT(MIN) - (0.7A/2)(RNCH + RIND))/VOUT

(EQ 3)

Where:

RIND is the inductor series resistance.

RNCH is the RDS(ON) of the N-channel MOSFET (0.3

Ω typ).

Note: Coils should be able to handle 500mARMS and have a ISAT

≥ 1A and should have a RIND ≤ 100mΩ.

Capacitor Selection

COUT Selection

Choose a COUT value to achieve the desired output ripple percentage. A 22µF ceramic capacitor is a good initial value.

The value for COUT can be determined by:

COUT > (L + 2.5µH) x VBATT(MAX)

/ (r% x 4)

(EQ 4)

Where:

r is the desired output ripple in %.

CIN Selection

CIN reduces the peak current drawn from the battery and can be the same value as COUT. A larger value for CIN can be

used to further reduce ripple and improve AS1321 efficiency.

External Diode

An external Schottky diode must be connected, in parallel with the on-chip synchronous rectifier, from LX to OUT. Use

diodes such as MBR0520L, EP05Q03L, or the generic 1N5817. The diode should be rated for 500mA, since it carries

current during startup and after the synchronous rectifier turns off. The Schottky diode must be connected as close to

the IC as possible. Ordinary rectifier diodes must not be used, since the slow recovery rate will compromise efficiency.

PC Board Layout and Grounding

Well-designed printed circuit-board layout is important for minimizing ground bounce and noise.

Place pin GND lead and the ground leads of CIN and COUT as close to the device as possible.

Keep the lead to pin LX as short as possible.

To maximize output power and efficiency and minimize output ripple voltage, use a ground plane and solder the

GND pin directly to the ground plane.

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