AS1321_1 AMSCO [austriamicrosystems AG], AS1321_1 Datasheet - Page 10

• Manufacturer Part Number: AS1321_1
• Description: 130mA Step-Up DC-DC Converter
• Manufacturer: AMSCO [austriamicrosystems AG]

AS1321
Datasheet - A p p l i c a t i o n I n f o r m a t i o n
9 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 I
power will be reduced.
For maximum output current calculate the value for L as:
Where:
R
R
Note: Coils should be able to handle 500mA
Capacitor Selection
C
Choose a C
The value for C
Where:
r is the desired output ripple in %.
C
C
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.
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!
!
!
IND
NCH
IN
OUT
IN
reduces the peak current drawn from the battery and can be the same value as C
Place pin GND lead and the ground leads of C
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
Selection
GND pin directly to the ground plane.
is the inductor series resistance.
is the R
Selection
OUT
DS(ON)
OUT
value to achieve the desired output ripple percentage. A 22µF ceramic capacitor is a good initial value.
can be determined by:
of the N-channel MOSFET (0.3Ω typ).
I
OUT(MAX)
(V
BATT(MAX)
C
= (0.7A/2)(V
OUT
> (L + 2.5µH) x V
(1µs))/0.7A < L < (V
RMS
BATT(MIN)
MAX
and have a I
IN
Revision 1.02
and C
from being reached before t
- (0.7A/2)(R
BATT(MAX)
OUT
BATT(MIN)
SAT
as close to the device as possible.
≥ 1A and should have a R
2
NCH
/ (r% x 4)
(7µs))/0.7A
+ R
IND
))/V
ON
OUT
OUT
expires, the AS1321 output
. A larger value for C
IND
≤ 100mΩ.
IN
can be
10 - 13
(EQ 2)
(EQ 3)
(EQ 4)