as1339 austriamicrosystems, as1339 Datasheet - Page 19

• Manufacturer Part Number: as1339
• Description: 650ma Rf Step-down Dc-dc For Pa, With Two Ldos
• Manufacturer: austriamicrosystems

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.
Table 7. Recommended Capacitors for the LDO’s
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 ( Δ I
In
tion current of the inductor should be rated higher than the maximum inductor current as calculated with
4). This is recommended because the inductor current will rise above the calculated value during heavy load tran-
sients.
The inductor current ripple Δ I
PFET on-time t
Figure 51. Ripple Current Diagram
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C
I
OUTmax
Equation (EQ 3)
IN2
I
Lmax
Name
C
, C
LDO2
I
L
LDO1
t
,
ON
ON
dt
dI
GRM155R61A105KE15
1 / f
(see EQ
the maximum inductor current in PWM mode under static load conditions is calculated. The satura-
C0402C104K4RAC
Part Number
2).
L
(see EQ 3)
ΔI
L
L
t
) decreases with higher inductance and increases with higher V
ON
=
is defined by the slope of the current (dI / dt)
DutyCycle
100nF
1µF
C
dI
---- -
dt
Revision 1.02
=
×
t
1
-- -
f
V
----------------------------
Voltage
IN
16V
10V
–
L
V
OUT
DutyCycle
Type
X7R
X5R
=
V
-------------
V
OUT
IN
0402
0402
Size
(see EQ 1)
KEMET
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Murata
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Manufacturer
multiplied by the
IN
to V
Equation (EQ
OUT
.
19 - 24
(EQ 1)
(EQ 2)