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Date: 6/21/06 Rev C SPX2815 1.5A Low Dropout Voltage Regulator © 2006 Sipex Corporation
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Frequency, Hz
Ripple Rejection, Db
Figure 5. Ripple Rejection; Vin=3.3V, Vout=1.8V (adj.),
Iload=200mA
Figure 6. Current Source
LOAD
ADJ
SPX2815
V
IN
C
1
OUT
IN
4.7μF
C
2
I
OUT
R
1
V
REF
R
1
I
OUT
=
+
V
OUT
Ripple Rejection
Ripple rejection can be improved by adding a
capacitor between the ADJ pin and ground as
shown in Figure 8. When ADJ pin bypassing is
used, the value of the output capacitor required
increases to its maximum. If the ADJ pin is not
bypassed, the value of the output capacitor can
be lowered to 22μF for an electrolytic aluminum
capacitor or 10μF for a solid tantalum capacitor
(Fig 7). However, the value of the ADJ-bypass
capacitor should be chosen with respect to the
following equation:
_______1______-_
C =
( 6.28 * F
R
* R
1
)
Where
C = value of the capacitor in Farads
(select an equal or larger standard value),
F
R
= ripple frequency in Hz,
R
1
= value of resistor R1 in Ohms.
If an ADJ-bypass capacitor is used, the ampli-
tude of the output ripple will be independent of
the output voltage. If an ADJ-bypass capacitor
is not used, the output ripple will be proportional
to the ratio of the output voltage to the reference
voltage:
M = V
OUT
/ V
REF
Where M = multiplier for the ripple seen when
the ADJ pin is optimally bypassed.
V
REF
=1.25V
Ripple rejection for the adjustable version is
shown in Figure 5.
Output Voltage
The output of the adjustable regulator can be set
to any voltage between 1.25V and 15V. The
value of V
OUT
can be quickly approximated using
the formula
V
OUT
=1.25 *(R
1
+ R
2
)/R
1
A small correction to this formula is required
depending on the values of resistors R
1
and R
2
,
since the adjustable pin current (approx 50μA)
flows through R
2
. When I
ADJ
is taken into ac-
count, the formula becomes
V
OUT
= V
REF
(1+ (R
2
/R
1
)) + I
ADJ
* R
2
where
V
REF
=1.25V
Layout Considerations
Parasitic line resistance can degrade load regu-
lation. In order to avoid this, connect R
1
directly
to V
OUT
as illustrated in Figure 13. For the same
reason, R
2
should be connected to the negative
side of the load.
APPLICATION INFORMATION: Continued
