SPX2815 SIPEX [Sipex Corporation], SPX2815 Datasheet - Page 4

• Manufacturer Part Number: SPX2815
• Description: 1.5A Low Dropout Voltage Regulator Adjustable & Fixed Output, Fast Response
• Manufacturer: SIPEX [Sipex Corporation]

Date: 5/17/04
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 capaci-
tor should be chosen with respect to the
following equation:
Where
If an ADJ-bypass capacitor is used, the
amplitude of the output ripple will be indepen-
dent 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:
Where M = multiplier for the ripple seen when
the ADJ pin is optimally bypassed.
Iload=200mA
Figure 5. Ripple Rejection; Vin=3.3V, Vout=1.8V (adj.),
-10
-20
-30
-40
-50
-60
-70
-80
-90
0
C = value of the capacitor in Farads
(select an equal or larger standard value),
F
R
10
R
1
= ripple frequency in Hz,
= value of resistor R1 in Ohms.
C = 1 / ( 6.28 * F
100
M = V
V
REF
1000
Frequency, Hz
OUT
=1.25V
/ V
10000
R
REF
* R
1
100000
)
SPX2815 1.5A Low Dropout Voltage Regulator
1000000
4
APPLICATION INFORMATION: Continued
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
mated using the formula
A small correction to this formula is required
depending on the values of resistors R
since the adjustable pin current (approx
50µA) flows through R
into account, the formula becomes
where
Layout Considerations
regulation. In order to avoid this, connect R
directly to V
For the same reason, R
to the negative side of the load.
Figure 6. Current Source
Parasitic line resistance can degrade load
V
IN
4.7µF
V
OUT
+
I
OUT
C
V
1
= V
=
OUT
OUT
V
IN
R
REF
OUT
REF
1
=1.25 *(R
as illustrated in Figure 13.
V
(1+ (R
can be quickly approxi-
SPX2815
REF
=1.25V
ADJ
© Copyright 2004 Sipex Corporation
2
2
. When I
2
should be connected
/R
1
+ R
1
)) + I
OUT
2
)/R
ADJ
ADJ
LOAD
1
R
is taken
* R
1
1
I
and R
OUT
2
C
2
V
1
2
OUT
,