APPLICATION INFORMATION: Continued
Ripple Rejection
Ripple rejection for the adjustable version is
shown in Figure 5.
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).
Output Voltage
The output of the adjustable regulator can be
set to any voltage between 1.25V and 15V.
The value of VOUT can be quickly approxi-
mated using the formula
VOUT=1.25 *(R1 + R2)/R1
However the value of the ADJ-bypass capaci-
tor should be chosen with respect to the
following equation:
A small correction to this formula is required
depending on the values of resistors R1 and R2,
since the adjustable pin current (approx
50µA) flows through R2. When IADJ is taken
into account, the formula becomes
C = 1 / ( 6.28 * FR * R1 )
Where
C = value of the capacitor in Farads
(select an equal or larger standard value),
FR = ripple frequency in Hz,
R1 = value of resistor R1 in Ohms.
VOUT = VREF(1+ (R2/R1)) + IADJ * R2
where
VREF=1.25V
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:
Layout Considerations
Parasitic line resistance can degrade load
regulation. In order to avoid this, connect R1
directly to VOUT as illustrated in Figure 13.
For the same reason, R2 should be connected
to the negative side of the load.
M = VOUT / VREF
Where M = multiplier for the ripple seen when
the ADJ pin is optimally bypassed.
VREF =1.25V
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
V IN
SPX2815
VOUT
IN
OUT
+
4.7µF
C1
C 2
ADJ
R1
IOUT
VREF
R1
LOAD
IOUT
=
10
100
1000
10000
100000
1000000
Frequency, Hz
Figure 6. Current Source
Figure 5. Ripple Rejection; Vin=3.3V, Vout=1.8V (adj.),
Iload=200mA
Date: 5/17/04
SPX2815 1.5A Low Dropout Voltage Regulator
© Copyright 2004 Sipex Corporation
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