SJ1117xxx
◈
Application Note
Input Bypass Capacitor (C
IN
)
An Input capacitor is recommended. A 10μF tantalum capacitor on the input is a suitable bypassing for all
applications.
Adjust Terminal Bypass Capacitor(C
ADJ
)
The adjust terminal can be bypassed to ground with a bypass capacitor to improve ripple rejection
This bypass capacitor prevents ripple from being amplified as the output voltage is increased. At
any ripple frequency, the impedance of the C
ADJ
should less than R1 to prevent the ripple from being
amplified
1
( 2
π
×
f
RIPPLE
×
C
ADJ
)
〈
R
1
The R1 is the resistor between the output and the adjust pin.
Output Capacitor(C
OUT
)
The output capacitor(C
OUT
) is critical in maintaining regulator stability, and must meet required
conditions for both minimum amount of capacitance and ESR. The minimum C
OUT
required is
10μF(tantalum capacitor). Any increase of the C
OUT
will merely improve the stability and transient
response. The ESR of the C
OUT
should range between 0.3Ω - 22Ω. In the case of the adjustable regulator,
when the C
ADJ
is used, a larger C
OUT
(22μF) is required.
Output Voltage
The SJ1117 develops a 1.25V reference voltage between the output and the adjust terminal (See Fig.9).
By placing a resistor between these two terminals, a constant current is caused to flow through R1 and
down through R2 to set the overall output voltage. Normally this current is chosen to be the specified
minimum load current. Because I
ADJ
is very small and constant when compared to the current through R1,
it represents a small error and can usually be ignored. For fixed voltage devices R1 and R2 are included in
the device.
VIN
IN
+ C
IN
ADJ
V
REF
R1
VOUT
SJ1117
OUT
+ C
O
I
ADJ
R2
V
OUT
=
V
REF
(
1
+
R
2
R
1
)
+
I
ADJ
×
R
2
Fig.13 Basic Adjustable Regulator
KSD-I0T005-008
8
KODENSHI [ KODENSHI KOREA CORP. ]
