RN5RG
OPERATION
In these ICs, Output Voltage V
OUT
is detected by
External PNP Transistor
Feed-back Registers RA, RB, and the detected Output
Voltage is compared with a reference voltage by Error
Amplifier, so that the base current of External PNP
OUT
IN
V
DD
4
2
EXT
RA
V
OUT
3
Error
Amplifier
+
–
Vref
Transistor is adjusted and Output Voltage V
OUT
is regu-
lated.
RB
1
GND GND
CE
5
SELECTION GUIDE FOR EXTERNAL COMPONENTS
1.External PNP Transistor
External PNP Transistor
V
IN
R1
R2
4
OUT
Select External PNP Transistor from the view-
points of output current, input voltage and power dis-
sipation.
Generally External PNP Transistor with low V
CE
(
SAT
)
and high h
FE
is suitable.
EXT
3
V
OUT
C
L
2.Base Resistor R2
EXT Pin of these ICs is protected by a current limit circuit from the destruction caused by excess current.
However, since this current limit circuit is provided for the purpose of protecting the IC, use a resistor R2 for the
protection of the External Transistor, although this IC can be operated without such a resistor. It is required that
the resistance of the Resistor R2 be determined with the input voltage, output voltage,output current, tempera-
ture, and the h
FE
value taken into consideration and the dispersion of these values. Before making such a deter-
mination, check the characteristics by calculating the respective values by using the following formula :
V
IN
[MIN.] –1.2(V)
R2
–
0.7(V)
R1
>
I
OUT
[MAX.]
h
FE
3.Phase Compensation
In these ICs, phase compensation is made for securing stable operation even when the load current is varied.
For this purpose, be sure to use a capacitor C
L
(tantalum type) with a capacitance of 10µF or more and a resistor
R1 with a resistance of about 10kΩ between the base and the emitter.There may be the case the loop oscillation
takes place when a tantalum capacitor C
L
with a large ESR is used, so select the C
L
carefully including the fre-
quency characteristics.
5
RICOH [ RICOH ELECTRONICS DEVICES DIVISION ]
