NJM2825 Просмотр технического описания (PDF) - Japan Radio Corporation
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NJM2825 Datasheet PDF : 6 Pages
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NJM2825
■Application Information
The NJM2825 creates a highly accurate reference voltage, enabling a low power consumption application circuit to be
configured.
In the basic application (Fig.1) of the shunt regulator, a voltage drop is created by resistor Rs connected between the
input voltage and the NJM2825, and the output voltage (cathode – anode voltage = VKA) is controlled to a constant
value. The voltage drop due to Rs is determined by the total of the output current and the cathode current.
The feedback to the output voltage is controlled by the FB terminal, and the cathode current changes so that the set
voltage is obtained.
As a result, Rs must conform to the following conditions.
VIN
RS
VOUT=VKA
*Minimum cathode current = 0.7 uA min
Conditions under which the input voltage is a minimum
and the output current is a maximum.
IK
R1
VREF
*Maximum cathode current = 12 mA max
CO
Conditions under which the input voltage is a maximum
R2
IFB
and the output current is a minimum.
The value of resistor Rs is obtained by means of the following formula.
[ ] RS
=
VIN
IK
− VOUT
+ IOUT
Ω
Fig.1 basic application
The output voltage can be set using any desired value between VREF and 13 V.
The output voltage is set according to the ratio between the values of the two external resistors, however an error
occurs depending upon the feedback current. The error can be minimized by combining two external resistors with low
resistance values. The formula for calculating the output voltage setting is shown below.
VOUT
=
R2
R1
+ 1 × VREF
+ IFB
× R2
As shown in the “reference voltage versus cathode voltage”
characteristics example, the reference voltage value has
VKA (V) R1 (kΩ) R2 (kΩ)
1.20
Open Short
negative characteristics. The reference voltage is corrected by
using ∆VREF/∆VKA stipulated by the electrical characteristics.
1.50
120
30.6
1.80
120
60.8
2.50
120
131
∆VREF
=
∆VREF
∆VKA
× VOUT
3.30
120
212
5.00
120
382
Table.1 Examples of output voltage settings at the standard
Table 1 shows an example of combining constants in the case where R1 is assumed to be 120 kΩ.
The error in the output voltage also varies with the accuracy of the resistors. In order to realize a highly accurate
application, the relative accuracy can be improved by either using accurate resistors or combining integrated resistors.
The NJM2825 contains an optimized phase compensation circuit. Consequently, in the basic application a stable
reference voltage is generated without the use of an output capacitor. As is indicated in the “dynamic impedance
versus frequency” characteristics, the impedance increases in proportion to the frequency. If necessary, connect an
output capacitor to reduce the high frequency impedance. You can connect a ceramic capacitor to obtain high stability,
but in this case be sure to use the NJM2825 in the stable operation region while referring to the “stable operation
boundary conditions” characteristics example.
Ver.2009-03-05
-5-
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