Datasheet

AD680
Rev. H | Page 10 of 12
4.5 V REFERENCE FROM A 5 V SUPPLY
The AD680 can be used to provide a low power, 4.5 V reference,
as shown in Figure 19. In addition to the AD680, the circuit
us
es a low power op amp and a transistor in a feedback con-
figuration that provides a regulated 4.5 V output for a power
supply voltage as low as 4.7 V. The high quality tantalum 10 μF
capacitor (C1) in parallel with the ceramic 0.1 μF capacitor (C2)
and the 3.9 Ω resistor (R5) ensure a low output impedance up to
approximately 50 MHz (see
Figure 19).
00813-019
6
3
2
7
4
OP90
+IN
V+
V–
OUT
–IN
R1
2kΩ
1%
CF
0.1μF
R4
3.57kΩ
R2
2.5kΩ
1%
AD680
V
OUT
GND
V
IN
0.1μF
R3
1kΩ
CC
3.3μF
2N2907A
4.7V TO 15V
C1
10μF
+
C2
0.1μF
R5
3.9Ω
Figure 19. 4.5 V Reference Running from a Single 5 V Supply
VOLTAGE REGULATOR FOR PORTABLE
EQUIPMENT
The AD680 is ideal for providing a stable, low cost, low power
reference voltage in portable equipment power supplies.
Figure 20 shows how the AD680 can be used in a voltage
r
egulator that not only has low output noise (as compared to a
switch mode design) and low power, but it also has a very fast
recovery after current surges. Some caution should be taken in
the selection of the output capacitors. Too high an ESR (effective
series resistance) could endanger the stability of the circuit. A solid
tantalum capacitor, 16 V or higher, and an aluminum electro-
lytic capacitor, 10 V or higher, are recommended for C1 and C2,
respectively. Also, the path from the ground side of C1 and C2
to the ground side of R1 should be kept as short as possible.
00813-020
2
3
7
6
4
OP777
+
–
R3
510kΩ
IRF9530
AD680
V
OUT
GND
V
IN
C1
68μF
TANT
+
C2
1000μF
ELECT
+
2
6
TEMP
3
4
0.1μF
CHARGE
R
INPUT
6V
LEAD-ACID
BATTERY
+
R2
402kΩ 1%
R1
402kΩ 1%
Figure 20. Voltage Regulator for Portable Equipment