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ADP1108-3.3 View Datasheet(PDF) - Analog Devices

Part Name
Description
Manufacturer
ADP1108-3.3 Datasheet PDF : 12 Pages
First Prev 11 12
ADP1108
PROGRAMMING THE GAIN BLOCK
The gain block of the ADP1108 can be used as a low battery de-
tector, error amplifier or linear post regulator. The gain block
consists of an op amp with PNP inputs and an open-collector
NPN output. The inverting input is internally connected to the
ADP1108’s 1.245 V reference, while the noninverting input is
available at the SET Pin. The NPN output transistor will sink
about 300 µA.
Figure 24a shows the gain block configured as a low-battery
monitor. Resistors R1 and R2 should be set to high values to re-
duce quiescent current, but not so high that bias current in the
SET input causes large errors. A value of 33 kfor R2 is a good
compromise.
The value for R1 is then calculated from the formula:
R1 = V LOBATT – 1.245 V
1.245 V
 R2 
where VLOBATT is the desired low battery trip point. Since the
gain block output is an open-collector NPN, a pull-up resistor
should be connected to the positive logic power supply.
VBAT
R1
ADP1108
1.245V
SET REF
7
R2
33k
GND
5
+5V
2
VIN
47k
AO
6
TO
PROCESSOR
R1 = VLB –1.245V
37.7µA
VLB = BATTERY TRIP POINT
Figure 24a. Setting the Low Battery Detector Trip Point
The circuit of Figure 24a may produce multiple pulses when ap-
proaching the trip point, due to noise coupled into the SET in-
put. To prevent multiple interrupts to the digital logic, hysteresis
can be added to the circuit (Figure 24b). Resistor R3, with a
value of 1 Mto 10 M, provides the hysteresis. The addition
of R3 will change the trip point slightly, so the new value for R1
will be:
R1 =
V LOBATT – 1.245 V

1.245 V
R2

V

L – 1.245 V
RL + R3

where VL is the logic power supply voltage, RL is the pull-up
resistor and R3 creates the hysteresis.
VBAT
R1
R2
33k
ADP1108
1.245V
SET REF
7
GND
5
+5V
2
VIN
47k
AO
6
TO
PROCESSOR
R3
1.6M
Figure 24b. Adding Hysteresis to the Low Battery Detector
VIN
6.5V TO 20V
C2
0.22
ZETEX
ZTX749
100100
L1*
100H
D1
1N5818
5VOUT
200mA AT 6.5V
500mA AT 8V
C1
220
2
1
3
VIN
ILIM
SW1
SENSE 8
ADP1108-5
GND SW2
5
4
*L1 = COILTRONICS CTX100-4
Figure 25. 6.5 V to 5 V Step-Down Converter
VIN
6.5V TO 20V
+
C2
220
2
1
3
VIN
ILIM SW1
ADP1108-5
GND
5
SENSE 8
SW2
4
L1*
300µH
MBRS130T3
*L1 = COILTRONICS CTX300-4
+
330µF
–5V OUTPUT
150mA
Figure 26. Positive to –5 V Converter
REV. 0
–11–
 

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