ADP1073 Просмотр технического описания (PDF) - Analog Devices
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ADP1073 Datasheet PDF : 16 Pages
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VIN
ADP1073 Q3
RLIM
(EXTERNAL)
ILIM
R1
80⍀
(INTERNAL)
SW1
DRIVER
OSCILLATOR
Q2
Q1
SW2
Figure 21. Current Limit Operation
Programming the Gain Block
The gain block of the ADP1073 can be used as a low battery
detector, 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
ADP1073’s 212 mV reference, while the noninverting input is
available at the SET pin. The NPN output transistor will sink
about 100 µA.
Figure 22a shows the gain block configured as a low-battery
monitor. Resistors R1 and R2 should be set to high values to
reduce quiescent current, but not so high that bias current in
the SET input causes large errors. A value of 100 kΩ for R2 is a
good compromise. The value for R1 is then calculated from the
formula:
R1 = V LOBATT − 212 mV
212 mV
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.
+5V
VBAT
R1 ADP1073
212mV
SET REF
VIN
AO
100k⍀
TO
PROCESSOR
GND
R2
( ) R1 = R2 VLB –1
212mV
VLB = BATTERY TRIP POINT
Figure 22a. Setting the Low Battery Detector Trip Point
ADP1073
+5V
VBAT
R1
R2
33k⍀
ADP1073
212mV
SET REF
GND
VIN
AO
47k⍀
TO
PROCESSOR
R3
1.6M⍀
Figure 22b. Adding Hysteresis to the Low Battery Detector
The circuit of Figure 22a may produce multiple pulses when
approaching the trip point, due to noise coupled into the SET
input. To prevent multiple interrupts to the digital logic, hyster-
esis can be added to the circuit (Figure 22b). Resistor RHYS, with
a value of 1 MΩ to 10 MΩ, provides the hysteresis. The addi-
tion of RHYS will change the trip point slightly, so the new value
for R1 will be:
R1= V LOBATT – 212 mV
212 mV
R2
–
VRL L–+21R2HmYSV
where VL is the logic power supply voltage, RL is the pull-up
resistor and RHYS creates the hysteresis.
The gain block can also be used as a control element to reduce
output ripple. The ADP3000 is normally recommended for low-
ripple applications, but its minimum input voltage is 2 V. The
gain-block technique using the ADP1073 can be useful for step-
up converters operating down to 1 V.
A step-up converter using this technique is shown in Figure 23.
This configuration uses the gain block to sense the output volt-
age and control the comparator. The result is that the compara-
tor hysteresis is reduced by the open loop gain of the gain block.
Output ripple can be reduced to only a few millivolts with this
technique, versus a typical value of 150 mV for a +5 V converter
using just the comparator. For best results, a large output
capacitor (1000 µF or more) should be specified. This tech-
nique can also be used for step-down or inverting applications,
but the ADP3000 is usually a more appropriate choice. See the
ADP3000 data sheet for further details.
VBAT
R3
680k⍀
L1
D1
ILIM VIN
SW1
R1
AO
ADP1073 SET
FB
GND SW2
R2
VOUT
C1
( ) ( ) VOUT =
R1 +1
R2
212mV
Figure 23. Using the Gain Block to Reduce Output Ripple
REV. 0
–11–
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