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ADP1110AN-3.3 View Datasheet(PDF) - Analog Devices

Part Name
Description
Manufacturer
ADP1110AN-3.3 Datasheet PDF : 16 Pages
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ADP1110
POSITIVE-TO-NEGATIVE CONVERSION
The ADP1110 can convert a positive input voltage to a negative
output voltage as shown in Figure 23. This circuit is essentially
identical to the step-down application of Figure 19, except that
the “output” side of the inductor is connected to power ground.
When the ADP1110’s internal power switch turns off, current
flowing in the inductor forces the output (–VOUT) to a negative
potential. The ADP1110 will continue to turn the switch on
until its FB pin is 220 mV above its GND pin, so the output
voltage is determined by the formula:
V OUT
= 220 mV
1
+
R1
R2
INPUT
CINPUT
RLIM
1
2
3
VIN ILIM SW1
SW2 4
ADP1110
FB 8
AO SET GND
6
7
5
D1
1N5818
L1
R1
R2
OUTPUT
CL
NC NC
NEGATIVE
OUTPUT
Figure 23. A Positive-to-Negative Converter
The design criteria for the step-down application also apply to
the positive-to-negative converter. The output voltage should be
limited to |6.2 V| unless a diode is inserted in series with the
SW2 pin (see Figure 21.) Also, D1 must again be a Schottky
diode to prevent excessive power dissipation in the ADP1110.
NEGATIVE-TO-POSITIVE CONVERSION
The circuit of Figure 24 converts a negative input voltage to a
positive output voltage. Operation of this circuit configuration is
similar to the step-up topology of Figure 19, except the current
through feedback resistor R1 is level-shifted below ground by a
PNP transistor. The voltage across R1 is VOUT – VBEQ1. However,
diode D2 level-shifts the base of Q1 about 0.6 V below ground
thereby cancelling the VBE of Q1. The addition of D2 also reduces
the circuit’s output voltage sensitivity to temperature, which other-
wise would be dominated by the –2 mV VBE contribution of Q1.
The output voltage for this circuit is determined by the formula:
V OUT
= 220 mV
R1
 R2
Unlike the positive step-up converter, the negative-to-positive
converter’s output voltage can be either higher or lower than the
input voltage.
CINPUT
L1
D1
RLIM
1
2
ILIM
VIN
SW1 3
ADP1110
FB 8
AO SET GND SW2
675 4
R1
Q1
2N3906
CL
D2
1N4148
10K
R2
POSITIVE
OUTPUT
NEGATIVE
INPUT
NC NC
Figure 24. A Negative-to-Positive Converter
LIMITING THE SWITCH CURRENT
The ADP1110’s RLIM pin permits the switch current to be
limited with a single resistor. This current limiting action occurs
on a pulse by pulse basis. This feature allows the input voltage
to vary over a wide range without saturating the inductor or
exceeding the maximum switch rating. For example, a particular
design may require peak switch current of 800 mA with a 2.0 V
input. If VIN rises to 4 V, however, the switch current will
exceed 1.6 A. The ADP1110 limits switch current to 1.5 A and
thereby protects the switch, but the output ripple will increase.
Selecting the proper resistor will limit the switch current to
800 mA, even if VIN increases. The relationship between RLIM
and maximum switch current is shown in Figure 6.
The ILIM feature is also valuable for controlling inductor current
when the ADP1110 goes into continuous-conduction mode.
This occurs in the step-up mode when the following condition is
met:
V

OUT +VD IODE
V IN V SW

<
1–
1
DC
where DC is the ADP1110’s duty cycle. When this relationship
exists, the inductor current does not go all the way to zero
during the time that the switch is OFF. When the switch turns
on for the next cycle, the inductor current begins to ramp up
from the residual level. If the switch ON time remains constant,
the inductor current will increase to a high level (see Figure 25).
This increases output ripple and can require a larger inductor
and capacitor. By controlling switch current with the ILIM
resistor, output ripple current can be maintained at the design
values. Figure 26 illustrates the action of the ILIM circuit.
100
90
200mA/div.
10
0%
10mV
10µs
Figure 25. ILIM Operation—IL Characteristic
100
90
200mA/div.
10
0%
10mV
10µs
Figure 26. ILIM Operation—IL Characteristic
REV. 0
–11–
 

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