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ADP1108AR-12 View Datasheet(PDF) - Analog Devices

Part Name
Description
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ADP1108AR-12 Datasheet PDF : 12 Pages
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ADP1108
Negative-to-Positive Conversion
The circuit of Figure 20 converts a negative input voltage to a posi-
tive output voltage. Operation of this circuit configuration is similar
to the step-up topology of Figure 15, except that 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 would
otherwise be dominated by the Ϫ2 mV/ЊC VBE contribution of Q1.
The output voltage for this circuit is determined by the formula:
V OUT
= 1.245 V
×

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.
D1
L1
1N5818
POSITIVE
OUTPUT
RLIM
R1
CL
1
2
3
Q1
ILIM VIN
SW1 2N3906
D2
C2
ADP1108 FB 8
1N4148
AO SET GND SW2
6
7
5
4
R2
10k
NEGATIVE
INPUT
NC NC
Figure 20. A Negative-to-Positive Converter
Limiting the Switch Current
The ADP1108’s RLIM pin permits the switch current to be lim-
ited 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 ex-
ceeding 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 ex-
ceed 1.6 A. The ADP1108 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 Figures 3 and 4.
The ILIM feature is also valuable for controlling inductor current
when the ADP1108 goes into continuous-conduction mode. This
occurs in the step-up mode when the following condition is met:
VOUT + V DIODE < 1
V IN V SW
1 – DC
where DC is the ADP1108’s duty cycle. When this relationship
exists, the inductor current does not go all the way to zero dur-
ing 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 21).
This increases output ripple and can require a larger inductor
and capacitor. By controlling switch current with the ILIM resis-
tor, output ripple current can be maintained at the design val-
ues. Figure 22 illustrates the action of the ILIM circuit.
50µs
100
90
VOUT = + 5V
IL = 100mA
L = 120µH
RLIM = 0
10
0%
200mA
VIN = 2.23V
Figure 21. (ILIM Operation, RLIM = 0 )
50µs
100
90
VOUT = + 5V
IL = 100mA
L = 120µH
RLIM = 120
VIN = 2.23V
10
0%
200mA
Figure 22. (ILIM Operation, RLIM = 120 )
The internal structure of the ILIM circuit is shown in Figure 23.
Q1 is the ADP1108’s internal power switch, which is paralleled
by sense transistor Q2. The relative sizes of Q1 and Q2 are
scaled so that IQ2 is 0.5% of IQ1. Current flows to Q2 through an
internal 80 resistor and through the RLIM resistor. These two
resistors parallel the base-emitter junction of the oscillator-
disable transistor, Q3. When the voltage across R1 and RLIM
exceeds 0.6 V, Q3 turns on and terminates the output pulse. If
only the 80 internal resistor is used (i.e. the ILIM pin is connected
directly to VIN), the maximum switch current will be 1.5 A.
VIN
ADP1108 Q3
RLIM
ILIM
(EXTERNAL)
R1
80
(INTERNAL)
SW1
DRIVER
OSCILLATOR
Q2
Q1
SW2
Figure 23. ADP1108 Current Limit Operation
The delay through the current limiting circuit is approximately
2 µs. If the switch ON time is reduced to less than 5 µs, accu-
racy of the current trip-point is reduced. Attempting to program
a switch ON time of 2 µs or less will produce spurious responses
in the switch ON time. However, the ADP1108 will still provide
a properly regulated output voltage.
–10–
REV. 0
 

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