ADP1108 Analog Devices, ADP1108 Datasheet - Page 10

• Manufacturer Part Number: ADP1108
• Description: Micropower DC-DC Converter Adjustable and Fixed 3.3 V/ 5 V/ 12 V
• Manufacturer: Analog Devices

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 (V
diode D2 level-shifts the base of Q1 about 0.6 V below ground,
thereby cancelling the V
the circuit’s output voltage sensitivity to temperature, which would
otherwise be dominated by the 2 mV/ C V
The output voltage for this circuit is determined by the formula:
Unlike the positive step-up converter, the negative-to-positive
converter’s output voltage can be either higher or lower than the
input voltage.
Limiting the Switch Current
The ADP1108’s R
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 V
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 V
and maximum switch current is shown in Figures 3 and 4.
The I
when the ADP1108 goes into continuous-conduction mode. This
occurs in the step-up mode when the following condition is met:
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 I
tor, output ripple current can be maintained at the design val-
ues. Figure 22 illustrates the action of the I
NEGATIVE
INPUT
LIM
Figure 20. A Negative-to-Positive Converter
C2
feature is also valuable for controlling inductor current
IN
rises to 4 V, however, the switch current will ex-
R
LIM
I
IN
AO SET
NC
LIM
LIM
1
6
increases. The relationship between R
V
ADP1108
V
OUT
pin permits the switch current to be lim-
V
BE
OUT
NC
7
V
IN
2
IN
of Q1. The addition of D2 also reduces
GND
– V
L1
V
5
1.245V
DIODE
SW
SW2
SW1
FB
4
3
1N5818
8
D1
2N3906
1 – DC
R2
R1
R1
R2
Q1
1
OUT
BE
LIM
contribution of Q1.
–V
circuit.
1N4148
10k
BEQ1
D2
). However,
LIM
C
L
POSITIVE
OUTPUT
resis-
LIM
–10–
The internal structure of the I
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 I
internal 80
resistors parallel the base-emitter junction of the oscillator-
disable transistor, Q3. When the voltage across R1 and R
exceeds 0.6 V, Q3 turns on and terminates the output pulse. If
only the 80
directly to V
The delay through the current limiting circuit is approximately
2 s. If the switch ON time is reduced to less than 5 s, accu-
racy of the current trip-point is reduced. Attempting to program
a switch ON time of 2 s or less will produce spurious responses
in the switch ON time. However, the ADP1108 will still provide
a properly regulated output voltage.
ADP1108
V
Figure 23. ADP1108 Current Limit Operation
IN
Figure 22. (I
Figure 21. (I
IN
OSCILLATOR
internal resistor is used (i.e. the I
resistor and through the R
100
100
), the maximum switch current will be 1.5 A.
0%
90
10
0%
90
10
Q2
Q3
is 0.5% of I
200mA
200mA
50µs
50µs
(EXTERNAL)
LIM
R
DRIVER
LIM
LIM
Operation, R
V
I
Operation, R
L
Q1
OUT
LIM
= 100mA
. Current flows to Q2 through an
= + 5V
circuit is shown in Figure 23.
I
LIM
Q2
R1
80
(INTERNAL)
V
V
I
L = 120µH
R
L
OUT
IN
LIM
V
LIM
= 100mA
L = 120µH
R
IN
LIM
= 2.23V
LIM
= 120
LIM
= 2.23V
= + 5V
resistor. These two
= 0
= 120 )
LIM
= 0
pin is connected
SW1
Q1
)
SW2
REV. 0
LIM