Datasheet

ADP1111
–11–
REV. 0
POSITIVE-TO-NEGATIVE CONVERSION
The ADP1111 can convert a positive input voltage to a negative
output voltage as shown in Figure 22. 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 ADP1111’s internal power switch turns off, current
flowing in the inductor forces the output (–V
OUT
) to a negative
potential. The ADP1111 will continue to turn the switch on
until its FB pin is 1.25 V above its GND pin, so the output
voltage is determined by the formula:
V
OUT
= 1. 25 V • 1 +
R2
R1
⎛
⎝
⎜
⎞
⎠
⎟
I
LIM
V
IN
SW1
SW2
FB
GNDSETAO
ADP1111
NC
L1
D1
1N5818
R
LIM
1
INPUT
2 3
6
7 5
4
8
NC
C
INPUT
+
R1
R2
C
L
+
OUTPUT
NEGATIVE
OUTPUT
Figure 22. 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 20.) Also, D1 must again be a Schottky
diode to prevent excessive power dissipation in the ADP1111.
NEGATIVE-TO-POSITIVE CONVERSION
The circuit of Figure 23 converts a negative input voltage to a
positive output voltage. Operation of this circuit configuration is
similar to the step-up topology of Figure 18, except the current
through feedback resistor R2 is level-shifted below ground by a
PNP transistor. The voltage across R2 is V
OUT
–V
BEQ1
. How-
ever, diode D2 level-shifts the base of Q1 about 0.6 V below
ground thereby cancelling the V
BE
of Q1. The addition of D2
also reduces the circuit’s output voltage sensitivity to tempera-
ture, which otherwise would be dominated by the –2 mV V
BE
contribution of Q1. The output voltage for this circuit is
determined by the formula:
V
OUT
= 1. 25 V •
R2
R1
Unlike the positive step-up converter, the negative-to-positive
converter’s output voltage can be either higher or lower than the
input voltage.
I
LIM
V
IN
SW1
SW2
FB
GNDSETAO
ADP1111
NC
D1
1N5818
1
2
3
6
7 5 4
8
NC
C2
+
R1
10kΩ
C
L
+
POSITIVE
OUTPUT
R2
MJE210
R
LIM
NEGATIVE
INPUT
L1
D2
2N3906
Q1
Figure 23. ADP1111 Negative-to-Positive Converter
LIMITING THE SWITCH CURRENT
The ADP1111’s R
LIM
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 V
IN
rises to 4 V, however, the switch current will
exceed 1.6 A. The ADP1111 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
IN
increases. The relationship between R
LIM
and maximum switch current is shown in Figure 6.
The I
LIM
feature is also valuable for controlling inductor current
when the ADP1111 goes into continuous-conduction mode.
Table I. Component Selection for Typical Converters
Input Output Output Circuit Inductor Inductor Capacitor
Voltage Voltage Current (mA) Figure Value Part No. Value Notes
2 to 3.1 5 90 mA 4 15 μH CD75-150K 33 μF*
2 to 3.1 5 10 mA 4 47 μH CTX50-1 10 μF
2 to 3.1 12 30 mA 4 15 μH CD75-150K 22 μF
2 to 3.1 12 10 mA 4 47 μH CTX50-1 10 μF
5 12 90 MA 4 33 μH CD75-330K 22 μF
51230mA 447μH CTX50-1 15 μF
6.5 to 11 5 50 mA 5 15 μH47μF**
12 to 20 5 300 mA 5 56 μH CTX50-4 47 μF**
20 to 30 5 300 mA 5 120 μH CTX100-4 47 μF**
5–57mA 656μH CTX50-4 47 μF
12 –5 250 mA 6 120 μH CTX100-4 100 μF**
NOTES
CD = Sumida.
CTX = Coiltronics.
**Add 47 Ω from I
LIM
to V
IN
.
**Add 220 Ω from I
LIM
to V
IN
.
REV. A