ADP1111 Analog Devices, ADP1111 Datasheet - Page 11

• Manufacturer Part Number: ADP1111
• Description: Micropower, Step-Up/Step-Down SW Regulator; Adjustable and Fixed 3.3 V, 5 V, 12 V
• Manufacturer: Analog Devices

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
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:
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
ever, diode D2 level-shifts the base of Q1 about 0.6 V below
ground thereby cancelling the V
REV. 0
INPUT
C
INPUT
Figure 22. Positive-to-Negative Converter
+
Input
Voltage
2 to 3.1
2 to 3.1
2 to 3.1
2 to 3.1
5
5
6.5 to 11
12 to 20
20 to 30
5
12
NOTES
CD = Sumida.
CTX = Coiltronics.
**Add 47 Ω from I
**Add 220 Ω from I
R
LIM
AO
I
NC
LIM
1
6
V
ADP1111
OUT
SET
V
NC
2
7
IN
Output
Voltage
5
5
12
12
12
12
5
5
5
–5
–5
GND
= 1.25 V • 1 +
SW1
3
5
LIM
LIM
SW2
1N5818
FB
to V
to V
BE
D1
4
8
IN
of Q1. The addition of D2
IN
.
.
⎛
⎜
⎝
Output
Current (mA)
90 MA
30 mA
7 mA
90 mA
10 mA
30 mA
10 mA
50 mA
300 mA
300 mA
250 mA
Table I. Component Selection for Typical Converters
L1
R2
R2
R1
R1
OUT
⎞
⎟
⎠
OUT
+
OUTPUT
–V
) to a negative
C
L
BEQ1
NEGATIVE
OUTPUT
. How-
Circuit
Figure
4
4
4
4
4
4
5
5
5
6
6
–11–
Inductor
Value
15 μH
47 μH
15 μH
47 μH
33 μH
47 μH
15 μH
56 μH
120 μH
56 μH
120 μH
also reduces the circuit’s output voltage sensitivity to tempera-
ture, which otherwise would be dominated by the –2 mV V
contribution of Q1. 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 ADP1111’s R
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
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
and maximum switch current is shown in Figure 6.
The I
when the ADP1111 goes into continuous-conduction mode.
NEGATIVE
Figure 23. ADP1111 Negative-to-Positive Converter
INPUT
LIM
C2
feature is also valuable for controlling inductor current
IN
+
Inductor
Part No.
CD75-150K
CTX50-1
CD75-150K
CTX50-1
CD75-330K
CTX50-1
CTX50-4
CTX100-4
CTX50-4
CTX100-4
rises to 4 V, however, the switch current will
R
LIM
AO
NC
I
1
LIM
6
IN
ADP1111
LIM
SET
increases. The relationship between R
NC
7
V
V
pin permits the switch current to be
2
IN
OUT
GND
5
SW2
= 1.25 V •
SW1
L1
Capacitor
Value
33 μF
10 μF
22 μF
10 μF
22 μF
15 μF
47 μF
47 μF
47 μF
47 μF
100 μF
4
FB
3
8
1N5818
MJE210
D1
R1
R2
R2
R1
Q1
Notes
*
**
**
**
**
ADP1111
10kΩ
2N3906
D2
+
C
L
POSITIVE
OUTPUT
LIM
BE