DN47 - Switching Regulator Generates Both Positive and Negative Supply with a Single Inductor - Design Note 47

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Switching Regulator Generates Both Positive and Negative
Supply with a Single Inductor – Design Note 47
Brian Huffman
Many systems require ±12V from a 5V input. Analog or
RS-232 driver power supplies are obvious candidates.
This requirement is usually solved by using a switcher
with a multiple-secondary transformer or multiple
switchers. These solutions can be complicated, requiring either transformer design or two inductors. An
alternative approach, shown in Figure 1, uses a single
inductor and charge pump to obtain the dual outputs.
This solution is particularly noteworthy because is uses
off-the-shelf components.
Figure 1 uses an LT®1172 to generate both the positive and negative supply. The LT1172 is configured as
a step-up converter to obtain the positive output. To
D1
MUR110
L1
50μH
+
+
C1
100μF
10V
VIN
5V
R1*
13.0k
VIN
E1
+VOUT
12V
100mA
L2**
5μH
VSW
+
LT1172CN8
C2
100μF
16V
+
C5**
100μF
16V +
VFB
E2
GND
VC
2k
0.02μF
C3
100μF
16V
R2*
1.5k
D2
1N5819
D3
1N5819
* 1% FILM RESISTORS
** OPTIONAL FILTER
D1 = MOTOROLA – MUR110
C1 = NICHICON – UPL1A101MRH6
C2, C3, C4, C5, C6 = NICHICON – UPL1C101MAH
L1 = COILTRONICS – CTX50-1-52
L2, L3 = COILTRONICS – CTX5-2-FR
VOUT = 1.25V (1 + R1/R2)
C4
100μF
16V
+
C6** +
100μF
16V
L3**
5μH
–VOUT
–12V
100mA
-5%/t5"
Figure 1. Inductor and Switch Capacitor Techniques
Provide Bipolar Output
??/??/47_conv
generate the negative output a charge pump is used.
The pump capacitor, C2, is charged up by the inductor
when D2 is forward biased and discharges into C4 when
the LT1172’s power switch pulls the positive side of C2
to ground. The output capacitor provides current to the
load during the charging cycle.
Figure 2 shows the regulator’s operating waveforms.
Since the LT1172 has a ground-referred power switch,
the inductor has the input voltage applied across it when
the switch is on. Trace A is the VSW pin voltage and
trace B is its current. The inductor current, trace C, rises
slowly as the magnetic field builds up. The current rate
of change is determined by the voltage applied across
the inductor and its inductance. During this interval,
energy is being stored in the inductor and no power
is transferred to the +12V output. When the switch is
turned off, energy is no longer transferred to the inductor, which causes the magnetic field to collapse. The
collapsing magnetic field induces a change in voltage
across the inductor causing the VSW pin to rise until
output diode D1 forward biases.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
A = 20V/DIV
VSW
B = 1A/DIV
C = 1A/DIV
ISW
IL1
D = 0.5A/DIV
ID1
E = 20V/DIV
VD2
F = 0.5A/DIV
IC3
G = 0.5A/DIV
ID2
H = 0.5A/DIV
ID3
HORIZ = 2μs/DIV
-5%/t5"
Figure 2. Switching Waveforms for ±12V Output
Converter
Trace D is the diode’s current waveform. The diode
provides a current path for the energy stored in the
inductor to be transferred between the load and the
output capacitor. When the diode is reverse biased,
the output capacitor provides the load current. The
LT1172’s error amplifier compares the feedback pin
voltage, from the 13kΩ–1.5kΩ divider, to its internal
1.24V reference and controls duty cycle. The output
voltage can be varied by changing the R1–R2 divider
ratio (see Equation 1). An RC network at the VC pin
provides loop compensation.
A charge pump is used to invert the +12V output to a
–12V output. When the LT1172’s power switch turns
off, the voltage on C2’s positive side rises until D1 is
forward biased. The inductor charges C2 when the
voltage on C2’s negative side rises enough to forward
bias D2. Trace F shows C2’s current waveform, trace
E is D2’s voltage waveform and trace G is its current.
The voltage across C2 will be equal to a diode drop
above +VOUT minus a Schottky diode drop. When the
LT1172’s power transistor turns on, the positive side of
C2 is pulled to ground. During this period diode D3 is
forward biased (trace H is its current waveform), and C4
is charged by C2. An optional LC filter is added to each
output to attenuated output voltage ripple. Efficiency
for this circuit generally exceeds 70%.
Diode junction losses (D2 and D3) preclude ideal
results, but performance is quite good. This circuit
will convert +VOUT to –VOUT with losses as shown in
Figure 3. Negative output load current should not exceed
the positive output load by more than a factor of 5,
otherwise the imbalance will cause the –12V transient
response to suffer.
Figure 4 can be used for a LCD display contrast control.
It is similar to the previous circuit except that all the load
current is drawn from the negative output. This requires
C3 to be small so negative output load fluctuations are
quickly reflected to the positive output. Resistor R3
adjusts output voltage between –12V to –21V.
The LT1172 provides an elegant solution to power
shutdown problems by integrating a shutdown feature;
eliminating the need to place a power MOSFET in series
with the input voltage. When the voltage of the VC pin
is pulled below 150mV, the IC shuts down pulling only
150μA. This is implemented by turning on Q1, reducing
the circuit’s quiescent current from 6mA to 150μA.
+
+
–12.0
C1
˜'
7
VIN
5V
+VOUT
12V TO 24V
L
VIN
E1
VSW
+
LT1172CN8
VOLTAGE LOSSES (V)
D1
1N4148
L1
˜)
C2
˜'
35V
+
VFB
E2
GND
VC
D2
1N5819
L
–11.5
C3
1μF
35V
681Ω
L
1μF
4)65%08/
D3
1N5819
Q1
–11.0
50
0
C4
˜'
35V
+
100
OUTPUT CURRENT (mA)
-5%/t5"
Figure 3. Losses for Charge Pump Converter
%%.05030-"o/
Q1 = VN2222LL
$/*$)*$0/o61-".")
$$/*$)*$0/o61-7.1)
-$0*-530/*$4o$59o
+VOUT
–12V TO –24V
N"50N"
-5%/t5"
Figure 4. LCD Display Contrast Control Power Supply
Data Sheet Download
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call (408) 432-1900
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