DN318 - Efficient Dual Polarity Output Converter Fits into Tight Spaces

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Efficient Dual Polarity Output Converter Fits into Tight Spaces
Design Note 318
Keith Szolusha
Introduction
This Design Note describes a compact and efficient ±5V
output dual polarity converter that uses a single buck
regulator. The topology shown features 3mm maximum
circuit height, high efficiency and low output voltage
ripple on a 5V output—important considerations for
many battery-powered, handheld and noise-sensitive
devices. This combination of features is not easily
achievable with other commonly used dual polarity
topologies. For instance, one alternative topology, a
flyback converter using a boost regulator, is relatively
inefficient, requires a bulky (5mm or taller) transformer
and generates high output voltage ripple. Another alternative, using two buck regulators, incurs both the
cost of the additional regulator and the cost of the PCB
real estate it occupies.
The single buck regulator topology shown here requires
few components. To reduce the maximum circuit height,
it uses two power inductors instead of a transformer.
In the absence of the transformer core, the coupling
capacitor allows energy to pass between the positive
and negative sides of the circuit while maintaining a
voltage potential between the two inductors, indirectly
regulating the negative output.
VIN
9V TO 16V
36V TRANSIENT
CIN
2.2μF
50V
CER
2.2k
3300pF
BOOST
VIN
VSW
SYNC VBIAS
LT1956
SHDN
VC
FB
GND
L1
0.1μF 1N4148W
CDRH4D28-150
V
SW
15.4k
4.99k
220pF
*SEE FIGURE 3 FOR MAXIMUM LOAD CURRENT
CCOUP
10μF
6.3V
0805
CER
X5R
B0540W
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.
VIN
9V TO 16V
36V TRANSIENT
VOUT
5V*
COUT
10μF 6.3V
0805 X5R
CER
COUT2
10μF 6.3V
0805 X5R
CER
VOUT2
DN318 F01 –5V*
B0540W
Figure 1. LT1956 9V to 16V Input (with 36V Transients),
±5V Output, 3mm Height All Ceramic Dual Polarity
Converter with High ΔI/Δt Crucial Layout Path Indicated
in Bold
08/03/318_conv
Typical Bucks with Second, Negative Outputs
The dual polarity output configuration is similar to
the typical buck regulator with a second, negative
polarity output added to the circuit using a coupling
capacitor and a second inductor, catch diode and
output capacitor. The duty cycle remains the same as
the typical buck regulator with the same VIN and VOUT.
The positive 5V output maintains its low output voltage ripple characteristic from the buck regulator, but
some of the current available for that output is rerouted
through the coupling capacitor to the second output.
The coupling capacitor charges up to and maintains a
voltage equal to the output voltage (5V). This induces
CIN
2.2μF
50V
CER
1.5k
10nF
L2
CDRH4D28-150
VSWN
12V Input, ±5V Output, Only 3mm High
A dual polarity output converter uses a buck regulator, such as the LT®1956 or LT3431. Both of these
are 500kHz, 1.5A/3A peak switch current monolithic
switchers. Figure 1 shows a 12V battery input (9V to
16V input with 36V transients) to ±5VOUT dual polarity
output converter using the LT1956EFE. Figure 2 shows
the same circuit with twice the load current rating using
the LT3431EFE.
BOOST
VIN
VSW
SYNC VBIAS
LT3431
SHDN
VC
FB
GND
0.22μF 1N4148W L1
CDRH6D28-100
VSW
15.4k
4.99k
220pF
*SEE FIGURE 6 FOR MAXIMUM LOAD CURRENT
CCOUP
10μF
6.3V
0805
CER
X5R
VOUT
5V*
B140
COUT
22μF 6.3V
X5R
CER
L2
CDRH6D28-100
COUT2
22μF 6.3V
X5R
CER
VOUT2
DN318 F02 –5V*
VSWN
B140
Figure 2. LT3431 9V to 16V Input (with 36V Transients),
±5V Output, 3mm Height All Ceramic Dual Polarity
Converter with High ΔI/Δt Crucial Layout Path Indicated
in Bold
the same voltage and hence the same current ripple
across both inductors. However, the average current
in L1 is the 5V load current while the average current
in L2 is the negative load current.
The maximum load current, shown in Figures 3 and 6, is
reached when the sum of the peak inductor currents is
equal to the peak switch current rating of the regulator,
1.5A (LT1956) or 3A (LT3430), or the negative output
loses regulation. The peak switch current region is to
the right of the peak in the curves. To the left of the
peak in the curves, 1.5A and 3A cannot be reached by
increasing the negative load current without losing over
3% regulation on VOUT2.
Extremely low negative load currents can also cause a
loss of regulation on VOUT2 as seen in Figures 4 and 7.
In order to maintain relatively good regulation in low
load applications, a preload resistor on VOUT2 of 12mA
(LT1956) or 25mA (LT3430) may be required. Feedback
is derived directly from VOUT so that its load current
can go to zero without a loss of regulation.
Conclusion
The LT1956- and LT3431-based dual polarity output
converters provide power for ±5V loads with a single
buck regulator. This design offers size and efficiency
advantages over other dual output designs, especially
those that require a transformer.
IOUT(MAX) [5V] = 1.5/3 – IOUT2 [–5V] – 2 • ILP-P/2
(for peak switch current region)
ILP-P = (VIN – 5VOUT )• DC/(L • 500kHz)
5.30
LT1956
500
90
5.20
12VIN
300
9VIN
200
5.10
5.05
750mA
5.00
4.95
250mA
4.85
0
1000
200
400
VOUT2 LOAD CURRENT (mA)
1200
5.30
1000
100
300
400
200
VOUT2 LOAD CURRENT (mA)
500
Figure 5. The Efficiency of Figure 1
100
LT3431
12VIN
LT3431
12VIN
95
5.20
16VIN
90
|VOUT2| (V)
600
9VIN
400
5.10
5.05
1.5A
5.00
4.95
1A
4.90
2000
1.5A
75
60
4.75
0
DN318 F06
Figure 6. Maximum Load Current
Conditions for Figure 2
1A
80
65
4.80
1500
500
1000
VOUT LOAD CURRENT (mA)
500mA
85
70
500mA
4.85
200
EFFICIENCY (%)
5.15
12VIN
0
0
DN318 F05
Figure 4. The Negative Supply
(VOUT2) Maintains ± 5% Regulation
5.25
0
40
DN318 F04
LT3431
800
50
0
600
DN318 F03
Figure 3. Maximum Load Current
Conditions for Figure 1.
60
10
4.75
200
400
600
800
VOUT LOAD CURRENT (mA)
250mA
70
20
500mA
4.80
1
LT1956
12VIN
750mA
30
4.90
100
500mA
80
5.15
EFFICIENCY (%)
16VIN
400
0
MAXIMUM VOUT2 LOAD CURRENT (mA)
100
LT1956
12VIN
5.25
|VOUT2| (V)
MAXIMUM VOUT2 LOAD CURRENT (mA)
600
200
400
600
800
VOUT2 LOAD CURRENT (mA)
1000
0
200
400
800
600
VOUT2 LOAD CURRENT (mA)
DN318 F08
DN318 F07
Figure 7. The Negative Supply
(VOUT2) Maintains ± 5% Regulation
Data Sheet Download
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Linear Technology Corporation
1000
Figure 8. The Efficiency of Figure 2
For applications help,
call (408) 432-1900
dn318f_conv LT/TP 0803 316.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 2003
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