advertisement 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 www.linear.com 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 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2003