Easy Balanced Load Sharing for Three or Four Supplies, Even with Unequal Supply Voltages Vladimir Ostrerov and Chris Umminger Using multiple small power supplies is often more economical and more reliable than using a single large power supply. For instance, separate batteries can be used for higher reliability. In a multi-supply system, it is important that the load is equally shared; otherwise, one supply may attempt to carry the entire load. This article shows how to easily load balance three or four supplies by cascading LTC4370 circuits. The LTC4370 controller enables current sharing between two supplies with a modest difference between the output voltages, as shown in Figure 1. To perfectly balance the current in both sides, the controller regulates the gate-source voltage of an N-channel MOSFET in whichever side has the higher voltage. This creates a voltage drop across the MOSFET’s RDS(ON) plus the current sense resistor. The LTC4370 can compensate for a voltage difference between two rails of up to 0.5V. If the voltage difference of the two supplies is somewhat less than 0.5V, the Figure 1. The LTC4370 currentbalancing controller enables balanced load sharing between two supplies, even when their voltage outputs are different. of the total load current equally. The output voltage at the load is less than the minimum of the supply voltages V1, V2 and V3. Because there are two stages of cascading, it is possible to have as much as 1V difference between V3 and V1 or V2, if the difference between V1 and V2 is already at the 0.5V limit. LTC4370 can regulate its output to match the lower value rail, set by adding an appropriate resistor on the RANGE pin. BALANCING THE LOAD BETWEEN THREE SUPPLIES WITH TWO CASCADED LTC4370s Figure 2 shows a 3-input, 12V system delivering 10A. Notice that one LTC4370 (U1) performs equal current sharing between supplies V1 and V2, while the second LTC4370 (U2) implements a 2:1 relation between the output current of U1 and the current of a third supply, V3. Thus, each supply contributes one third 39nF* 0.1µF NC VIN1 GATE1 OUT1 VCC OUT2 EN2 CPO2 VIN2 *OPTIONAL, FOR FAST TURN-ON 2mΩ 11.875V 0.18µF SUM85N03-06P + 25mV – 5A 26 | May 2016 : LT Journal of Analog Innovation 10A GATE2 COMP 39nF* 11.9V 2mΩ FETON2 RANGE •Sense resistor tolerance, worst-case for 1% resistors is 2% overall. 11.875V FETON1 LTC4370 GND LIMITATIONS •LTC4370 error amplifier input offset, ±2mV (maximum) + 325mV – SUM85N03-06P EN1 CPO1 Cascading three LTC4370 controllers (Figure 2) allows four supplies to share the load. In the first stage, U1 and U2 force equal sharing between a pair of supplies, where the output current of U1 is I12 = I1 + I2 , and the output current of U2 is I34 = I3 + I4 . A third LTC4370, the second stage, keeps I12 = I34 . Thus, each supply contributes one fourth of the total load current. The two stages, as above, allow the possibility of as much as 1V difference between the four supply voltages. The main error sources that affect perfect current sharing are: 5A 12.2V BALANCING THE LOAD BETWEEN FOUR SUPPLIES Sharing error attributed to the error amplifier input offset decreases with increasing sense voltage, but power dissipation increases. For the simple LTC4370 circuit with two supplies, this error is expressed as an imbalance in the supplies’ sharing of current: design ideas 12.4V EN1 SUM85N03-06P ∆I = I1 − I2 39nF 50V EN1 CPO1 VIN1 10k Using the worst-case errors, above, the error is: GATE1 OUT1 VCC VCC 0.1µF U1 LTC4370 GND NC 2mΩ FETON2 2mΩ RANGE 10k VIN2 GATE2 COMP 39nF 50V 12.0V Figure 2. Two LTC4370s can be cascaded to enable current sharing of three supplies. 2mV ∆I ≤ + 0.01• ILOAD [A] RSENSE I12 = I1 + I2 = ILOAD I2 = ILOAD OUT2 EN2 CPO2 EN2 I1 = ILOAD FETON1 For the circuit of Figure 2, where ideal load sharing means the load is distributed into 1⁄3ILOAD and 2⁄3ILOAD , it is easier to estimate the worst-case imbalance via an expression of the maximum and minimum current of each supply: CCOMP 0.18µF RCOMP 15k SUM85N03-06P SUM85N03-06P 39nF 50V EN1 EN1 CPO1 VIN1 10k 2mV IMAX = 0.672 • ILOAD + [A] 3.01• RSENSE GATE1 OUT1 VCC VCC 0.1µF U2 LTC4370 GND NC 2mΩ FETON2 4mΩ VIN2 10k GATE2 COMP 39nF 50V 12.0V V1 EN1 SUM85N03-06P VIN1 GATE1 OUT1 VCC U1 LTC4370 GND NC 2mΩ FETON2 2mΩ OUT2 VIN2 EN2 CPO2 GATE2 COMP 39nF 50V Figure 3. Four supplies can each support an equal share of a load by using three LTC4370s in a 2-stage cascade. V2 V3 I1 = ¼ILOAD FETON1 RANGE 10k By cascading the shared output of one LTC4370 with another LTC4370, three or more supplies can be efficiently controlled to provide equal current to the load. With errors on the order of the sense resistor tolerance, the voltage drop is minimal. n 39nF 50V EN1 CPO1 EN2 CCOMP 0.18µF RCOMP 15k SUM85N03-06P SUM85N03-06P EN1 EN1 CPO1 OUT1 VCC VCC 0.1µF U2 LTC4370 GND NC 10k 2mΩ FETON2 2mΩ VIN2 GATE2 COMP 39nF 50V V4 GND I12 = ½ILOAD FETON1 2mΩ FETON2 2mΩ RANGE VIN2 GATE2 COMP 39nF 50V ILOAD I34 = ½ILOAD OUT2 I3 = ¼ILOAD FETON1 OUT2 RANGE EN2 CPO2 EN2 GATE1 U3 LTC4370 EN2 CPO2 10k GATE1 OUT1 VCC NC VIN1 VIN1 10k 39nF 50V EN1 CPO1 39nF 50V VCC 0.1µF SUM85N03-06P 10k I12 = I1 + I2 = ½ILOAD I1 = ¼ILOAD EN2 EN1 CONCLUSION CCOMP 0.18µF RCOMP 15k SUM85N03-06P 10k VCC 0.1µF ILOAD I3 = ILOAD OUT2 EN2 CPO2 EN2 FETON1 RANGE 2mV IMIN = 0.328 • ILOAD + [A] 3.01• RSENSE I12 = I1 + I2 = ILOAD CCOMP 0.18µF RCOMP 15k SUM85N03-06P I4 = ¼ILOAD I34 = I3 + I4 = ½ILOAD CCOMP 0.18µF RCOMP 15k SUM85N03-06P May 2016 : LT Journal of Analog Innovation | 27