Increasing Output Voltage and Current Range Using Series-Connected Isolated µModule Converters Jesus Rosales The LTM®8057 and LTM8058 UL60950recognized 2kV AC isolated µModule converters are used here to demonstrate this design approach, which can also be applied to the LTM8046, LTM8047 and LTM8048. Let’s assume an output of 10V at 300m A is desired from a 20V input. Reviewing the maximum output current curve from Figure 1, we VIN 5V TO 28V GND + CIN1 10µF 35V C4 4.7µF 50V C6 0.1µF 25V R2 6.98k 1% Figure 2. Two LTM8057 µModule regulators with outputs connected in series, supporting a 10V, 300mA output application from a 20V input 24 | October 2014 : LT Journal of Analog Innovation However, upon noticing that a single LTM8057 can deliver 300m A at 5V from a 20V input, a solution becomes apparent. Since the output voltage is isolated from the input, the outputs of two VIN RUN C1 10µF 50V C3 10µF 50V notice that a single LTM8057 is insufficient to meet the output current requirement under these conditions. C8 4.7µF 50V C7 0.1µF 25V R4 6.98k 1% VOUT VOUT– BIAS VOUT C210V 22µF 16V 450 400 OUTPUT CURRENT (mA) Linear Technology’s isolated µModule® converters are compact solutions for breaking ground loops. These converters employ a flyback architecture whose maximum output current varies with input voltage and output voltage. Although their output voltage range is limited to a maximum of 12V, one can increase the output voltage or the output current range. The solution simply involves connecting the secondary side of two or more isolated µModule converters in series. 350 300 250 200 150 100 50 0 2.5V 3.3V 5V 7.5V 10V 12V 4 6 8 10 12 14 16 18 20 22 23 24 28 30 VIN (V) Figure 1. Typical maximum output current vs input voltage LTM8057s set at 5V can be connected in series to achieve a 10V output at 300m A (Figure 2). The same circuit in Figure 2 can also be used to increase the output voltage range when more than 12V is needed. By adjusting the feedback resistors to provide a 7.5V nominal output voltage, the combined output voltage has increased to 15V. The output current capability for U1 LTM8057EY SS ADJ GND VOUT VIN RUN BIAS VOUT – C5 22µF 16V RTN U2 LTM8057 SS ADJ GND Figure 3. Two LTM8057 µModule regulators with outputs connected in series deliver more than 160mA at 15V output, 12V input. design ideas The output capabilities of isolated μModule converters can be increased by adding one or more isolated µModule converters with the outputs tied in series while preserving the output noise characteristics. the 15V is the same as that of the individual 7.5V µModule regulator (Figure 3). VIN 5V TO 28V GND The circuit shown in Figure 2 supports a third option: providing positive and negative outputs with a common return. The return node for both outputs is the common connection in the middle of the output stack. With this approach the circuit in Figure 2 would have 5V and –5V outputs. Each output can be of different magnitude, since the output voltages for each converter are determined independently. C1 10µF 50V 1210 C6 0.1µF 25V R5 5.9k 1% VOUT1 VIN RUN C14 4.7µF 50V C9 0.1µF 25V R7 5.9k 1% C2 22µF 16V VOUT– BIAS U1 LTM8058 VOUT2 SS C5 0.01µF 25V BYP ADJ1 GND ADJ2 C3 10µF 16V C8 22µF 16V VOUT– BIAS VOUT2 10V R4 162k 1% VOUT1 VIN RUN C7 10µF 50V U1 LTM8058 SS VOUT2 C15 0.01µF 25V BYP ADJ1 GND ADJ2 R10 162k 1% C16 10µF 16V RTN Figure 4. Two LTM8058 µModule regulators connected with VOUT2 in series for 10V output 0 0 –20 –20 –40 –40 INTENSITY (dBm) INTENSITY (dBm) Linear Technology’s isolated μModule converters provide a simple and compact solution for isolated power at regulated output voltages. The LTM8057 and LTM8058 solutions shown here successfully demonstrate that the output capabilities of isolated μModule converters can be increased by adding one or more isolated μModule converters with the outputs tied in series while preserving the output noise characteristics. n CIN1 10µF 35V C4 4.7µF 50V LOW OUTPUT NOISE SERIESCONNECTED CONVERTERS The low output spectrum noise advantage of the LTM8058 with its integrated LDO post regulator can be maintained with series-connected outputs. Figure 4 shows the schematic for two LTM8058s with VOUT2, the output of the LDO connected in series for 10V output. Figures 5 and 6, respectively, show the output noise spectrum of the LTM8058 under a 100m A load at 10V with the LDO outputs connected in series (Figure 4 schematic) and the flyback outputs connected in series. + –60 –80 –80 –100 –100 –120 –60 0 0.5 1 1.5 2 2.5 3 3.5 4 FREQUENCY (MHz) 4.5 5 Figure 5. Noise spectrum for two LTM8058s with the LDO outputs connected in series under a 100mA, 10V output load –120 0 0.5 1 1.5 2 2.5 3 3.5 4 FREQUENCY (MHz) 4.5 5 Figure 6. Noise spectrum for two LTM8058s with the flyback outputs connected in series under a 100mA, 10V output load October 2014 : LT Journal of Analog Innovation | 25