Increasing Output Voltage and Current Range Using SeriesConnected Isolated µModule Converters – Design Note 530 Jesus Rosales and Willie Chan 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. The LTM ®8057 and LTM8058 UL60950-recognized 2kV AC isolated µModule converters will be used to demonstrate this design approach, which can also be applied to the LTM8046, LTM8047 and LTM8048. Let’s assume an output of 10V at 300mA is desired from a 20V input. Reviewing the maximum output current curve from Figure 1, we notice that a single LTM8057 is insufficient to meet the output current requirement under these conditions. However, upon noticing that a single LTM8057 can deliver 300mA at 5V from a 20V input, a solution becomes apparent. Since the output voltage is isolated from the input, the outputs of two LTM8057s set at 5V can be connected in series to achieve a 10V output at 300mA (Figure 2). 450 OUTPUT CURRENT (mA) 400 350 300 250 200 150 100 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) DN4JR F01 Figure 1. Typical Maximum Output Current vs Input Voltage 09/14/530 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 the 15V is the same as that of the individual 7.5V modules (Figure 3). 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. Low Output Noise Series-Connected Converters The low output spectrum noise advantage of the LTM8058 with its integrated LDO post regulator can still 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 10VOUT. Figures 5 and 6, respectively, show the output noise spectrum of the LTM8058 under a 100mA load at 10V with the LDO outputs connected in series (Figure 4 schematic) and the flyback outputs connected in series. Linear Technology’s isolated μModule converters provide a simple and compact solution for isolated power at regulated output voltages. The LTM8057 and LTM8058 successfully demonstrate that the output capabilities of isolated μModule converters can be increased by adding one or more isolated modules with the outputs tied in series while still preserving the output noise characteristics. L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. + GND VIN RUN C1 10µF 50V CIN1 10µF 35V C4 4.7µF 50V C6 0.1µF 25V C3 10µF 50V C8 4.7µF 50V C7 0.1µF 25V U1 LTM8057EY SS GND VOUT VIN RUN BIAS VOUT C5 22µF 16V – RTN Figure 3. Two LTM8057 Modules with Outputs Connected in Series Deliver More Than 160mA at 15VOUT from 12VIN U2 LTM8057 SS ADJ R4 6.98k 1% VOUT– BIAS ADJ R2 6.98k 1% VOUT C210V 22µF 16V VOUT GND 0 DN4JR F02 –20 VIN 5V TO 28V GND + C1 10µF 50V 1210 CIN1 10µF 35V C4 4.7µF 50V C6 0.1µF 25V R5 5.9k 1% VOUT1 VIN RUN – VOUT C14 4.7µF 50V C9 0.1µF 25V R7 5.9k 1% –40 –60 –80 –100 C2 22µF 16V –120 BIAS U1 LTM8058 VOUT2 SS BYP ADJ1 GND ADJ2 VOUT1 VIN RUN C7 10µF 50V INTENSITY (dBm) Figure 2. Two LTM8057 Modules with Outputs Connected in Series, Supporting a 10V, 300mA Output Application from 20VIN VOUT– BIAS C5 0.01µF 25V VOUT2 10V C3 10µF 16V R4 162k 1% VOUT2 C8 22µF 16V BYP ADJ1 GND ADJ2 C15 0.01µF 25V R10 162k 1% Data Sheet Download Linear Technology Corporation 4.5 5 DN4JR F05 –20 C16 10µF 16V RTN DNRJR F04 Figure 4. Two LTM8058 Modules Connected with VOUT2 in Series for 10VOUT www.linear.com/LTM8057 www.linear.com/LTM8058 1.5 2 2.5 3 3.5 4 FREQUENCY (MHz) 0 U1 LTM8058 SS 0 0.5 1 Figure 5. Noise Spectrum for Two LTM8058s with the LDO Outputs Connected in Series Under a 100mA, 10VOUT Load INTENSITY (dBm) VIN 5V TO 28V –40 –60 –80 –100 –120 0 0.5 1 1.5 2 2.5 3 3.5 4 FREQUENCY (MHz) 4.5 5 DN4JR F06 Figure 6. Noise Spectrum for Two LTM8058s with the Flyback Outputs Connected in Series Under a 100mA, 10VOUT Load For applications help, call (408) 432-1900 dn530f LT/AP 0914 111K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2014