### DN530 - Increasing Output Voltage and Current Range Using Series- Connected Isolated μModule Converters

```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
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
R4
6.98k
1%
VOUT–
BIAS
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
GND
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
GND
C15
0.01µF
25V
R10
162k
1%
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,
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,