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

```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
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
GND
VOUT
VIN
RUN
BIAS
VOUT
–
C5
22µF
16V
RTN
U2
LTM8057
SS
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
GND
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
GND
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,