lx1675 Microsemi Corporation, lx1675 Datasheet - Page 10

• Manufacturer Part Number: lx1675
• Description: Multiple Output Loadshare?? Pwm Production Data Sheet
• Manufacturer: Microsemi Corporation

Copyright © 2004
Rev. 1.2a, 2006-02-16
equivalent series resistance, (ESR). As can be seen in the
previous example, if the offset error is zero and the ESR of the
two inductors are identical, then the two inductor currents will be
identical.
inductors, the value of the inductor’s ESR can be changed to
allow more current to flow through one inductor than the other.
The inductor with the lower ESR value will have the larger
current. The inductor currents are directly proportional to the
ratio of the inductor’s ESR value.
inductor ESR for each phase where a different amount of power
is taken from two different input power supplies. A typical setup
will have a +5V power supply connected to the phase 1 half
bridge driver and a +3.3V power supply connected to the phase 2
half bridge driver. The combined power output for this core
voltage is 18W (+1.5V @ 12A). For this example the +5V power
supply will supply 7W and the +3.3V power supply will supply
the other 11W. 7W @ 1.5V is a 4.67A current through the phase
1 inductor. 11W @ 1.5V is a 7.33A current through the phase 2
inductor.
the power level split.
the ESR of the phase 1 inductor is selected as 10mΩ, then the
ESR value of the phase 2 inductor is calculated as:
inductors can be chosen from standard vendor tables with an ESR
ratio close to the required values. Inductors can also be designed
for a given application so that there is the least amount of
compromise in the inductor’s performance.
B
The first method is to change the ratio of the inductors
The following circuit description shows how to select the
The higher current inductor will have the lower ESR value. If
Depending on the required accuracy of this power sharing;
I
-P
+3.3V @ 11W
HASE
46.7mV
TM
1.5V +
+5V @ 7W
The ratio of inductor ESR is inversely proportional to
To change the ratio of current between the two
, L
Figure 7 –LoadSHARE Using Inductor ESR
OAD
SHARE (ESR M
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11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
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L2
L1
A
A
ESR
⎞
⎟
⎠
ESR
6.4mΩ
×
10
10mΩ
1
2
mΩ
=
T H E O R Y O F O P E R A T I O N
2 I
1 I
ETHOD
=
4.67A
6
7.33A
4 .
mΩ
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1.5V @ 12A
Integrated Products Division
18W
Microsemi
®
while having a much larger current in one phase versus the other.
A simple resistor divider can be used on the input side of the Low
Pass Filter that is taken off of the switching side of the inductors. If
the Phase 2 current is to be larger than the current in Phase 1; the
resistor divider is placed in the feedback path before the Low Pass
Filter that is connected to the Phase 2 inductor. If the Phase 2
current needs to be less than the current in Phase 1; the resistor
divider is then placed in the feedback path before the Low Pass
Filter that is connected to the Phase 1 inductor.
divider network in the feedback path, appears as a voltage
generator between the ESR of the two inductors.
voltage generator to be positive at Phase 2. With a divider in the
feedback path of Phase 1 the voltage generator becomes positive at
Phase 1. The Phase with the positive side of the voltage generator
will have the larger current. Systems that operate continuously
above a 30% power level can use this method.
inductors still flows during a no load condition. This produces a
low efficiency condition during a no load or light load state, this
method should not be used if a wide range of output power is
required.
the value of the resistor divider network required to generate the
offset voltage necessary to produce the different current ratio in the
two output inductors. The power sharing ratio is the same as that
of Figure 7. The Offset Voltage Generator is symbolic for the DC
voltage offset between Phase 1 & 2. This voltage is generated by
small changes in the duty cycle of Phase 2. The output of the LPF
is a DC voltage proportional to the duty cycle on its input. A small
amount of attenuation by a resistor divider before the LPF of Phase
2 will cause the duty cycle of Phase 2 to increase to produce the
added offset at V2. The high DC gain of the error amplifier will
force LPF2 to always be equal to LPF1. The following calculations
determine the value of the resistor divider necessary to satisfy this
example.
B
Sometimes it is desirable to use the same inductor in both phases
A divider in the feedback path from Phase 2 will cause the
A down side is that the current difference between the two
The following description and Figure 8 show how to determine
Multiple Output LoadSHARE™ PWM
As in Figure 7, the millivolts of DC offset created by the resistor
I
-P
HASE
( C O N T I N U E D )
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P
OAD
RODUCTION
SHARE (F
EEDBACK
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ATA
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HEET
IVIDER
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ETHOD
LX1675
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Page 10