LX1675

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 10

Copyright

© 2004

Rev. 1.2a, 2006-02-16

Multiple Output LoadSHARE™ PWM

TM

®

The first method is to change the ratio of the inductors

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.

To change the ratio of current between the two

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.

The following circuit description shows how to select the

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 ratio of inductor ESR is inversely proportional to

the power level split.

1

I

2

I

2

ESR

1

ESR =

The higher current inductor will have the lower ESR value. If

the ESR of the phase 1 inductor is selected as 10m

Ω, then the

ESR value of the phase 2 inductor is calculated as:

mΩ

4

.

6

mΩ

10

A

33

.

7

A

67

.

4

=

×

⎟

⎠

⎞

⎜

⎝

⎛

Depending on the required accuracy of this power sharing;

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.

1.5V @ 12A

18W

6.4m

Ω

4.67A

7.33A

10m

Ω

1.5V +

46.7mV

L1

L2

+5V @ 7W

+3.3V @ 11W

Figure 7 –LoadSHARE Using Inductor ESR

Sometimes it is desirable to use the same inductor in both phases

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.

As in Figure 7, the millivolts of DC offset created by the resistor

divider network in the feedback path, appears as a voltage

generator between the ESR of the two inductors.

A divider in the feedback path from Phase 2 will cause the

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.

A down side is that

the current difference between the two

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 following description and Figure 8 show how to determine

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.