Microsemi

Linfinity Microelectronics Division

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

Page 11

Copyright

© 2000

Rev. 0.3m, 2005-04-12

LX1672

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

2

2

1

I

I

ESR

ESR

=

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

the ESR of the phase 1 inductor is selected as 10mΩ, then the ESR

value of the phase 2 inductor is calculated as:

mΩ

4

.

6

mΩ

10

33

.

7

67

.

4

=

×

⎟

⎠

⎞

⎜

⎝

⎛

A

A

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

6.4mΩ

4.67A

7.33A

10mΩ

1.5V +

46.7mV

L1

L2

+5V @ 7W

+3.3V @ 11W

VOUT

Figure 7

– Ratio 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

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.