lx1672 Microsemi Corporation, lx1672 Datasheet - Page 11

• Manufacturer Part Number: lx1672
• Description: Multiple Output Loadshare Pwm
• Manufacturer: Microsemi Corporation

Copyright © 2000
Rev. 1.0, 2005-08-10
B
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.
ratio of inductor ESR is inversely proportional to 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.
I
-P
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;
Figure 7 – Ratio LoadSHARE™ Using Inductor ESR
HASE
+3.3V @ 11W
46.7mV
1.5V +
ESR
+5V @ 7W
ESR
TM
, L
To change the ratio of current between the two
1
2
OAD
=
SHARE ( ESR M
I
I
1
2
⎛
⎜
⎝
. 4
. 7
L2
L1
67
33
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
A
A
6.4m Ω
⎞
⎟
⎠
10m Ω
×
10
mΩ
ETHOD
T H E O R Y O F O P E R A T I O N
4.67A
7.33A
=
6
)
4 .
1.5V @ 12A
mΩ
VOUT
18W
Integrated Products Division
®
Microsemi
The
B
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, a down side is that
that the current difference between the two inductors still flows
during a no load condition.
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.
determine the value of the resistor divider necessary to satisfy this
example.
I
-P
Sometimes it is desirable to use the same inductor in both phases
This produces a low efficiency condition during a no load or light
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
A divider in the feedback path from Phase 2 will cause the
HASE
, L
( C O N T I N U E D )
OAD
P
RODUCTION
SHARE ( F
EEDBACK
D
ATA
D
S
The following calculations
IVIDER
HEET
M
ETHOD
LX1672
)
Page 11