L6615 STMicroelectronics, L6615 Datasheet - Page 13

• Manufacturer Part Number: L6615
• Description: High/low side load share controller
• Manufacturer: STMicroelectronics

The last point is the design of the compensation network Z
ground.
Besides the power supply feedback loop, the current sharing system introduces another, outer loop. To
avoid interaction between them it is important to design the bandwidth of the sharing loop at least one or-
der of magnitude lower than the bandwidth of the power supply loop.
For the total system, the loop gain is:
where
Typically the compensation network is built by a R-C series.
A resistor in series with C
at the load share loop crossover frequency, f
If f
5
Several are the methods to sense the power supply output current; the simplest one is to use a power
resistor (fig. 13a) but increasing load current could require expensive resistor to support the inherent pow-
er dissipation, imposing the use of several paralleled resistor.
Other methods to sense the output current are showed in fig. 13b and 13c:
1. R
2. Current transformer: in case of very high load currents, a transformer allows sensing a smaller cur-
C(SH)
CURRENT SENSE METHODS
as sense resistor (fig 13a): the L6615 sense pins will be connected, through R
and to the source of the MOS. Besides providing the sense resistor, the FET is used as "ORing" el-
ement: driving properly its gate, it is possible isolate the power supply output from the load (the body
diode is reversed biased so it doesn't conduct).
This is useful whenever features like hot swap or hot plug are required; compared with the well-known
solution using ORing diode, the ORing FET greatly reduces the power dissipation, in particular:
where V
rent, obtained through a scaling factor equal to the transformer turn ratio. In this way, the sense re-
sistor power dissipation requirements can be less tight: obviously this is paid with the cost of the
transformer.
In fig. 13c it is showed the simplified output stage of a power supply in forward configuration: through
two current transformers the load current is reproduced in the sensing circuit scaled by a factor N.
R
that doesn't affect the correct behaviour of the current sharing system because its loop gain is de-
signed with a low bandwidth - at least 2 order of magnitude lower than the switching frequency - that
will cut this high frequency.
SENSE
DS(ON)
is the share loop crossover frequency, then:
R
A
LOAD
PWR
: a power MOS is placed in series to the output and its channel resistance (R
F
will read a ripple (at the switching frequency) superimposed on the average current value
(s) is the transfer function of PWM controller and power stage (see fig. 11)
is the forward drop across the diode.
is the equivalent load resistance
G
P
P
LOOP s
C
MOS
DIO DE
C
=
C
=
------------------------------- -
2
is required to boost the phase margin of the load share loop. The zero is placed
=
R
=
DS O N
V
F
R
1
f
SENSE
C SH
I
OUT
I
2
OUT
+
R
R
---------------------------- -
R
SE NSE
R
--------------- G
CGA
C
R
CGA
R
G
=
C(SH)
G
-------------------------------------------- -
2
G
I
2
OUT
M
.
M
f
R
------------- -
C SH
Z
R
ADJ
1
C
A
s
C
R
--------------------- - A
C
R
(s) connected between the COMP pin and
R
------------- - A
SENSE
C
LOAD
R
ADJ
A
PWR
PWR f
s
C SH
------------------
R
LOA D
G
1
resistors to the drain
DS(ON)
) is used
L6615
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