L6918A STMicroelectronics, L6918A Datasheet - Page 22

• Manufacturer Part Number: L6918A
• Description: 5 BIT PROGRAMMABLE MULTIPHASE CONTROLLER
• Manufacturer: STMicroelectronics

L6918 L6918A
Figure 13. Current Sharing Control Loop.
The current sharing between devices uses the droop function. Each device can be modeled with its Thevenin
equivalent circuit (that is an ideal voltage source equal to the programmed voltage by VIDs and its related output
resistance R
output resistance R
Considering this modelization reported in figure 14, it can be seen that the recirculating current between devices
depends on the accuracy of the regulation.
The accuracy of the voltage source is given by the offset of the master error amplifier Vos (6mV typ) and de-
pends on the ratio between this offset and the output voltage variation with load (R
between the regulated voltages causes a converter to source a current that is sunk by the other one. The accu-
racy related to droop resistance depends on precision of feedback current of the device I
R
The current sharing error (CSE) results:
Considering the external resistors tolerance of 1%, the typical current feedback accuracy of 2.5 A/50 A (5%),
4 phases operation, Error Amplifier offset Vos=6mV, droop resistance R
I
Figure 14. Equivalent Circuit for current sharing error calculation
22/35
LOAD
SENSE
=60A (I
, Transconductance resistors Rg and feedback resistors R
CSE
OUT
OUT
CSE
I
), while the whole converter is modeled by the same ideal voltage source and an equivalent
=I
OUT
=
LOAD
DROOP
=
V
PROG
--------------- -
1
-- -
2
I
I
O UT
R
/2), it results:
OUT
--------------------------------- -
1.5m
OUT
=R
0.006V
I
OUT
OUT
COMP
L6918
1
-- -
2
60A
/2;
------------------------------- -
R
+
+
O UT
R
OUT
Vos
2
Recirculating Current
+
I
1
-- -
2
OUT
PWM1
PWM2
2.5 A
--------------- -
1/5
1/5
50 A
V
2
OUT
+
1
-- -
2
2
D02IN1393
+
----------- -
I
CORRECTION
I
F B
DUTY CYCLE
1
-- - 0.01
2
FB
CURRENT
SHARING
I
2
LOAD
+
1
-- -
2
2
R
+
LOAD
-------------- -
R
2
-- - 0.01
2
R
FB
F B
I
I
INFO2
INFO1
FB
L 1
L 2
.
2
+
2
DROOP
2
-- -
2
+
4
-- - 0.01
2
V
------------------------- -
R
OUT
R
VID
SE NSE
SENSE
=1.5m (R
R
DROOP
2
=
2
OUT
V
0.062 6.2%
+
OUT
4
-- -
2
I
,I
LOAD
OUT
OUT
---------- -
R
FB
R
g
g
, sense resistors
=2,R
). The mismatch
R
2
LOAD
DROOP
) and