L6918D View Datasheet(PDF) - STMicroelectronics
Part Name
Description
Manufacturer
L6918D Datasheet PDF : 35 Pages
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L6918 L6918A
Figure 13. Current Sharing Control Loop.
+
PWM1
1/5
1/5
L1
CURRENT
SHARING
DUTY CYCLE
CORRECTION
IINFO2
IINFO1
+
COMP
PWM2
D02IN1393
L2
VOUT
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 ROUT), while the whole converter is modeled by the same ideal voltage source and an equivalent
output resistance RDROOP=ROUT/2;
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 (ROUT,IOUT). The mismatch
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 IFB, sense resistors
RSENSE, Transconductance resistors Rg and feedback resistors RFB.
The current sharing error (CSE) results:
CSE = ∆--I--O-I-O--U--U--T--T-- ≡
12--
R-----O----U--V--T--o--⋅--s-I--O----U----T-
2
+
12--
-∆--I-F-I--F-B--B-
2
+
12--
-∆-R--R---F--F-B--B--
2
+
22--
-∆--R--R--S--S--E--E-N--N--S--S--E-E--
2
+
42--
-∆--R--R--g--g-
2
Considering the external resistors tolerance of 1%, the typical current feedback accuracy of 2.5µA/50µA (5%),
4 phases operation, Error Amplifier offset Vos=6mV, droop resistance RDROOP=1.5mΩ (ROUT=2,RDROOP) and
ILOAD=60A (IOUT=ILOAD/2), it results:
CSE =
12--
1----.-5---0-m--.--0--Ω-0----6-⋅--V-6---0----A--
2
+
12--
2-5---.-05---µ-µ---A-A---
2
+
12--
(0.01
)2
+
22-- (0.01)2 + 42-- (0.01)2
=
0.062( 6.2%)
Figure 14. Equivalent Circuit for current sharing error calculation
IOUT
ROUT
Recirculating Current
VPROG
IOUT
ROUT
L6918
VOUT
ILOAD
RLOAD
RDROOP
ILOAD
VID
VOUT
RLOAD
22/35
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