L6917B
s Current Sharing (CS) Control Loop
Active current sharing is implemented using the information from Tran conductance differential amplifier in an
average current mode control scheme. A current reference equal to the average of the read current (IAVG) is
internally built; the error between the read current and this reference is converted to a voltage with a proper gain
and it is used to adjust the duty cycle whose dominant value is set by the error amplifier at COMP pin (See fig.
11).
The current sharing control is a high bandwidth control loop allowing current sharing even during load transients.
The current sharing error is affected by the choice of external components; choose precise Rg resistor (±1% is
necessary) to sense the current. The current sharing error is internally dominated by the voltage offset of Tran
conductance differential amplifier; considering a voltage offset equal to 2mV across the sense resistor, the cur-
rent reading error is given by the following equation:
∆----II--MR----EA----AX---D-- = -R----S----E---N--2--S--m--E---V--⋅---I-M-----A----X-
Where ∆IREAD is the difference between one phase current and the ideal current (IMAX/2).
For Rsense = 4mΩ and Imax = 40A the current sharing error is equal to 2.5%, neglecting errors due to Rg and
Rsense mismatches.
Figure 11. Current Sharing Control Loop
L1
+
PWM1
1/5
CURRENT
IINFO2
SHARING
DUTY CYCLE IINFO1
1/5
CORRECTION
+
COMP
PWM2
D02IN1393
L2
VOUT
s Average Current Mode (ACM) Control Loop
The average current mode control loop is reported in figure 12. The current information IFB sourced by the FB
pin flows into RFB implementing the dependence of the output voltage from the read current.
The ACM control loop gain results (obtained opening the loop after the COMP pin):
GLOOP(s) = (---Z----P---(---s-P--)--W--+----M-Z----L--⋅-(--Zs---)-F--)-(---s⋅---)---Z-⋅--A----F(----(--R--(--s--s--D--)----)-R--+--O----O--1---P--+--+---A-----Z----(1----P-s-------()--s---)-⋅--)-R-----F---B-----
Where:
– RDROOP = R-----s-R--e---gn---s---e- ⋅ RFB is the equivalent output resistance determined by the droop function;
– ZP(s) is the impedance resulting by the parallel of the output capacitor (and its ESR) and the applied
load Ro;
17/33