L6918A STMicroelectronics, L6918A Datasheet - Page 21

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

MAIN CONTROL LOOP
The four phases control loop is composed by two dual phases devices that are independent each other. So, the
compensation network and the control loop stability of each device don't depend on the other except for the fact
that the other converter represents a load for this one.
The L6918/A control loop is composed by the Current Sharing control loop and the Average Current Mode con-
trol loop. Each loop gives, with a proper gain, the correction to the PWM in order to minimize the error in its
regulation: the Current Sharing control loop equalize the currents in the inductors while the Average Current
Mode control loop fixes the output voltage equal to the reference programmed by VID. Figure 12 reports the
block diagram of the main control loop
Figure 12. Main Control Loop Diagram
CURRENT SHARING (CS) CONTROL LOOP
The devices are configured to work in a four synchronized phase application. Since the application is composed
by two-phase devices that share reference and synchronization signals, the current sharing between the phases
is realized in two different steps:
The Current Sharing between phases of the same device uses the internal current information to correct the
PWM signal in order to equalize the current. 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 (I
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. 13).
The current sharing control is a high bandwidth control 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 mismatch of
Tran conductance differential amplifier between phases; considering a voltage mismatch equal to 2mV across
the sense resistor, the current reading error is given by the following equation:
Where I
For Rsense=4m
Rsense mismatches.
READ
is the difference between one phase current and the ideal current (I
1. Sharing between the phases of the same device;
2. Sharing between devices.
and Imax=40A the current sharing error is equal to 2.5%, neglecting errors due to Rg and
D02IN1392
AVG
+
+
4/5
) is internally built; the error between the read current and this reference
COMP
AMPLIFIER
PWM1
PWM2
------------------- -
ERROR
1/5
1/5
I
I
MAX
REA D
Z
F(S)
+
-
=
CORRECTION
DUTY CYCLE
CURRENT
SHARING
PROGRAMMED
-------------------------------------- -
R
REFERENCE
S ENSE
BY VID
FB
2mV
R
FB
I
I
INFO2
INFO1
I
MAX
L 1
L 2
C
O
R
O
MAX/2
).
L6918 L6918A
21/35