L6918A STMicroelectronics, L6918A Datasheet - Page 14

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

L6918 L6918A
Figure 4. Drivers peak current: High Side (left) and Low Side (right)
CURRENT READING AND OVER CURRENT
Each device involved in the four phase conversion has its own current reading circuitry and over current protec-
tion. As a results, the OCP network design for each device must be performed fort half of the maximum output
current.
The current flowing trough each phase is read using the voltage drop across the low side mosfets R
across a sense resistor (R
by the external resistor Rg placed outside the chip between ISENx and PGNDSx pins toward the reading points.
The full differential current reading rejects noise and allows to place sensing element in different locations with-
out affecting the measurement's accuracy. The current reading circuitry reads the current during the time in
which the low-side mosfet is on (OFF Time). During this time, the reaction keeps the pin ISENx and PGNDSx
at the same voltage while during the time in which the reading circuitry is off, an internal clamp keeps these two
pins at the same voltage sinking from the ISENx pin the necessary current (Needed if low-side mosfet R
sense is implemented to avoid absolute maximum rating overcome on ISENx pin).
The proprietary current reading circuit allows a very precise and high bandwidth reading for both positive and
negative current. This circuit reproduces the current flowing through the sensing element using a high speed
Track & Hold Tran conductance amplifier. In particular, it reads the current during the second half of the OFF
time reducing noise injection into the device due to the high side mosfet turn-on (See fig. 5). Track time must
be at least 200ns to make proper reading of the delivered current.
This circuit sources a constant 50 A current from the PGNDSx pin and keeps the pins ISENx and PGNDSx at
the same voltage. Referring to figure 5, the current that flows in the ISENx pin is then given by the following
equation:
Where R
resistor used between ISENx and PGNDSx pins toward the reading points; I
phase.
The current information reproduced internally is represented by the second term of the previous equation as
follow:
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SENSE
CH3 = HGATE1; CH4 = HGATE2
is an external sense resistor or the R
SENSE
I
I SENx
) and internally converted into a current. The transconductance ratio is issued
=
50 A
+
R
---------------------------------------------
SENSE
dsON
R
g
of the low side mosfet and Rg is the transconductance
I
P HASE
=
CH3 = LGATE1; CH4 = LGATE2
50 A
+
I
INF Ox
PHASE
is the current carried by each
DSON
dsON
or