L6918 L6918A

14/35

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.

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

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

µA 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:

phase.

The current information reproduced internally is represented by the second term of the previous equation as

follow:

CH3 = HGATE1; CH4 = HGATE2

CH3 = LGATE1; CH4 = LGATE2

I

I SEN x

50

µA

R

SEN SE

I

P H ASE

⋅

R

g

---------------------------------------------

50

µAI

IN F Ox

+

=

+

=