L6712ADTR(2004) 데이터 시트보기 (PDF) - STMicroelectronics

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L6712ADTR
(Rev.:2004)

TWO-PHASE INTERLEAVED DC/DC CONTROLLER

STMicroelectronics 

L6712A L6712

CURRENT READING AND OVER CURRENT

The current flowing trough each phase is read using the voltage drop across the low side mosfets RdsON

or across a sense resistor (RSENSE) in series to the LS mosfet and internally converted into a current. The

transconductance ratio is issued by the external resistor Rg placed outside the chip between ISENx and

PGNDSx pins toward the reading points. The differential current reading rejects noise and allows to place

sensing element in different locations without 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 RdsON sense is implemented to avoid absolute max-

imum 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 transconductance amplifier. In particular, it reads the current during the second half

of the OFF time reducing noise injection into the device due to the mosfet turn-on (See fig. 4-left). Track

time must be at least 200ns to make proper reading of the delivered current.

This circuit sources a constant 50µA current from the PGNDSx pin: it must be connected through the Rg

resistor to the ground side of the sensing element (See Figure 4-right). The two current reading circuitries

use this pin as a reference keeping the ISENx pin to this voltage.

The current that flows in the ISENx pin is then given by the following equation:

IISENx

=

50µA + -R----S---E----N----S----E----⋅---I--P----H----A----S---E----x-

Rg

=

50 µA + IINFOx

Where RSENSE is an external sense resistor or the RdsON of the low side mosfet and Rg is the transcon-

ductance resistor used between ISENx and PGNDSx pins toward the reading points; IPHASEx is the current

carried by the relative phase. The current information reproduced internally is represented by the second

term of the previous equation as follow:

IINFOx

=

R-----S----E---N----S----E----⋅---I--P----H----A----S---E----x-

Rg

Since the current is read in differential mode, also negative current information is kept; this allow the de-

vice to check for dangerous returning current between the two phases assuring the complete equalization

between the phase's currents. From the current information of each phase, information about the total cur-

rent delivered (IFB = IINFO1 +IINFO2) and the average current for each phase (IAVG = (IINFO1 +IINFO2)/2 ) is

taken. IINFOX is then compared to IAVG to give the correction to the PWM output in order to equalize the

current carried by the two phases.

Figure 4. Current reading timing (left) and circuit (right)

ILS1

ILS2

IFB

Track & Hold

LGATEx

ISENx

PGNDSx

Rg

IISENx

Rg

50µA

11/27

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