L6701 datasheet(20/44 Pages) STMICROELECTRONICS

20 / 44 page

9 Current Sharing Loop and Current Reading

L6701

20/44

Current Sharing Loop and Current Reading

9.1

Current Sharing Loop

L6701 embeds two separate Current Reading circuitries used to perform Current Sharing and

OCP through ISENx pins and Voltage Positioning (Droop Function) through CS+ and CS- pins

(See Section 10).

Current sharing control loop and connections are reported in Figure 6: the current read through

the ISENx pins is converted into a current IINFOx proportional to the current delivered by each

phase and the information about the average current IAVG = ΣIINFOx / 3 is internally built into the

device. The error between the read current IINFOx and the reference IAVG is then converted into

a voltage that with a proper gain is used to adjust the duty cycle whose dominant value is set by

the voltage error amplifier in order to equalize the current carried by each phase.

9.2

Current Reading for Current Sharing

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

MOSFETs RDS(on) or across a sense resistor in its series and it is internally converted into a

current. The trans-conductance ratio is issued by the external resistor RISEN placed outside the

chip between ISENx and the reading point (usually the LS MOSFET Drain).

The current sense circuit tracks the current information for a time TTRACK centered in the

middle of the LS conduction time and holds the tracked information during the rest of the

period. The current that flows from the ISENx pin is the current information used by the device to

perform current sharing and OCP and it is given by:

where RDS(on) is the ON resistance of the low side MOSFET and RISEN is the trans-

conductance resistor connected between the ISENx pins and the LS Drain; IPHASEx is the

current carried by the relative phase and IINFOx is the current information signal reproduced

internally. RISENx is designed according to the Over Current Protection: see Section 13.6 for

details.

Figure 6.

Current Sharing Loop and Current Reading Connections

ISENx

dsO N

ISEN

----------------- IPHASEx

⋅

INFO x

INFO1

PWM1 Out

From EA

INFO2

AVG

PWM2 Out

INFO3

PWM3 Out

AVG

ISENx

LGATEx

PHASEx

ISEN

IISENx

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