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L6917

5 BIT PROGRAMMABLE DUAL-PHASE CONTROLLER

STMicroelectronics 

L6917B

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.

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). Track time must be at least

200ns to make proper reading of the delivered current.

Figure 4. Current Reading Timing (Left) and Circuit (Right)

ILS1

ILS2

Total current

information

Track & Hold

LGATEX

ISENX

PGNDSX

Rg

IISENx

Rg

50µA

This circuit sources a constant 50µA current from the PGNDSx pin and keeps the pins ISENx and PGNDSx at

the same voltage. Referring to figure 4, the current that flows in the ISENx pin is then given by the following

equation:

IISENx = 50µA + -R----S----E---N----S----ER-----⋅g---I-P----H----A----S---E-- = 50µ A + IINFOx

Where RSENSE is an external sense resistor or the rds,on of the low side mosfet and Rg is the transconductance

resistor used between ISENx and PGNDSx pins toward the reading points; IPHASE is the current carried by each

phase and, in particular, the current measured in the middle of the oscillator period

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--

Rg

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

The transconductance resistor Rg has to be designed in order to have current information of 25µA per phase

at full nominal load; the over current intervention threshold is set at 140% of the nominal (IINFOx = 35µA).

According to the above relationship, the limiting current (ILIM) for each phase, which has to be placed at one half

of the total delivered maximum current, results:

ILIM

=

3----5----µ----A-----⋅---R-----g--

RSENSE

Rg = -I-L---I--M-----3-⋅--5-R---µ-S---A-E---N----S----E--

An over current is detected when the current flowing into the sense element is greater than 140% of the nominal

11/33

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