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L6918

5 BIT PROGRAMMABLE MULTIPHASE CONTROLLER

STMicroelectronics 

L6918 L6918A

IINFOx = R-----S---E----N----S---E-R----⋅g---I--P---H----A----S---E--

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.

Figure 5. Current reading timing (left) and circuit (right)

ILS1

ILS2

Total

current

information

Track & Hold

LGATEX

ISENX

PGNDSX

Rg

IISENx

Rg

50µA

From the current information for 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 can 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). Accord-

ing to the above relationship, the over current threshold (IOCPx) for each phase, which has to be placed at one

half of the total delivered maximum current, results:

IOCPx = 3---R-5---S-µ---EA---N---⋅-S--R--E---g-

Rg = I--O----C----P---x-3---5⋅---R-µ----AS---E----N----S---E--

An over current is detected when the current flowing into the sense element is greater than IOCP (IINFOx>35µA):

the device enters in Quasi-Constant-Current operation. The low-side mosfets stays ON until IINFO becomes

lower than 35µA skipping clock cycles. The high side mosfets can be turned ON with a TON imposed by the

control loop at the next available clock cycle and the device works in the usual way until another OCP event is

detected.

The device limits the bottom of the inductor current triangular waveform. So the average current delivered can

slightly increase also in Over Current condition since the current ripple increases. In fact, the ON time increases

due to the OFF time rise because of the current has to reach the IOCP bottom. The worst-case condition is when

the duty cycle reaches its maximum value (d=50% internally limited). When this happens, the device works in

Constant Current and the output voltage decrease as the load increase. Crossing the UVP threshold causes the

Slave device to pull down the SLAVE_OK line. All mosfets are turned off and all the devices involved in the reg-

ulation stop working. Cycle the power supply to restart operation.

Figure 6 shows the constant current working condition

15/35

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