L6919CD datasheet(12/32 Pages) STMICROELECTRONICS

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L6919C

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follow:

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 com-

pared 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 25

µA 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 over current threshold (IOCPx) for each phase, which has to be placed at one half

of the total delivered maximum current, results:

Since the device senses the output current across the low-side mosfets (or across a sense resistors in series

with them) the device limits the bottom of the inductor current triangular waveform: an over current is detected

when the current flowing into the sense element is greater than IOCPx (IINFOx > 35µA).

Introducing now the maximum ON time dependence with the delivered current (where T is the switching period

T=1/fSW):

This linear dependence has a value at zero load of 0.80·T and at maximum current of 0.40·T typical and results

in two different behaviors of the device:

1. TON Limited Output Voltage.

This happens when the maximum ON time is reached before the current in each phase reaches IOCPx (IINFOx

< 35

µA).

Figure 6a shows the maximum output voltage that the device is able to regulate considering the TON limitation

imposed by the previous relationship. If the desired output characteristic crosses the TON limited maximum output

voltage, the output resulting voltage will start to drop after crossing. In this case, the device doesn't perform con-

stant current limitation but only limits the maximum ON time following the previous relationship. The output volt-

age follows the resulting characteristic (dotted in Figure 6b) until UVP is detected or anyway until IFB = 70µA.

Figure 6. TON Limited Operation

IN FO x

SENSE

PHASE

⋅

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

OCPx

µARg

⋅

SEN SE

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

OC P x

SE NSE

⋅

µA

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

ON,MAX

0.80

5.73 k

⋅

() T

⋅

–

0.80

SENSE

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

OUT

5.73k

⋅⋅



 T

0.80 T I

⋅

µA

0.40 T I

⋅

µA







⋅

–

a) Maximum output Voltage

b) TON Limited Output Voltage

0.80·VIN

0.40·VIN

VOUT

IOUT

IOCP=2·IOCPx

(IFB=70µA)

TON Limited Output

characteristic

0.80·VIN

0.40·VIN

VOUT

IOUT

IOCP=2·IOCPx

(IFB=70µA)

Desired Output

characteristic and

UVP threshold

Resulting Output

characteristic

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