L6712A L6712

12/27

The transconductance resistor Rg can be designed in order to have current information of 25

µA per phase

µA).

placed at 1/2 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

µA).

The maximum duty cycle is limited as a function of the measured current and, since the oscillator frequen-

cy is fixed once programmed, imply a maximum on-time limitation as follow (where T is the switching pe-

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:

Figure 5. TON Limited Operation

1.TON Limited Output Voltage.

µA).

maximum output voltage, the output resulting voltage will start to drop after crossing. In this case, the de-

vice doesn't perform constant current limitation but only limits the maximum duty cycle following the pre-

vious relationship. The output voltage follows the resulting characteristic (dotted in Figure 5b) until UVP is

µA.

I

OCPx

35

µARg

⋅

R

SENSE

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

Rg

I

⋅

35

µA

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

=

=

T

ON,MAX

0.80

I

FB

–

5.73k

⋅

() T0.80

R

SENSE

Rg

---------------------- I

⋅⋅

–





T =

T

0.80 T I

FB

0

µA

=

⋅

=

T

0.40 T I

FB

70

µA

=

⋅

=











⋅

=

⋅

=

µA)

characteristic

µA)

Desired Output

characteristic and

UVP threshold

Resulting Output

characteristic

a

) Maximum output Voltage

b) TON Limited Output Voltage