13/29

L6712A L6712

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

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 9. TON Limited Operation

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 9b) until UVP is

Constant Current Operation

35

µA).

The device enters in Quasi-Constant-Current operation: the low-side mosfets stays ON until the current

ON with a TON imposed by the control loop at the next available clock cycle and the device works in the

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

OUT

5.73k

⋅⋅

–

⎝⎠

⎛⎞

T =

T

0.80 T I

FB

0

µA

=

⋅

=

T

0.40 T I

FB

70

µA

=

⋅

=

⎩

⎪

⎨

⎪

⎧

⋅

=

⋅

=

characteristic

Desired Output

characteristic and

UVP threshold

Resulting Output

characteristic

a

) Maximum output Voltage

b) TON Limited Output Voltage