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L6919C
2. Constant Current Operation
This happens when ON time limitation is reached after the current in each phase reaches I
OCPx
(I
INFOx
>35
μ
A).
The device enters in Quasi-Constant-Current operation: the low-side mosfets stays ON until the current read
becomes lower than I
OCPx
(I
INFOx
< 35
μ
A) skipping clock cycles. The high side mosfets can be turned ON with
a T
ON
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.
This means that the average current delivered can slightly increase also in Over Current condition since the cur-
rent ripple increases. In fact, the ON time increases due to the OFF time rise because of the current has to reach
the I
OCPx
bottom. The worst-case condition is when the ON time reaches its maximum value.
When this happens, the device works in Constant Current and the output voltage decrease as the load increase.
Crossing the UVP threshold causes the device to latch (FAULT pin is driven high).
Figure 7 shows this working condition
Figure 7. Constant Current operation
It can be observed that the peak current (Ipeak) is greater than the I
OCPx
but it can be determined as follow:
V
Vout
–
L
Where V
outMIN
is the minimum output voltage (VID-30% as follow).
The device works in Constant-Current, and the output voltage decreases as the load increase, until the output
voltage reaches the Under-Voltage threshold (V
outMIN
). When this threshold is crossed, all mosfets are turned
off, the FAULT pin is driven high and the device stops working. Cycle the power supply to restart operation. The
maximum average current during the Constant-Current behavior results:
In this particular situation, the switching frequency results reduced. The ON time is the maximum allowed
(T
onMAX
) while the OFF time depends on the application:
Ipeak
I
–
V
OUT
Over current is set anyway when I
INFOx
reaches 35
μ
A (I
FB
= 70
μ
A). The full load value is only a convention
to work with convenient values for I
FB
. Since the OCP intervention threshold is fixed, to modify the percent-
age with respect to the load value, it can be simply considered that, for example, to have on OCP threshold
of 170%, this will correspond to I
INFOx
= 35
μ
A (I
FB
= 70
μ
A). The full load current will then correspond to
I
INFOx
= 20.6
μ
A (I
FB
= 41.1
μ
A).
a) Maximum current for each phase
b) Output Characteristic
TonMAX
TonMAX
I
OCPx
Ipeak
I
MAX
Vout
Iout
(I
FB
=50
μ
A)
I
OCP
=2·I
OCPx
(I
FB
=70
μ
A)
I
MAX,TOT
UVP
Droop effect
Ipeak
I
OCPx
--------------------------------------
Ton
MAX
+
I
OCPx
V
-----------–
0.40 T
Vout
L
+
=
=
I
MAX,TOT
2 I
MAX
2
I
OCPx
Ipeak
-------------------------------------
I
–
2
+
+
=
T
OFF
L
-------------------------------------
=
f
1
T
ONmax
T
OFF
+
------------------------------------------
=