L6917B
µ
>35 A): the device enters in Quasi-Constant-Current operation. The low-side mosfets stays ON
INFOx
current (I
µ
becomes lower than 35 A skipping clock cycles. The high side mosfets can be turned ON with a T
ON
until I
INFO
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 140% bottom. The worst-case condition is
when the duty cycle reaches its maximum value (d=75% 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 device to latch (FAULT pin is driven high).
Figure 5 shows this working condition
Figure 5. Constant Current operation
Ipeak
Vout
Droop
effect
IMAX
140%
UVP
TonMAX
TonMAX
Iout
MAX
OCP
I
Inom
I
It can be observed that the peak current (Ipeak) is greater than the 140% but it can be determined as follow:
–
V
Vout
MIN
IN
--------------------------------------
+
=
Ipeak
1.4 I
Ton
MAX
NOM
L
Where I
is the nominal current and Vout
is the minimum output voltage (VID-40% as explained below).
MIN
NOM
The device works in Constant-Current, and the output voltage decreases as the load increase, until the output
voltage reaches the under-voltage threshold (Vout ). When this threshold is crossed, all mosfets are turned
MIN
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:
–
Ipeak 1.4 I
NOM
------------------------------------------------
=
+
2
I
1.4 I
MAX
NOM
2
In this particular situation, the switching frequency results reduced. The ON time is the maximum allowed (Ton-
MAX) while the OFF time depends on the application:
–
Ipeak 1.4 I
NOM
1
------------------------------------------------
=
= ------------------------------------------
f
T
L
OFF
Vout
+
T
Ton
MAX
OFF
µ
reaches 35 A. The full load value is only a convention to work with con-
Over current is set anyway when I
INFOx
venient values for I . Since the OCP intervention threshold is fixed, to modify the percentage with respect to
FB
the load value, it can be simply considered that, for example, to have on OCP threshold of 170%, this will cor-
µ
µ
µ µ
= 20.5 A (I = 41 A).
INFOx FB
respond to I
= 35 A (I = 70 A). The full load current will then correspond to I
INFOx
FB
12/33
STMICROELECTRONICS [ ST ]
