L6917 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6917' PDF : 33 Pages View PDF
L6917B
current (IINFOx>35µA): the device enters in Quasi-Constant-Current operation. The low-side mosfets stays ON
until IINFO becomes lower than 35µA skipping clock cycles. The high side mosfets can be turned ON with a TON
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
IMAX
Droop
effect
140%
UVP
TonMAX
TonMAX
Iout
Inom IOCP IMAX
It can be observed that the peak current (Ipeak) is greater than the 140% but it can be determined as follow:
Ipea k = 1.4 ⋅ INOM + V-----I-N-----–-----V--L--o----u----t--M----I--N-- ⋅ To nMAX
Where INOM is the nominal current and VoutMIN is the minimum output voltage (VID-40% as explained below).
The device works in Constant-Current, and the output voltage decreases as the load increase, until the output
voltage reaches the under-voltage threshold (VoutMIN). 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:
IMAX
=
1.4
⋅
INOM
+
2
⋅
I---p---e----a----k-----–----1---.--4-----⋅---I--N----O----M--
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:
TOFF = L ⋅ -I--p---e----a----k----V–-----o1---u.--4--t---⋅---I--N----O----M--
f = ---------------------1---------------------
TonMAX + TOFF
Over current is set anyway when IINFOx reaches 35µA. The full load value is only a convention to work with con-
venient values for IFB. Since the OCP intervention threshold is fixed, to modify the percentage with respect to
the load value, it can be simply considered that, for example, to have on OCP threshold of 170%, this will cor-
respond to IINFOx = 35µA (IFB = 70µA). The full load current will then correspond to IINFOx = 20.5µA (IFB = 41µA).
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