PM6685
Device description
Figure 34. Current waveforms in current limit conditions
Current DC current limit = maximum load
Maximum load current
is influenced by the
inductor current ripple
Inductor current
Valley current threshold
Time
Being fixed the valley threshold, the greater the current ripple is, greater the DC output
current is
The valley current limit can be set with resistor RCSENSE:
Equation 6
RCSENSE
=
RDSon × ILvalley
ICSENSE
Where ICSENSE = 100uA, RDSon is the drain-source on resistance of the low side switch.
Consider the temperature effect and the worst case value in RDSon calculation.
The accuracy of the valley current threshold detection depends on the offset of the internal
comparator (∆VOFF) and on the accuracy of the current generator(∆ICSENSE):
Equation 7
∆ILvalley
ILvalley
=
∆ICSENSE
ICSENSE
+
⎡
⎢
⎣
∆VOFF
RCSENSE × ICSENSE
⎤
× 100⎥
⎦
+
∆RCSENSE
RCSENSE
+ ∆RSNS
RSNS
Where RSNS is the sensing element (RDSon).
PM6685 provides also a fixed negative peak current limit to prevent an excessive reverse
inductor current when the switching section sinks current from the load in PWM mode. This
negative current limit threshold is measured between PHASE and SGND pins, comparing
the magnitude drop on the PHASE node during the conduction time of the low side
MOSFET with an internal fixed voltage of 120mV.
If the current is sensed on the low side MOSFET, the negative valley-current limit INEG (if
the device works in PWM mode) is given by:
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