PM6681A STMicroelectronics, PM6681A Datasheet - Page 24

• Manufacturer Part Number: PM6681A
• Description: Dual synchronous step down controller with adjustable LDO
• Manufacturer: STMicroelectronics

Device description
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Figure 36. Current waveforms in current limit conditions
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 R
Equation 7
Where I
Consider the temperature effect and the worst case value in R
The accuracy of the valley current threshold detection depends on the offset of the internal
comparator (∆V
Equation 8
Where R
PM6681A 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 120 mV.
The negative valley-current limit INEG (if the device works in PWM mode) is given by:
Equation 9
Current
CSENSE
SNS
∆
I
Lvalley
I
is the sensing element (R
Lvalley
OFF
= 100 µA, R
DC current limit = maximum load
) and on the accuracy of the current generator(∆I
=
∆
I
CSENSE
I
CSENSE
DS(on)
+
(R
⎡
⎢
⎣
R
R
DS(on)
is the drain-source on resistance of the low side switch.
Valley current threshold
CSENSE
Inductor current
CSENSE
I
DS(on)
NEG
∆
sensing technique)
V
=
OFF
×
=
I
R
).
CSENSE
120
DSon
R
I
DSon
CSENSE
CSENSE
mV
×
I
×
Lvalley
100
:
⎤
⎥
⎦
+
∆
R
DS(on)
R
CSENSE
Maximum load current is
influenced by the inductor
current ripple
CSENSE
CSENSE
calculation.
+
∆
R
R
):
SNS
SNS
Time
PM6681A