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PM6680 数据表(PDF) 27 Page - STMicroelectronics

部件名 PM6680
功能描述  No Rsense dual step-down controller with adjustable voltages for notebook system power
PDF  49 Pages
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制造商  STMICROELECTRONICS [STMicroelectronics]
网页  http://www.st.com
标志 STMICROELECTRONICS - STMicroelectronics

PM6680 数据表(HTML) 27 Page - STMicroelectronics

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PM6680
Device description
27/49
An internal 100
µA current source is connected to CSENSE pin and determines a voltage
drop on RCSENSE. If the voltage across the sensing element is greater than this voltage
drop, the controller doesn't initiate a new cycle. A new cycle starts only when the sensed
current goes below the current limit.
Since the current limit circuit is a valley current limit, the actual peak current limit is greater
than the current limit threshold by an amount equal to the inductor ripple current. Moreover
the maximum output current is equal to the valley current limit plus half of the inductor ripple
current:
Equation 6
The output current limit depends on the current ripple, as shown in Figure 34:
Figure 34.
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 RCSENSE:
Equation 7
Where ICSENSE = 100 uA, 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 (
∆V
OFF) and on the accuracy of the current generator(∆VCSENSE):
Equation 8
2
I
I
(max)
I
L
Lvalley
LOAD
∆
+
=
Inductor current
Valley current threshold
DC current limit = maximum load
Current
Time
Maximum load current
is influenced by the
inductor current ripple
CSENSE
Lvalley
DSon
CSENSE
I
I
R
R
×
=
SNS
SNS
CSENSE
CSENSE
CSENSE
CSENSE
OFF
CSENSE
CSENSE
Lvalley
Lvalley
R
R
R
R
100
I
R
V
I
I
I
I
∆
+
∆
+
⎥
⎦
⎤
⎢
⎣
⎡
×
×
∆
+
∆
=
∆



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