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A5974AD データシート(PDF) 39 Page - STMicroelectronics

部品番号 A5974AD
部品情報  Zero load current operation
PDF  49 Pages
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メーカー  STMICROELECTRONICS [STMicroelectronics]
ホームページ  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

A5974AD データシート(HTML) 39 Page - STMicroelectronics

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A5974AD
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Doc ID 018762 Rev 1
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Figure 28.
350 mA LED boost current source
The device is powered from the output voltage so the maximum voltage drop across the
LEDs and resistor sense is 36 V.
The output voltage is given by:
Equation 42
where the ideal duty cycle D for the boost converter is:
Equation 43
As for positive and inverting buck-boost (see Section 8.6 and Section 8.7) the measured
real duty cycle must be used to calculate the switch current level.
The peak current flowing in the embedded switch is:
Equation 44
while its average current is equal to:
Equation 45
This is due to the fact that the current flowing through the internal power switch is delivered
to the output only during the OFF phase.
The switch peak current must be lower than the minimum current limit of the overcurrent
protection (see Table 4 for details) while the average current must be lower than the rated
DC current of the device.
V
OUT
V
IN
1D
–
-------------
=
D
V
OUT
V
IN
–
V
OUT
------------------------------
=
I
SW
I
LOAD
1D
–
---------------
I
RIPPLE
2
--------------------
+
I
LOAD
1D
–
---------------
V
IN
2L
⋅
-----------
D
f
SW
---------
⋅
+
==
I
SW
I
LOAD
1D
–
---------------
=



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