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LED5000 Datasheet(PDF) 41 Page - STMicroelectronics

Part # LED5000
Description  3 A monolithic step-down current source with dimming capability
PDF  52 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

LED5000 Datasheet(HTML) 41 Page - STMicroelectronics

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LED5000
Application notes - alternative topologies
Doc ID 023951 Rev 1
41/51
Equation 66
where VOP_MIN is the minimum operating voltage.
The equations for the floating boost are:
Equation 67
The ideal duty cycle DIDEAL for the boost converter is:
Equation 68
As seen for the buck-boost topologies (
Chapter 6.1 and Chapter 6.2), due to power losses
the real duty cycle is always higher than the ideal. The real value (that can be measured in
the application) should be used in the following formulas to estimate the switch current.
The peak current flowing in the embedded switch is:
Equation 69
while its average current level is equal to:
Equation 70
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
Section Table 5.: Electrical characteristics for details) while the average
current must be lower than the rated DC current of the device.
As a consequence, the maximum output current depends on the application conditions:
Equation 71
where ISW MAX represents the rated current of the device.
The current capability is reduced by the term (1-DREAL) and so, for example, with a duty
cycle of 0.5, and considering an average current through the switch of 3 A, the maximum
output current deliverable to the load is 1.5 A.
V
IN_START
V
OP_MIN
V
DIODE
+
5.5V
V
DIODE
+
==
V
OUT
V
IN
1D
IDEAL
–
----------------------------
=
D
IDEAL
V
OUT
V
IN
–
V
OUT
-----------------------------
=
I
SW
I
LOAD
1D
REAL
–
---------------------------
I
RIPPLE
2
--------------------
+
I
LOAD
1D
REAL
–
---------------------------
V
IN
2L
⋅
-------------
D
f
SW
---------
⋅
+
==
I
SW
I
LOAD
1D
REAL
–
---------------------------
=
I
LOAD MAX
I
SW MAX
1D
REAL
–
()
⋅
≅



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