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LED5000 数据表(PDF) 22 Page - STMicroelectronics

部件名 LED5000
功能描述  3 A monolithic step-down current source with dimming capability
PDF  52 Pages
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制造商  STMICROELECTRONICS [STMicroelectronics]
网页  http://www.st.com
标志 STMICROELECTRONICS - STMicroelectronics

LED5000 数据表(HTML) 22 Page - STMicroelectronics

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Application notes - buck conversion
LED5000
22/51
Doc ID 023951 Rev 1
5.8.1
Dimming frequency vs. dimming depth
As seen in
Chapter 5.8 the LEDs current rising and falling edge time mainly depends on the
system bandwidth (TRISE) and the selected output capacitor value (TRISE and TFALL).
The dimming performance depends on the minimum current pulse shape specification of
the final application. The ideal minimum current pulse has rectangular shape, in any case it
degenerates into a trapezoid or, at worst, into a triangle, depending on the ratio (TRISE +
TFALL)/ TDIM
Equation 23
The small signal response in
Figure 14 is considered as an example.
Equation 24
Assuming the minimum current pulse (TMIN_PULSE) shape specification as:
Equation 25:
where TDIMMING represents the dimming period and DMIN the minimum duty cycle which
gives the TMIN_PULSE charge. In the given example TMIN_PULSE=9µs
Figure 15.
dimming signal
Given TMIN_PULSE it is possible to calculate the maximum dimming depth given the dimming
frequency or vice versa.
For example, assuming a 10 KHz dimming frequency the maximum dimming depth is 9% or
given a 5% dimming depth it follows a 5.5 KHz maximum fDIM.
The LED5000 dimming performance is strictly dependent on the system small signal
response. As a consequence, an optimized compensation network (good phase margin and
bandwidth maximized) and minimized COUT value are crucial for best performance. Once
rec
gle
tan
T
RISE
T
FALL
+
T
DIM
---------------------------------------------
1
«
→
trapezoid
T
RISE
T
FALL
+
T
DIM
---------------------------------------------
1
<
→
triangle
T
RISE
T
FALL
+
T
DIM
---------------------------------------------
1
=
T
RISE
5
μs
≅
T
FALL
2
μs
≅
⎩
⎨
⎧
T
RISE
T
FALL
+
0.75
T
MIN_PULSE
⋅
0.75
D
MIN
T
DIMMING
⋅⋅
==
AM13499v1



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