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AN2823 датащи(PDF) 23 Page - STMicroelectronics

номер детали AN2823
подробное описание детали  Current source for LED driving based on the L5973AD
PDF  33 Pages
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производитель  STMICROELECTRONICS [STMicroelectronics]
домашняя страница  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2823 датащи(HTML) 23 Page - STMicroelectronics

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AN2823
The L5973AD in LED driving applications
Doc ID 15015 Rev 1
23/33
Figure 22.
Dimming signal
From Figure 20 and 21 (VIN = 18 V, string composed of two LEDs) the following data can be
inferred:
Equation 30
Considering a dimming frequency of 250 Hz, the minimum duty cycle of the PWM dimming
satisfies the equation:
Equation 31
so the minimum D is about 5%.
By relaxing the edge requirements or increasing the transient performance of the circuit,
which depend on VIN and the number of LED composing the string and inductor value, a
lower D can be achieved.
2.2.3
Frequency compensation
Most of the theory explained in Section 2.1.3 on page 13 is still valid for the circuit in
Figure 19. The equation of the loop transfer function is:
Equation 32
where the terms GPWM, AOTA and GPWM are already known.
Here, the term
αLED(s) is slightly different and can be calculated as:
Equation 33
T
RISE
40
μs
=
T
FALL
6
μs
=
⎩
⎨
⎧
T
RISE
T
FALL
+
0.2 T
MIN_PULSE
⋅
0.2 D
MIN
T
DIMMING
⋅⋅
==
G
LOOP s
() G
PWM fSW
() Ks
() A
OTA s
() G
PWR s
() α
LED nLED
()
⋅
⋅
⋅
⋅
=
α
LED nLED
()
R
SENSE
n
LED
r
LED
R
SENSE
+
⋅
----------------------------------------------------------
=



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