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AN4501 Datasheet(PDF) 28 Page - STMicroelectronics

Part # AN4501
Description  Design of a boost LED driver using L99LD01
PDF  48 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN4501 Datasheet(HTML) 28 Page - STMicroelectronics

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Selection of the output capacitor
AN4501
28/48
DocID026394 Rev 1
Figure 12. Typical current waveforms of the output capacitor current
Equation 18:
The contribution ESRCOUT to the output voltage ripple is:
Equation 19:
6.1.3
Total output voltage ripple
Summing both contributions, we obtain:
The substitution of IL,PP by its expression from Equation 11 yields:
Equation 20:
6.2
Output current ripple
The output current ripple is the ratio between the total output voltage ripple and the dynamic
resistance of the LED string (note that RSENSE and RONM2 can be neglected compared to
the dynamic resistance of the LED string).
RD,OUT ~ NbLED x RD,LED where NbLED is the number of LEDs in the string and RD,LED is
the dynamic resistance one LED at the considered output current.
I
CO UT PP
,
I
DPP
,
I
L PEAK
,
I
OU T
1D
–
-------------
V
OUT
D
×
1D
–
()
×
2L
×
F
SW
×η
×
---------------------------------------------------
+
==
=
ΔV
COUT,ESR
ESR
CO U T
I
L PEAK
,
×
=
V
OUT PP
,
ΔV
COUT
ΔV
CO UT ESR
,
I
OUT
D
×
C
OUT
F
SW
×
----------------------------------
ESR
COUT
I
L PEAK
,
×
+
==
=
V
OUT PP
,
I
OU T
D
×
C
OUT
F
SW
×
----------------------------------
ESR
COUT
I
OUT
1D
–
-------------
V
OU T
D
×
1D
–
()
×
2L
×
F
SW
×η
×
---------------------------------------------------
+



×
+
=



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