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LM3421 Datasheet(PDF) 32 Page - Texas Instruments

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Part # LM3421
Description  N-Channel Controllers for Constant Current LED Drivers
PDF  65 Pages
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

LM3421 Datasheet(HTML) 32 Page - Texas Instruments

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LIM
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245 mV
1-DMAX
DMAX
ICO-RMS = ILED x
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RMS
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12
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LM3421, LM3421-Q1
LM3423, LM3423-Q1
SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011
www.ti.com
Boost and Buck-boost
(47)
5. LED RIPPLE CURRENT
Set the nominal LED ripple current (
ΔiLED-PP), by solving for the output capacitance (CO):
Buck
(48)
Boost and Buck-boost
(49)
To set the worst case LED ripple current, use DMAX when solving for CO. Remember, when PWM dimming it is
recommended to use a minimum of 40 µF of output capacitance to improve performance.
The minimum allowable RMS output capacitor current rating (ICO-RMS) can be approximated:
Buck
(50)
Boost and Buck-boost
(51)
6. PEAK CURRENT LIMIT
Set the peak current limit (ILIM) by solving for the transistor path sense resistor (RLIM):
(52)
7. LOOP COMPENSATION
Using a simple first order peak current mode control model, neglecting any output capacitor ESR dynamics, the
necessary loop compensation can be determined.
First, the uncompensated loop gain (TU) of the regulator can be approximated:
Buck
(53)
Boost and Buck-boost
(54)
32
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Copyright © 2008–2011, Texas Instruments Incorporated
Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1



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