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LM3421 Datasheet(PDF) 13 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) 13 Page - Texas Instruments

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ILED =
RSNS
1.24V
RSNS
VSNS
RCSH
RHSP
=
x
VSNS = 1.24V x
RCSH
RHSP
ICSH =
RHSP
VSNS
RSNS
ILED
RHSP
RHSN
HSN
HSP
High-Side
Sense Amplifier
CSH
1.24V
CCMP
RCSH
COMP
Error Amplifier
VSNS
To PWM
Comparator
LM3421/23
ICSH
RT
CT
LM3421/23
RCT
Start tON
VIN/25
Reset timer
RSNS
VIN
LED-
RT
CT
VSW
LM3421/23
RCT
Start tON
VIN/25
Reset timer
LM3421, LM3421-Q1
LM3423, LM3423-Q1
www.ti.com
SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011
Figure 16. Off-timer Circuitry for Boost and Buck-
Figure 17. Off-timer Circuitry for Buck Regulators
boost Regulators
Figure 18. LED Current Sense Circuitry
AVERAGE LED CURRENT
The LM3421/23 uses an external current sense resistor (RSNS) placed in series with the LED load to convert the
LED current (ILED) into a voltage (VSNS) as shown in Figure 18. The HSP and HSN pins are the inputs to the
high-side sense amplifier which are forced to be equal potential (VHSP=VHSN) through negative feedback.
Because of this, the VSNS voltage is forced across RHSP to generate the signal current (ICSH) which flows out of
the CSH pin and through the RCSH resistor. The error amplifier will regulate the CSH pin to 1.24V, therefore ICSH
can be calculated:
(7)
This means VSNS will be regulated as follows:
(8)
ILED can then be calculated:
(9)
Copyright © 2008–2011, Texas Instruments Incorporated
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Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1



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