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LM3421, LM3421-Q1
LM3423, LM3423-Q1
www.ti.com
V SW
LM3421/23
V IN
SNVS574E – JULY 2008 – REVISED MAY 2013
R T
V IN /25
RCT
Start t ON
R SNS
R T
LM3421/23
C T
Reset timer
LED-
V IN /25
RCT
Start t ON
C T
Reset timer
Figure 16. Off-timer Circuitry for Boost and Buck-
Figure 17. Off-timer Circuitry for Buck Regulators
boost Regulators
LM3421/23
I LED
R HSP
HSP
High-Side
Sense Amplifier
V SNS
R SNS
R HSN
HSN
R CSH
C CMP
CSH
COMP
I CSH
1.24V
Error Amplifier
To PWM
Comparator
Figure 18. LED Current Sense Circuitry
AVERAGE LED CURRENT
The LM3421/23 uses an external current sense resistor (R SNS ) placed in series with the LED load to convert the
LED current (I LED ) into a voltage (V SNS ) 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 (V HSP =V HSN ) through negative feedback.
Because of this, the V SNS voltage is forced across R HSP to generate the signal current (I CSH ) which flows out of
the CSH pin and through the R CSH resistor. The error amplifier will regulate the CSH pin to 1.24V, therefore I CSH
can be calculated:
I CSH =
V SNS
R HSP
(7)
This means V SNS will be regulated as follows:
V SNS = 1.24V x
R HSP
R CSH
(8)
I LED can then be calculated:
I LED =
V SNS
R SNS
=
1.24V
R SNS
x
R HSP
R CSH
(9)
Copyright ? 2008–2013, Texas Instruments Incorporated
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