Texas Instruments LM3423 Evaluation Board LM3423BBLSCSEV/NOPB LM3423BBLSCSEV/NOPB Datenbogen
Produktcode
LM3423BBLSCSEV/NOPB
I
LED
=
R
SNS
1.24V
R
SNS
V
SNS
R
CSH
R
HSP
=
x
V
SNS
= 1.24V x
R
CSH
R
HSP
I
CSH
=
R
HSP
V
SNS
R
SNS
I
LED
R
HSP
R
HSN
HSN
HSP
High-Side
Sense Amplifier
CSH
1.24V
C
CMP
R
CSH
COMP
Error Amplifier
V
SNS
To PWM
Comparator
Comparator
LM3421/23
I
CSH
R
T
C
T
LM3421/23
RCT
Start t
ON
V
IN
/25
Reset timer
R
SNS
V
IN
LED-
R
T
C
T
V
SW
LM3421/23
RCT
Start t
ON
V
IN
/25
Reset timer
SNVS574E – JULY 2008 – REVISED MAY 2013
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 (R
SNS
) placed in series with the LED load to convert the
LED current (I
LED
) into a voltage (V
SNS
) as shown in
. 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:
(7)
This means V
SNS
will be regulated as follows:
(8)
I
LED
can then be calculated:
(9)
Copyright © 2008–2013, Texas Instruments Incorporated
13
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