Datasheet
V
SNS
= V
REF
x
R17
©
§
¹
·
R7
I
LED
=
R9
©
§
¹
·
V
SNS
=
R9
©
§
¹
·
V
REF
x
R17
©
§
¹
·
R7
V
CSH
= (R17 x I
CSH
) = V
SNS
x
R7
©
§
¹
·
R17
I
CSH
=
V
SNS
R7
©
§
¹
·
I
LED
=
1.24V
R9
©
§
¹
·
R7
R17
©
§
¹
·
x
+
-
1.24V
19
R7
20
+
-
4
2
HSP
HSN
CHS
COMP
I
CHS
+
-
V
SNS
+
-
R8
R9
V
OUT
C5
R21
V
CHS
I
LED
R17
Board Features
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Figure 2. High-Side Sensing Circuit
This establishes a current gain determined by a resistor ratio consisting of R17 and R7 along with R9 as
described in the equation:
(1)
Substituting in the resistor values as listed in the board schematic gives a fixed I
LED
current of 1A.
5.2 Setting the Current Sense Voltage
By substituting in different resistor values, the LED average current can be user adjusted. The LM3423
controller uses a high-side sense amplifier to regulate LED average current. The CSH pin is regulated by
the error amplifier to be V
REF
. Understanding how average LED current is regulated comes down to
understanding the relationship between V
CSH
and V
SNS
, because V
SNS
and R
SNS
set the LED current. The
high side amplifier in forces its input terminals to equal potential. Because of this, the V
SNS
voltage is
forced across R
HSP
. Another way to view this is that the amplifier’s output transistor pulls current through
R7 (R
HSP
) until V
HSP
= V
HSN
and this happens when the voltage across R7 is equal to V
SNS
.
The current flowing down to the CSH pin is given by,
(2)
and the voltage at the CSH pin is then given by,
(3)
The CSH voltage is the sense voltage gained up by the ratio of R17 to R7. In addition, the control
system’s error amplifier regulates the CSH voltage to V
REF
. Using equation 14, the following equations are
derived,
(4)
The above equations show how current in the LED relates to the regulated voltage V
REF
, which is
approximately 1.25V for the LM3423.
4
AN-1907 LM3423 Buck-Boost Configuration Evaluation Board SNVA376A–December 2008–Revised May 2013
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