Download Calculated Industries 3423 Specifications

LM3421, LM3421-Q1
LM3423, LM3423-Q1
www.ti.com
SNVS574E – JULY 2008 – REVISED MAY 2013
VIN
VSW
LM3421/23
RSNS
VIN/25
RT
Start tON
RCT
RT
LM3421/23
CT
VIN/25
Reset timer
LED-
Start tON
RCT
CT
Reset timer
Figure 16. Off-timer Circuitry for Boost and Buckboost Regulators
Figure 17. Off-timer Circuitry for Buck Regulators
LM3421/23
ILED
VSNS
RHSP
RSNS
RHSN
RCSH
HSP
High-Side
Sense Amplifier
HSN
CSH
ICSH
Error Amplifier
To PWM
Comparator
1.24V
CCMP
COMP
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:
ICSH =
VSNS
RHSP
(7)
This means VSNS will be regulated as follows:
RHSP
VSNS = 1.24V x
RCSH
(8)
ILED can then be calculated:
VSNS
1.24V RHSP
x
ILED =
=
RSNS
RSNS
RCSH
(9)
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