Datasheet
CST
5
V
=
ADJ
V
5
5
=
EXT
RA1 x
=
#
EXT
RA x#
=
5
R
=
x
248 mV
5
1.24V
==
V
ADJ
R5 x
V
ADJ
V
CST
CSP
5V
1.24V
5
#A
5R
R
R
+
-
-
+
I
T
ENDS
V
IN
+
-
CSN
IADJ
+
-
GND
Optional
PGATE
LED+
LED-
R
SNS
+-
V
SNS
D1
L1
Q1
V
ADJ
R
EXT
I
L
V
CST
t
ON
LM3409/09HV
LM3409, LM3409HV, LM3409-Q1
SNVS602J –MARCH 2009–REVISED MAY 2013
www.ti.com
Figure 22. Peak Current Control Circuit
There are three different methods to set the current sense threshold (V
CST
) using the multi-function IADJ pin:
1. IADJ pin left open: 5µA internal current source biases the Zener diode and clamps the IADJ pin voltage
(V
ADJ
) at 1.24V causing the maximum threshold voltage:
(2)
2. External voltage (V
ADJ
) of 0V to 1.24V: Apply to the IADJ pin to adjust V
CST
from 0V to 248mV. If the V
ADJ
voltage is adjustable, analog dimming can be achieved.
3. External resistor (R
EXT
) placed from IADJ pin to ground: 5µA current source sets the V
ADJ
voltage and
corresponding threshold voltage:
(3)
Controlled Off-Time
Once Q1 is turned off, it remains off for a constant time (t
OFF
) which is preset by an external resistor (R
OFF
), an
external capacitor (C
OFF
), and the output voltage (V
O
) as shown in Figure 23. Since I
LED
is tightly regulated, V
O
will remain nearly constant over widely varying input voltage and temperature yielding a nearly constant t
OFF
.
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