Datasheet
( )
BASESSSUTSUSS
ttC
--
-x
=
V2.0
A10P
OCMPBYPBASESSSU
CCk28C168t x
+
x:
+
x:
=
--
O
V
LED
I
COCMPVCCSU
tttt
++=
( )
RR +x
2UV1UV
A20 P
( )
x
HYS
A20V xP- R
2UV
R
1UV
UVH
R =
R
UV1
=
1.24VV
ONTURN
-
-
R1.24V
UV2
x
R
UV2
=
A20 P
V
HYS
LM3424
www.ti.com
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009
A small filter capacitor (C
OVP
= 47 pF) should be added from the OVP pin to ground to reduce coupled switching
noise.
14. INPUT UVLO
For all topologies, input UVLO is programmed with the turn-on threshold voltage (V
TURN-ON
) and the desired
hysteresis (V
HYS
).
Method #1: If no PWM dimming is required, a two resistor network can be used. To set V
HYS
, solve for R
UV2
:
(95)
To set V
TURN-ON
, solve for R
UV1
:
(96)
Method #2: If PWM dimming is required, a three resistor network is suggested. To set V
TURN-ON
, assume R
UV2
=
10 kΩ and solve for R
UV1
as in Method #1. To set V
HYS
, solve for R
UVH
:
(97)
15. SOFT-START
For all topologies, if soft-start is desired, find the start-up time without C
SS
(t
SU
):
(98)
Then, if the desired total start-up time (t
TSU
) is larger than t
SU
, solve for the base start-up time (t
SU-SS-BASE
),
assuming that a C
SS
greater than 40% of C
CMP
will be used:
(99)
Then solve for C
SS
:
(100)
16. PWM DIMMING METHOD
PWM dimming can be performed several ways:
Method #1: Connect the dimming MosFET (Q
3
) with the drain to the nDIM pin and the source to GND. Apply an
external PWM signal to the gate of Q
DIM
. A pull down resistor may be necessary to properly turn off Q
3
.
Method #2: Connect the anode of a Schottky diode to the nDIM pin. Apply an external inverted PWM signal to
the cathode of the same diode.
The DDRV pin should be connected to the gate of the dimFET with or without level-shifting circuitry as described
in the PWM DIMMING section. The dimFET should be rated to handle the average LED current and the nominal
output voltage.
17. ANALOG DIMMING METHOD
Analog dimming can be performed several ways:
Method #1: Place a potentiometer in place of the thermistor in the thermal foldback circuit shown in the
THERMAL FOLDBACK / ANALOG DIMMING section.
Method #2: Place a potentiometer in series with the R
CSH
resistor to dim the LED current from the nominal I
LED
to near zero.
Method #3: Connect a controlled current source as detailed in the THERMAL FOLDBACK / ANALOG DIMMING
section to the CSH pin. Increasing the current sourced into the CSH node will decrease the LEDs from the
nominal I
LED
to zero current in the same manner as the thermal foldback circuit.
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