LM3424 Datasheet, PDF (38/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
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Typical Applications (continued)
A small filter capacitor (C_OVP = 47 pF) should be added from the OVP pin to ground to reduce coupled switching
noise.
8.2.1.2.14 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:
R_UV2
=
V_HYS
20 2A
(95)
To set V_TURN-ON, solve for R_UV1:
R_UV1
=
1.24V x
V_{TURN- ON} -
R_UV2
1.24V
(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:
R_UVH
=
R_UV1 x (V_HYS - 20
20 2A x (R_UV1
2A x ) R_UV2
+ ) R_UV2
(97)
8.2.1.2.15 Soft-Start
For all topologies, if soft-start is desired, find the start-up time without C_SS (t_SU):
t_SU = t _VCC + t_CMP + t_CO
(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:
t_SU-
SS
-
BASE
=168:
x
C_BYP
+
28
k:
x
C_CMP
+
V_O
I_LED
x
C_O
(99)
Then solve for C_SS:
( ) C_SS
=
10 2A
0.2V
x
t_TSU - t_SU-SS-BASE
(100)
8.2.1.2.16 PWM Dimming Method
PWM dimming can be performed several ways:
Method 1: Connect the dimming MOSFET (Q₃) 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 Q₃.
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.
8.2.1.2.17 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 and 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 and 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.
38
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