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LM3421, LM3421-Q1
LM3423, LM3423-Q1
SNVS574E – JULY 2008 – REVISED MAY 2013
www.ti.com
Method #2: If PWM dimming is required, a three resistor network is suggested. To set V TURN-ON , assume R UV2 =
10 k ? and solve for R UV1 as in Method #1. To set V HYS , solve for R UVH :
R UVH =
R UV 1 x ( V HYS - 23 P A x R UV 2 )
23 P A x ( R UV 1 + R UV 2 )
(94)
13. 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 AGND. 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.
14. ANALOG DIMMING METHOD
Analog dimming can be performed several ways:
Method #1: Place a potentiometer in series with the R CSH resistor to dim the LED current from the nominal I LED
to near zero.
Method #2: Connect a controlled current source as detailed in the 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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