LM3429_14 Datasheet, PDF (29/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

LM3429
www.ti.com
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
11. OUTPUT OVLO
For boost and buck-boost regulators, output OVLO is programmed with the turn-off threshold voltage (V_TURN-OFF)
and the desired hysteresis (V_HYSO). To set V_HYSO, solve for R_OV2:
R_OV2
=
V_HYSO
20 PA
(82)
To set V_TURN-OFF, solve for R_OV1:
Boost
R_OV1
=
1.24V x R_OV2
V_{TURN- OFF} - 1.24V
(83)
Buck-boost
R_OV1 =
1.24V x R_OV2
V_{TURN- OFF} - 620 mV
(84)
A small filter capacitor (C_OVP = 47 pF) should be added from the OVP pin to ground to reduce coupled switching
noise.
12. 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 PA
(85)
To set V_TURN-ON, solve for R_UV1:
R_UV1
=
1.24V x R_UV2
V_{TURN- ON} - 1.24V
(86)
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 PA x (R_UV1
PA x ) R_UV2
+ R ) _UV2
(87)
13. 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.
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.
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