LM3429_14 Datasheet, PDF (18/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

LM3429
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
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The hysteresis (V_HYS) is defined as follows:
UVLO only
V_HYS = 20 PA x R_UV2
(24)
PWM dimming and UVLO
V_HYS
=
20
PA
x
¨¨©§^RUV2
+
R_UVH
x (R_UV1 +
R_UV1
R_UV2)_{¸¸¹·}
(25)
PWM DIMMING
The active low nDIM pin can be driven with a PWM signal which controls the main NFET (Q1). The brightness of
the LEDs can be varied by modulating the duty cycle of this signal. LED brightness is approximately proportional
to the PWM signal duty cycle, so 30% duty cycle equals approximately 30% LED brightness. This function can
be ignored if PWM dimming is not required by using nDIM solely as a V_IN UVLO input as described in the Input
Under-Voltage Lockout section or by tying it directly to V_CC or V_IN (if less than 76VDC).
Inverted
V_IN
PWM
LM3429
R_UV2
R_UVH
R_UV1
D_DIM
nDIM
Q_DIM
Standard
PWM
Figure 14. PWM Dimming Circuit
Figure 14 shows two ways the PWM signal can be applied to the nDIM pin:
1. Connect the dimming MosFET (Q_DIM) with the drain to the nDIM pin and the source to GND. Apply an
external logic-level PWM signal to the gate of Q_DIM. A pull down resistor may be necessary to properly turn
off Q_DIM if no signal is present.
2. Connect the anode of a Schottky diode (D_DIM) to the nDIM pin. Apply an external inverted logic-level PWM
signal to the cathode of the same diode.
A minimum on-time must be maintained in order for PWM dimming to operate in the linear region of its transfer
function. Because the controller is disabled during dimming, the PWM pulse must be long enough such that the
energy intercepted from the input is greater than or equal to the energy being put into the LEDs. For boost and
buck-boost regulators, the following condition must be maintained:
2 x I_LED x V_O X L1
t_PULSE =
V_IN²
(26)
In the previous equation, t_PULSE is the length of the PWM pulse in seconds.
STARTUP REGULATOR (V_CC LDO)
The LM3429 includes a high voltage, low dropout (LDO) bias regulator. When power is applied, the regulator is
enabled and sources current into an external capacitor connected to the V_CC pin. The V_CC output voltage is 6.9V
nominally and the supply is internally current limited to 20 mA (minimum). The recommended bypass
capacitance range for the V_CC regulator is 2.2 µF to 3.3 µF. The output of the V_CC regulator is monitored by an
internal UVLO circuit that protects the device during startup, normal operation, and shutdown from attempting to
operate with insufficient supply voltage.
18
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