LM3464 Datasheet, PDF (18/33 Pages) National Semiconductor (TI) – 4 Channel LED Driver Evaluation Board

LM3464, LM3464A
SNVS652F – APRIL 2010 – REVISED MAY 2013
www.ti.com
I_LED(AVG) = D_PWM x I_LED
(20)
PWM DIMMING CONTROL WITH THERMAL FOLDBACK
The PWM dimming control can coexist with thermal foldback by applying PWM dimming control signal and
thermal control signal to the DIM and Thermal pins concurrently. Normally, the dimming frequency for thermal
foldback control should be much higher than the frequency of the PWM dimming control signal. Figure 24
presents the relationship among V_Thermal, V_{Thermal_Cap}, V_DIM and I_LED. As shown in the figure, when thermal
foldback is functioning, the average output current can be further decreased linearly according to the duty cycle
of the PWM dimming signal being applied to the DIM pin. In order to synchronize the dimming signals, the C_THM
is discharged on every rising edge of the PWM dimming signal on DIM pin, notice as t₁, t₂ and t₃ in Figure 24.
Figure 24. Thermal Foldback + PWM Dimming Control
LOW POWER STANDBY
The LM3464/64A will enter low power standby mode when the EN pin is pulled to GND. The EN pin is internally
biased thus no external pull-up resistor or bias is required. Under standby mode, all the output channels are cut-
off and part of the internal circuitries are disabled to maintain low power consumption. Upon the EN pin is pulled
low, the OutP pin stopa sinking current from the feedback node of the primary power stage. This causes the rail
voltage fall back to V_RAIL(nom) slowly as the output capacitors of the primary power supply are being discharged
by the LEDs. Pulling the EN pin low will not disable the V_CC regulator. When the EN pin is released (floating), the
LM3464/64A exits low power standby mode and the startup sequence begins as described in Figure 16.
FAULT HANDLING and INDICATION
The LM3464/64A features a complete mechanism for fault handling and indication. The LM3464/64A detects
LED failures and raises fault indication signal at the Faultb pin upon open or short circuits of LED strings,
insufficient supply voltage and so on. In order to avoid false triggering the fault detection circuitry, the
LM3464/64A features a timer for fault recognition. When a fault condition arises and sustains longer than the
time constant preset by the capacitor , C_FLT, a fault is confirmed. The Faultb pin is then pulled low as an
indication. The time constant for fault detection is defined by the value of the capacitor connects across the
FAULT_CAP pin and GND, C_FLT. Normally, a 2.2 nF C_FLT that set a 264 us delay time is suitable for most
application. For those applications with slow response primary power supply, the value of C_FLT may need to
increase accordingly. The time delay for fault detection is governed by the following equation:
C_FLT x 3.0V
T_FAULT = _{25 PA}
(21)
18
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