LM3464 Datasheet, PDF (17/33 Pages) National Semiconductor (TI) – 4 Channel LED Driver Evaluation Board

LM3464, LM3464A
www.ti.com
SNVS652F – APRIL 2010 – REVISED MAY 2013
If the voltage at the Thermal pin is driven to exceed 3.25V, all output channels will be enabled with 100% thermal
dimming duty cycle. If the Thermal pin voltage is set below 0.4V, all output channels will be disabled with 0%
thermal dimming duty cycle. The dimming frequency and duty cycle with thermal foldback control are governed
by the following equations:
50 PA
f_{Thermal-foldback} = _{(3.25 - 0.4) x CTHM}
(14)
D_{Thermal-foldback} = (T_{LED_ON} x f_{Thermal-foldback}) x 100%
= [(V_THERMAL - 0.4) X 35]%
for 0.4 ≤ V_THERMAL ≤ 3.25V
(15)
(16)
(17)
SETTING MINIMUM THERMAL DIMMING DUTY CYCLE
In applications that need to ensure minimum illumines under high temperature environments, the minimum
dimming duty cycle for thermal foldback may need to be limited. Such limit is defined by the voltage at the DMIN
pin. When the Thermal pin voltage falls below the voltage at the DMIN pin, the thermal foldback dimming duty
cycle will maintain at the level which set by the voltage of the DMIN pin (V_DMIN), as shown in Figure 23.
V_Thermal
V_CC
3.25V
I_LED (AVG)
100%
V_DMIN
0.4V
0V
Thermal
Min. thermal
foldback begin foldback dimming
Min. I_LED (AVG)
Approximately 0.5%
(Limited by t_{THM_MIN})
Temperature
Figure 23. Thermal Foldback Control with Minimum Dimming Duty Cycle Limit
To define the minimum thermal dimming duty cycle, V_DMIN should be set in between 0.4V to 3.25V. The minimum
duty cycle is governed by the following equation:
D_MINIMUM = [(V_DMIN - 0.4) X 35]%
for 0.4 ≤ V_DMIN ≤ 3.25V
(18)
(19)
When V_DMIN is below 0.4V (e.g. connect to GND), the minimum thermal dimming duty cycle limit is disabled. In
applications that thermal foldback control is not required, the DMIN pin can be tied to GND to reduce power
consumption.
PWM DIMMING
The LM3464/64A provides a DIM pin that accepts TTL logic level signal for PWM dimming. When the DIM pin is
pulled low, all LED current regulators will turn off while maintaining V_CC regulator and part of the internal
circuitries operating. External pull up resistor is required if the DIM pin is driven by an open collector / drain
driver. PWM dimming ensures uniform color temperature of the light throughout the entire dimming range. The
average current of every output channel is decided by the dimming duty cycle and follows the equation below:
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