LM3464_10 Datasheet, PDF (14/22 Pages) National Semiconductor (TI) – LED Driver with Dynamic Headroom Control and Thermal Control Interfaces

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FIGURE 9. Signals Facilitating Thermal Foldback Control
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 ther-
mal foldback control are governed by the following equations:
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
SETTING MINIMUM THERMAL DIMMING DUTY CYCLE
In applications that need to guarantee minimum illumines un-
der 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 10.
30115042
FIGURE 10. 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
When V_DMIN is below 0.4V (e.g. connect to GND), the mini-
mum thermal dimming duty cycle limit is disabled. In applica-
tions 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 guarantees
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:
I_LED(AVG) = D_PWM x I_LED
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 11 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 dis-
charged on every rising edge of the PWM dimming signal on
DIM pin, notice as t₁, t₂ and t₃ in Figure 11.
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