LM3464 datasheet(14/22 Pages) NSC

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30115041

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:

Thermal-foldback = (TLED_ON x fThermal-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:

MINIMUM = [(VDMIN - 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

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:

LED(AVG) = DPWM x ILED

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,

Thermal_Cap, VDIM and ILED. 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 t

1, t2 and t3 in Figure 11.

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