LM3464 datasheet(13/22 Pages) NSC

13 / 22 page

30115011

FIGURE 6. LED Current vs R

ISNSx

RESPONSE OF THE LM3464/64A DRIVER STAGE

In order to ensure good operation stability of the entire sys-

tem, the response of the LM3464/64A circuitry must be set

slower than the primary power supply. The response of the

LM3464/64A is decided by the value of the capacitor, C

DHC.

In general, a higher capacitance C

DHC will result in slower re-

sponse of the LM3464/64A driver stage.

Generally, a first order integrator that consists of C

DHC and a

transconductance amplifier with g

m = 76umho and +/– 15uA

current limit as shown in Figure 7 defines the frequency re-

sponse of the LM3464/64A driver stage.

30115030

FIGURE 7. Simplified Circuit of the Frequency Response

Setting Mechanism

The transconductance amplifier serves as a voltage to current

converter that charges C

DHC with a current proportional to the

difference in voltage between the DRx and VDHC pins.

As the voltage of the OutP pin is equal to V

CC – VCDHC, the

capacitance of C

DHCdecide the rate of change of the OutP pin

voltage and eventually limits the frequency response of the

whole system . The higher capacitance the C

DHC has, the

longer time the OutP pins takes for certain voltage change.

Thus the value of C

DHC

decides the response of the

LM3464/64A driver stage.

If the response of the LM3464/64A driver stage is set faster

than that of the primary power supply, the entire system will

suffer from unstable operation. However, setting the re-

sponse of the LM3464/64A driver stage unnecessarily slow

will worsen transient performance of the system and false

trigger the fault detection mechanism of the LM3464/64A.

Practically, the minimum value of the C

DHC can be found out

by means of ‘try and error’. In most cases, a 1uF 16V ceramic

capacitor is a good starting point that sets the response of the

LM3464/64A driver stage slow enough for initial trial.

The value of the C

DHC capacitor can be reduced to speed up

the response of the LM3464/64A driver stage. Otherwise, in

case the system is unstable with 1uF C

DHC, the capacitance

of the C

DHC capacitor should be increased until the entire

system get into stable operation.

This approach is effectively setting the cut-off frequency of the

LM3464/64A driver stage lower than that of the primary power

supply. Usually, setting the cut-off frequencies of the two

stages apart can help avoiding unstable operation. The cut-

off frequency of the LM3464/64A driver stage is governed by

the follow equation:

THERMAL FOLDBACK INTERFACE

The thermal foldback function of the LM3464/64A helps in re-

ducing the average LED currents and prolonging the LED

lifetime under high temperature. The Thermal pin of the

LM3464/64A is an analog input for thermal foldback control

that accepts DC voltage from 0V to V

CC. The thermal foldback

control circuitry reduces the average LED currents by means

of PWM dimming as shown in Figure 8:

30115040

FIGURE 8. Average LED Current Reduces According to

Thermal

The dimming frequency is defined by a sawtooth waveform

that generated by charging and discharging the capacitor

THM which connects across the Thermal_Cap pin and GND.

The LM3464/64A charges the C

THM up to 3.25V with 50uA

constant current and discharge the C

THM by pulling the

Thermal_Cap pin to ground until the pin voltage equals 0.4V.

By comparing the voltage at the Thermal pin to the sawtooth

voltage being generated at the Thermal_Cap pin of the

LM3464/64A, a PWM dimming signal for thermal foldback is

generated as shown in Figure 9:

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