LM3464 Datasheet, PDF (15/33 Pages) National Semiconductor (TI) – 4 Channel LED Driver Evaluation Board

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LM3464, LM3464A
SNVS652F – APRIL 2010 – REVISED MAY 2013
Figure 20. 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 – V_CDHC, 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 response 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:
1
f_{LM3464(-3 dB)} = _{2S(1.2 x 106) x CDHC}
(11)
THERMAL FOLDBACK INTERFACE
The thermal foldback function of the LM3464/64A helps in reducing the average LED currents and prolonging the
LED lifetime under high temperature. By applying a DC voltage to the Thermal pin, the average output current is
adjustable from 100% down to a minimum value limited by the discharge time of the C_THM. The Thermal pin of
the LM3464/64A is an analog input for thermal foldback control that accepts a DC voltage in the range of 0V to
V_CC. The thermal foldback control circuitry reduces the average LED currents by means of PWM dimming as
shown in Figure 21:
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