Datasheet

f
LM3464(-3 dB)
=
1
2S(1.2 x 10
6
) x C
DHC
LM3464, LM3464A
www.ti.com 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
DHC
decide 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:
(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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