LM3464MHX National Semiconductor Corporation, LM3464MHX Datasheet - Page 13

• Manufacturer Part Number: LM3464MHX
• Description: Lm3464 Led Driver With Dynamic Headroom Control And Thermal Control Interfaces
• Manufacturer: National Semiconductor Corporation

RESPONSE OF THE LM3464 DRIVER STAGE
In order to ensure good operation stability of the entire sys-
tem, the response of the LM3464 circuitry must be set slower
than the primary power supply. The response of the LM3464
is decided by the value of the capacitor C
higher capacitance C
LM3464 driver stage.
Generally, a first order integrator that consists of C
transconductance amplifier with g
current limit as shown in Figure 7 defines the frequency re-
sponse of the LM3464 driver stage.
FIGURE 7. Simplified Circuit of the Frequency Response
The transconductance amplifier serves as a voltage to current
converter that charges C
voltage difference between V
As the voltage of the OutP pin is equal to V
speed of voltage change of the OutP pin is limited by C
The higher capacitance the C
OutP pins takes for certain voltage change. Thus the value of
C
If the response of the LM3464 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 driver stage unnecessarily slow will worsen transient
performance of the system and false trigger the fault detection
mechanism of the LM3464. Practically, the minimum value of
the C
cases, a 1uF 16V ceramic capacitor is a good starting point
that sets the response of the LM3464 driver stage slow
enough for initial trial.
The value of the C
the response of the LM3464 driver stage. Otherwise, in case
the system is unstable with 1uF C
C
get into stable operation.
This approach is effectively setting the cut-off frequency of the
LM3464 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 driver stage is governed by the
follow equation:
THERMAL FOLDBACK INTERFACE
The thermal foldback function of the LM3464 helps in reduc-
ing the average LED currents and prolonging the LED lifetime
under high temperature. The Thermal pin of the LM3464 is an
analog input for thermal foldback control that accepts DC
voltage from 0V to V
DHC
DHC
DHC
decides the response of the LM3464 driver stage.
capacitor should be increased until the entire system
can be found out by means of ‘try and error’. In most
DHC
Setting Mechanism
CC
DHC
capacitor can be reduced to speed up
. The thermal foldback control circuitry
DHC
will result in slower response of the
with current proportional to the
DRx
DHC
and V
DHC
m
has, the longer time the
= 76umho and +/– 15uA
, the capacitance of the
VDHC
DHC
.
CC
. In general, a
– V
DHC
CDHC
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and a
, the
DHC
.
13
reduces the average LED currents by means of PWM dim-
ming as shown in Figure 8:
The dimming frequency is defined by a sawtooth waveform
that generated by charging and discharging the capacitor
C
LM3464 charges the C
current and discharge the C
pin to ground until the pin voltage equals 0.4V. By comparing
the voltage at the Thermal pin to the sawtooth wave, a PWM
dimming signal for thermal foldback is obtained as shown in
Figure 9:
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:
FIGURE 8. Average LED Current Reduces According to
THM
connected across the Thermal_Cap pin and GND. The
THM
V
Thermal
up to 3.25V with 50uA constant
THM
by pulling the Thermal_Cap
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