LM3464MHX National Semiconductor Corporation, LM3464MHX Datasheet - Page 11

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

System Operation
In order to provide failure protection to the LEDs, the rail volt-
age is pulled up by the LM3464 from a relatively low voltage
level until reaching certain preset level. Figure 3 shows the
change of the rail voltage of the LM3464 LED lighting system
upon the primary power source is powered.
Figure 1 shows the typical application circuit of powering the
LM3464 LED driver circuitry with an off-the-shelf fly-back AC/
DC converter. In this application, output voltage feedback of
the AC/DC converter is mainly governed by a voltage refer-
ence IC, LM431 and a voltage divider consists of R
The LM3464 influences the output voltage of the AC/DC con-
verter by sinking current from the junction of the voltage
divider (R
The operation of the LM3464 upon startup can be divided into
several phases according to the change of the rail voltage as
shown in Figure 3. When the AC/DC converter is powered up,
the rail voltage increases and stays steady when its native
nominal output voltage, V
defined by the output voltage feedback resistor divider of the
AC/DC converter. At this voltage level, the LM3464 is pow-
ered already. After certain delay defined by C
starts to push up the rail voltage by sinking current into the
OutP pin from the voltage feedback node of the AC/DC con-
verter until V
rail voltage in normal operation and the voltage level that the
LED strings turns on and DHC begins. As the LEDs turn on,
the output capacitor of the primary power supply discharges
and the rail voltage decreases to certain level that system ef-
ficiency is maximized (V
FIGURE 3. Changes of Rail Voltage Upon Power Up
1
and R
DHC_READY
2
) to realize dynamic headroom control.
is reached. This is the possible highest
LED
RAIL(nom)
).
is reached. This voltage is
DHC
, the LM3464
1
and R
30115039
2
.
11
Application Information
SETTING (V
The nominal rail voltage V
age of the primary power supply (AC/DC) prior to DHC begins.
The selection of V
ward voltages of the LED arrays and should follow the equa-
tion shows below:
In the equation, V
all the LED arrays under all possible temperature. And V
HC
VDHC pin. Normally, the forward voltage of an LED drops as
the ambient temperature increases. This could create large
variation of total forward voltage of a LED sting under different
temperature. In order to ensure proper system startup, the
variation of LED forward voltage against temperature and for-
ward current must be considered in calculations.
SETTING V
DHC begins when the voltage at VLedFB pin reaches 2.5V,
which is defined by the values of R
Where
At this stage, the current of the LED strings are regulated and
the rail voltage decreases to reduce the voltage drop and
power dissipation on the MOSFETs.
In case the OutP pin is accidentally shorten to ground, the rail
voltage will increase and end up exceeding V
possible peak output voltage, V
verter can be roughly defined by the forward voltage of the
LED strings and must set below the rated voltage of the com-
ponents at the output of the AC/DC converter. It is suggested
to limit V
ward voltage of the LED string. The following equations define
the maximum output voltage of the AC/DC converter that can
be pushed up by the LM3464:
for V
also since
V
is the voltage headroom that defines by the voltage at the
RAIL(peak)
REF(AC/DC)
RAIL(peak)
DHC_READY
RAIL(nom)
= V
= 2.5V
V
RAIL(nom)
R1
f(all_temp)
to no more than 10VDC higher than the for-
V
RAIL(nom)
+ V
DHC_READY
)
AND V
REF(AC/DC)
≤
is the lowest forward voltage among
RAIL(nom)
is primarily dependent on the for-
V
f(all_temp)
RAIL(peak)
< V
RAIL(peak)
= (R
RAIL(peak)
is the nominal output volt-
FB1
+ V
1
and R
x I
VDHC
R1
of the AC/DC con-
) + V
FB2
DHC_READY
:
REF(AC/DC)
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. The
VD-