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LM3464AMHX 数据表(PDF) 11 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部件名 LM3464AMHX
功能描述  LED Driver with Dynamic Headroom Control and Thermal Control Interfaces
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制造商  NSC [National Semiconductor (TI)]
网页  http://www.national.com
标志 NSC - National Semiconductor (TI)

LM3464AMHX 数据表(HTML) 11 Page - National Semiconductor (TI)

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System Operation
In order to provide failure protection to the LEDs, the rail volt-
age is pulled up by the LM3464/64A from a relatively low
voltage level at system startup until the rail voltage reaches
certain preset level. Figure 3 shows the change of the rail
voltage of the LM3464/64A LED lighting system upon the pri-
mary power source is powered.
The Lm3464/64A can be interfaced to an off-the-shelf con-
verter to form a LED lighting system with simple connections.
Figure 1 shows the typical application circuit of a lighting sys-
tem using the LM3464/64A with a fly-back AC/DC converter.
In this application, the output voltage of the AC/DC converter
is mainly governed by a voltage reference IC, LM431 and a
voltage divider consists of R
1 and R2. The LM3464/64A influ-
ences the output voltage of the AC/DC converter by sinking
current from the junction of the voltage divider (R
1 and R2) to
realize dynamic headroom control.
The operation of the LM3464/64A upon startup can be divided
into several phases according to the changes of the rail volt-
age as shown in Figure 3. When the AC/DC converter is
powered, the rail voltage increases and stays steady when its
native nominal output voltage, V
RAIL(nom) is reached. This volt-
age is defined by the output voltage feedback resistor divider
of the AC/DC converter. At this voltage level, the LM3464/64A
is powered already. After certain delay defined by C
DHC, the
LM3464/64A starts to push the rail voltage up by sinking cur-
rent into the OutP pin from the voltage feedback node of the
AC/DC converter until the rail voltage reaches V
DHC_READY.
V
DHC_READY is the highest rail voltage in normal operation and
should be enough to turn on all the LED strings with current
regulation (defined by R
SNSx). As VRAIL reaches VDHC_READY,
the LM3464/64A turns on all the output channels. This dis-
charges the output capacitor of the primary power supply and
causes the rail voltage to decrease to certain level that system
efficiency is maximized (V
LED).
30115039
FIGURE 3. Changes of Rail Voltage Upon Power Up
Application Information
SETTING (V
RAIL(nom))
The nominal rail voltage V
RAIL(nom) is the nominal output volt-
age of the primary power supply (AC/DC) prior to DHC begins.
The selection of V
RAIL(nom) is primarily depend on the forward
voltages of the LED arrays and should follow the equation
shows below:
V
RAIL(nom) Vf(all_temp) + VVDHC
In the equation, V
f(all_temp) is the lowest forward voltage among
all the LED strings under all possible temperature. And V
VD-
HC is the voltage headroom which equals to the voltage at the
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 must be
considered in calculations.
SETTING V
DHC_READY AND VRAIL(peak)
DHC begins when the voltage at VLedFB pin reaches 2.5V,
which is defined by the values of R
FB1 and RFB2:
Where
V
DHC_READY < VRAIL(peak)
At this stage, the current of the LED strings are regulated and
the rail voltage decreases in order to maintain minimum volt-
age 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 exceeds V
DHC_READY. To
avoid damaging the AC/DC converter, the possible peak out-
put voltage, V
RAIL(peak) can be roughly defined by the forward
voltage of the LED strings and must set below the rated volt-
age of the components at the output of the AC/DC converter.
In order to limit the power dissipation on the external MOS-
FETs, V
RAIL(peak) is set to to no more than 10VDC higher than
the forward 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 LM3464/64A:
V
RAIL(peak) = VR1 + VREF(AC/DC) = (R1 x IR1) + VREF(AC/DC)
for V
REF(AC/DC) = 2.5V
also since
11
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