Datasheet

R
DHC
=
1.7V
I
R1
-
2.5V
R
2
I
R1
=
V
REF(AC/DC)
R
D2
+
V
REF(AC/DC)
- V
D1
- V
outP(min)
R
DHC
=
2.5V
R
D2
+
2.5V - 0.5V - 0.3V
R
DHC
I
R1
=
V
RAIL(peak)
- 2.5V
R
1
2.5V = V
DHC_READY
x
R
FB2
R
FB1
+ R
FB2
LM3464, LM3464A
SNVS652F –APRIL 2010–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
SETTING (V
RAIL(nom)
)
The nominal rail voltage V
RAIL(nom)
is the nominal output voltage 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)
≤ V
f(all_temp)
+ V
VDHC
(1)
In the equation, V
f(all_temp)
is the lowest forward voltage among all the LED strings under all possible temperature.
And V
VDHC
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 V
RAIL(peak)
DHC begins when the voltage at VLedFB pin reaches 2.5V, which is defined by the values of R
FB1
and R
FB2
:
(2)
Where
V
DHC_READY
< V
RAIL(peak)
(3)
At this stage, the current of the LED strings are regulated and the rail voltage decreases in order to maintain
minimum 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 exceeds
V
DHC_READY
. To avoid damaging the AC/DC converter, the possible peak output voltage, V
RAIL(peak)
can be roughly
defined by the forward voltage of the LED strings and must set below the rated voltage of the components at the
output of the AC/DC converter. In order to limit the power dissipation on the external MOSFETs, 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)
= V
R1
+ V
REF(AC/DC)
= (R
1
x I
R1
) + V
REF(AC/DC)
(4)
for V
REF(AC/DC)
= 2.5V
(5)
also since
(6)
(7)
As the system enters steady state, the rail voltage V
RAIL
decreases and finally settles to an optimal level that
maintains the maximum power efficiency of the system. The voltage level of V
RAIL
under steady state can be
calculated following this equation:
V
RAIL
= V
f(highest)
+ V
VDHC
(8)
In the equation, V
RAIL
is the rail voltage in steady state and V
f(highest)
is the total forward voltage of the LED string
which carry the highest forward voltage among the LED stings.
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