LM3464MH National Semiconductor, LM3464MH Datasheet - Page 12

• Manufacturer Part Number: LM3464MH
• Description: IC, LED DRIVER, BUCK, 28TSSOP
• Manufacturer: National Semiconductor

Specifications of LM3464MH

• No. Of Outputs: 4
• Input Voltage: 12V To 80V
• Driver Case Style: TSSOP
• Svhc: No SVHC (15-Dec-2010)
• No. Of Pins: 28
• Operating Temperature Range: -40°C To +125°C
• Topology: Linear
• Efficiency: 98%
• Led Driver Application: Streetlights, Solid State Lighting Solutions
• Dimming Control Type: PWM / Analogue
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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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
voltages of the LED arrays and should follow the equation
shows below:
In the equation, V
all the LED strings 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 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 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
avoid damaging the AC/DC converter, the possible peak out-
put voltage, V
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
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:
for V
also since
is the voltage headroom which equals to the voltage at the
V
RAIL(peak)
REF(AC/DC)
RAIL(peak)
DHC_READY
RAIL(nom)
= V
RAIL(peak)
= 2.5V
V
RAIL(nom)
R1
is set to to no more than 10VDC higher than
f(all_temp)
RAIL(nom)
V
+ V
DHC_READY
)
AND V
can be roughly defined by the forward
REF(AC/DC)
≤
is the lowest forward voltage among
RAIL(nom)
is primarily depend on the forward
V
f(all_temp)
RAIL(peak)
< V
= (R
RAIL(peak)
is the nominal output volt-
FB1
+ V
1
and R
x I
VDHC
R1
) + V
FB2
DHC_READY
:
REF(AC/DC)
. To
VD-
12
As the system enters steady state, the rail voltage V
creases and finally settles to an optimal level that maintains
the maximum power efficiency of the system. The voltage
level of V
this equation:
In the equation, V
V
carry the highest forward voltage among the LED stings.
V
the headroom voltage for the LM3464/64A driver stage and
equals to the minimum V
MOSFETs under steady state. The VDHC pin is internally bi-
ased to 0.9V which also set the default voltage headroom to
0.9V. In applications that the output of the AC/DC converter
contains more than 0.9V peak-to-peak ripple voltage, the volt-
age headroom can be increased by applying external bias to
the VDHC pin.
DEFINING VOLTAGE HEADROOM
The voltage headroom is the rail voltage margin that reserve
for precision linear current regulation under steady state. Un-
der steady state, the voltage headroom is always minimized
by the LM3464/64A to reduce power losses on the MOSFETs
till one of the drain voltage (V
voltage on VDHC pin (0.9V typical).
With external bias, the voltage of the VDHC pin can be ad-
justed up or down to adapt to different types of primary power
supply. Figure 4 shows a simple resistor based biasing circuit
that derives biasing voltage from the output of the internal
voltage regulator, the VCC pin.
With the additional resistors, the VDHC pin voltage is ad-
justable in between 0.8V and 2V. The values of R
should be at least 10 times lower than the typical values of
the internal resistor divider of the VDHC pin (see Figure 4).
However, it is recommended not to set the voltage headroom
too low because the ripple voltage of the primary power sup-
FIGURE 4. Adjusting Voltage Headroom with Resistors
f(highest)
VDHC
is the voltage at the VDHC pin. This voltage decides
is the total forward voltage of the LED string which
RAIL
under steady state can be calculated following
V
RAIL
RAIL
is the rail voltage in steady state and
= V
DRx
f(highest)
DRx
among the drain voltages of the
) of the MOSFETs equals the
+ V
VDHC
30115013
A
RAIL
and R
de-
B