LM3464_10 Datasheet, PDF (18/22 Pages) National Semiconductor (TI) – LED Driver with Dynamic Headroom Control and Thermal Control Interfaces

For V_RAIL(peak) = 150V, the maximum value of R_DRx is:
And the minimum value of R_DRx is:
are 3mA and 700uA respectively, in order to ensure the volt-
age of the VIN pin is clamped close to 75V even when the
LM3464/64A is disabled, a 1.5W/75V zener diode CMZ5946B
from Central Semiconductor is used to ensure adequate con-
duction current for Z_IN. The reverse current of the CMZ5946B
is specified 5mA at 75V, so the allowable current flows
through Z_IN is in between 5mA to 20mA.
The value of R_IN is governed by the following equations:
Thus, the value of R_DRx must be selected in the range:
To minimize power dissipation on the zener diodes, a stan-
dard 42.2kΩ resistor can be used for the R_DRx. Because the
resistors, R_DRx are used to absorb the power being introduced
by the voltage difference between V_RAIL and V_Zx, the maxi-
mum power dissipation on every R_DRx equals to:
Maximum value of R_IN:
Minimum value of R_IN:
So the value of R_IN must be in the range:
To minimize power dissipations on both the Z_IN and R_IN, a
standard 9.31kΩ resistor can be selected for the R_IN. Then
the maximum power dissipation on R_IN is:
Thus, a standard 42.2kΩ resistor with 0.25W power rating
(1206 package) and 1% tolerance can be used.
Calculating the Values of Z_IN and R_IN:
Similar to the requirements of selecting the Zx and R_DRx, the
voltage at the VIN pin of the LM3464/64A is clamped to 75V
by a voltage clamping circuit consists of Z_IN and R_IN. Because
the maximum operating and shut-down current (V_EN < 2.1V)
Thus, a standard 9.38kΩ resistor with 2512 package (1W)
and 1% tolerance can be used.
www.national.com
18