Texas Instruments LM3464, Application Note, Page: 15 / 20

rent of the CMHZ5267B is 1.7mA at 75V zener voltage, the
maximum allowable reverse current is 6.67mA at 500mW
power dissipation.
Given that the input current of the DRx pins of the LM3464A
at 100V is 63uA maximum, if the DRx pin voltage is below
100V, the current flowing into the DRx pin (I
DRx
) is below
63uA. In the following calculations, I
DRx
is assumed to 63uA
to reserve operation margin to compensate the characteris-
tics variations of the components.
Because V
RAIL(peak)
is the possible highest voltage at the DRx
pins, the maximum resistance of R
DRx
can be calculated fol-
lowing this equation:
Where V
Z
and I
Z
are the reverse voltage and current of the
zener diode Zx respectively.
For V
RAIL(peak)
= 150V, the maximum value of R
DRx
is:
And the minimum value of R
DRx
is:
Thus, the value of R
DRx
must be selected in the range:
To minimize power dissipation on the zener diodes, a stan-
dard 42.2k
Ω
resistor can be used for the R
DRx
. The maximum
power dissipation on the R
DRx
is then equals to:
Thus, a standard 42.2k
Ω
resistor with 0.25W power rating
(1206 package) and 1% tolerance can be used.
Calculating the Values of Z
IN
and R
IN
:
Assume the VIN pin of the LM3464A is about to be clamped
to 75V, a 1.5W/75V zener diode CMZ5946B from Central
Semiconductor is used to ensure adequate conduction cur-
rent for Z
IN
. Because the reverse current of the CMZ5946B is
5mA at 75V, the allowable current flows through Z
IN
is in be-
tween 5mA to 20mA. Similar to the requirements of selecting
the Zx and R
DRx
, the voltage at the VIN pin of the LM3464A
is clamped to 75V by a voltage clamping circuit consists of
Z
IN
and R
IN
. Also since the maximum operating and shut-
down current (V
EN
< 2.1V) are 3mA and 700uA respectively,
to ensure the voltage of the VIN pin is clamped close to 75V
even when the LM3464A is disabled, the value of R
IN
should
be calculated following the equations below:
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 9.31k
Ω
resistor can be selected for the R
IN
. Then
the maximum power dissipation on R
IN
is:
Thus, a standard 9.38k
Ω
resistor with 2512 package (1W)
and 1% tolerance can be used.
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