Datasheet

LM3464, LM3464A
SNVS652F –APRIL 2010–REVISED MAY 2013 www.ti.com
In Figure 26, Z
1
, Z
2
, Z
3
, Z
4
and Z
IN
are zener diodes for clamping the DRx pin voltage and input voltage (VIN pin)
of the LM3464/64A. The reverse voltage of the zener diodes must be below 80V for LM3464 and 95V for
LM3464A. The resistors R
DR1
, R
DR2
, R
DR3
, R
DR4
and R
IN
are resistors for absorbing the voltage difference
between the clamping voltage of the zener diodes and the rail voltage.
Calculating the Values of Z
x
and R
DRx
:
The resistance of the R
DRx
must be properly selected according to the reverse current of the zener diode and
input current of the DRx pins of the LM3464/64A to secure the allowable pin voltages are not violated. For
instant, the DRx pins are required to clamp at 75V and a 500mW/75V zener diode CMHZ5267B from Central
Semiconductor is used. The reverse current of the CMHZ5267B is specified 1.7mA at 75V zener voltage. The
maximum allowable reverse current is 6.67mA as the power rating of the CMHZ5276B is 500mW.
Given that the input current of the DRx pins of the LM3464/64A at 100V is 63uA maximum, if the DRx pin voltage
is below 100V the current flows into the DRx pin (I
DRx
) is below 63uA. In order to reserve operation margin for
component variations, I
DRx
is assumed equal to 63uA in the following calculations.
Because V
RAIL(peak)
is the possible highest voltage at the DRx pins, the maximum resistance of R
DRx
can be
obtained following this equation:
(22)
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:
(23)
And the minimum value of R
DRx
is:
(24)
Thus, the value of R
DRx
must be selected in the range:
(25)
To minimize power dissipation on the zener diodes, a standard 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 maximum power dissipation on every R
DRx
equals to:
(26)
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) are 3mA and 700uA respectively, in order to ensure the voltage 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 conduction 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.
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