LM3464 datasheet(17/22 Pages) NSC

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30115061

FIGURE 13. Additional Voltage Clamping Circuits for V

RAIL(peak) > 80V/95V (LM3464/64A)

APPLICATIONS WITH HIGH RAIL VOLTAGE

The normal operation voltage of the LM3464 and LM3464A

are rated to 80V and 95V respectively, applying voltage over

the operation voltage limit to the LM3464/64A can damage

the device permanently. In applications that the rail voltage is

higher than the operation voltage limited of the device (80V

for LM3464, 95V for LM3464A), voltage clamping circuits

must be added externally to ensure the voltage limits of all the

pins of the LM3464/64A are not violated. Figure 13 shows a

typical application circuit with 150V peak rail voltage.

In figure 13, Z

1, Z2, Z3, Z4 and ZIN are zener diodes for clamp-

ing 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 resis-

tors R

DR1, RDR2, RDR3, RDR4 and RIN are resistors for absorb-

ing the voltage difference between the clamping voltage of the

zener diodes and the rail voltage.

Calculating the Values of Z

x and RDRx:

The resistance of the R

DRx must be properly selected accord-

ing 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 re-

verse 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 volt-

age is below 100V the current flows into the DRx pin (I

DRx) is

below 63uA. In order to reserve operation margin for compo-

nent 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 fol-

lowing this equation:

Where V

Z and IZ are the reverse voltage and current of the

zener diode Zx respectively.

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