Data Sheet

LT4322

analog.com

Rev. 0

15 of 29

that the selected device is suited to operate within the required frequency, temperature, and load current of the

application.

Input Snubber

A small percentage of application circuits may suffer from undamped oscillations due to the parasitic input

inductance and any input bypass capacitance CIN. In those cases, utilize an RC snubber network, as shown in Figure

22, to prevent oscillations. Users must estimate their parasitic inductance LIN and choose a resistor RSN according to

the following equation:

RSN = √

������������������

������������������

(7)

Choose CSN as follows:

CSN = 2 × π × √������������������ × ������������������ / RSN

(8)

Figure 22. Input Snubber Circuitry

Operation at Voltages Above 60V

An extra blocking device is necessary to protect the VDDC and/or CATHODE pins when the peak-to-peak AC input

voltage exceeds 120V for full-wave rectifier and full-bridge applications. The blocking device is also needed when the

peak-to-peak AC voltage exceeds 60V for half-wave rectifier applications and when the DC input voltage exceeds 60V

for DC input applications. Depletion N-Channel MOSFET M2 in Figure 23 extends the input operating voltage range to

600V. It safely clamps the VDDC and CATHODE voltages to about 2V higher than the VDDA voltage. M2 must be a depletion

mode device whose VGS(OFF) ≈ -2.5V and IDSS ≥ 75mA to ensure the circuit starts up and regulates properly. Power

MOSFET M1 must also be rated to handle the maximum input voltage. The DN3765 depletion MOSFET comes in a

large package due to its 600V rating. Depletion MOSFETs rated for lower voltages (e.g., 350V) are available in smaller

packages.

Figure 23. M1 BVDSS Selection and Addition of Depletion Mode MOSFET, M2, Extends Operating Range to 600V

VIN

CIN

0.01µF

CSN

0.01µF

Ω

RSN

LIN

AC

INPUT

LT4322

VSSA

VOUT

VIN

IPT60R050G7

CVDDA

GATE

CATHODE

VSSA

VDDA

VDDC

ANODE

LT4322

M1

M2

DN3765

DEPLETION HVNMOS