Data Sheet

LT4322

analog.com

Rev. 0

13 of 29

Figure 21. LT4322 in a 48VP-P, 60Hz, 1A Half-Wave Rectifier Application

The LT4322 eliminates thermal design problems and costly heat sinks by maximizing available voltage and reducing

power dissipation, which greatly improves efficiency and reduces PC board area. The LT4322 operates with input

frequencies ranging from DC to 100kHz. Thanks to its floating architecture, the maximum operating voltage is limited

only by the voltage ratings of the external MOSFETs in the particular application circuit where it is used. The following

sections cover power-on, ideal diode operation, external component selection, a design example, and

recommended PCB layout techniques.

VDDA Capacitor Selection

In its primary applications, the LT4322 draws its input power from the VDDC pin and generates a 12V supply voltage

between VDDA and VSSA. For AC input supply voltages, the VDDC voltage is nearly zero during the forward bias phase of

the AC cycle. The LT4322 draws its power from a reservoir capacitor, CVDDA, during that time. Choose CVDDA as follows:

CVDDA(µF) = 1,500× t (1)

where t is the period of the AC input supply. Use capacitors with the appropriate voltage rating and temperature

coefficient to ensure that the true capacitance matches the value in equation 1. In low-frequency applications having

large MOSFET gate capacitance and/or high output current, use a 0.1µF ceramic capacitor in parallel with CVDDA for

high-frequency bypassing. Place the 0.1µF capacitor as close to the VDDA and VSSA pins as possible. For frequencies

greater than 200Hz where CVDDA will most likely be a ceramic capacitor, CVDDA is sufficient, and a parallel ceramic

capacitor is not needed.

When powering VDDC with a DC input supply, connect a 1µF or larger capacitor between VDDA and VSSA. When powering

VDDA with a DC input supply, connect a 1µF or larger capacitor between VDDA and VSSA and connect VDDC to VSSA.

MOSFET Selection

The LT4322 drives N-Channel MOSFETs to conduct the load current. The important characteristics of the MOSFET are

on-resistance, RDS(ON), the maximum drain-source voltage, BVDSS, the gate threshold voltage VGS(TH), the continuous

body diode current rating IS and the single pulse avalanche energy rating EDS,AL(R).

The maximum allowable drain-source voltage, BVDSS, must be higher than the maximum CATHODE-to-ANODE

voltage seen in the application circuit. For half-wave rectifier applications, the full peak-to-peak AC input voltage

appears across the MOSFET when the input voltage is at its minimum value. For full-wave rectifier applications, each

of the four MOSFETs sees one-half of the peak-to-peak input voltage. An avalanche-rated MOSFET ensures

robustness during momentary drain-to-source overvoltage conditions.

The MOSFET’s on-resistance, RDS(ON), directly affects the forward voltage drop and power dissipation during heavy

load. The desired forward voltage drop should be less than that of a diode for reduced power dissipation; 100mV is

a good starting point. Given the average output load current IAVG, choose a MOSFET for DC inputs which has:

VIN

VOUT

CVDDA

22µF

CG

10nF

COUT

3000µF

GATE CATHODE

VSSA

VDDA

VDDC

ANODE

LT4322

48V p-p

AC

60Hz

BUK7M15-60EX

1A

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