Data Sheet

LT4322

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Rev. 0

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RDS(ON)< 100 mV/IAVG

(2)

For AC full-bridge rectifier applications, use:

RDS(ON)< 100 mV/(3 × IAVG) (3)

The AC input calculation assumes the duration of the current conduction occupies 1/3 of the AC period.

For very high output current applications, users may connect several MOSFETs in parallel to reduce the overall RDS(ON).

The LT4322’s high-speed transconductance (GM AMP) amplifier is capable of driving up to 100nF MOSFET gate

capacitance at frequencies up to 100kHz. At initial power-up, the application output capacitance is initially charged

via the body diode of the power MOSFET until the LT4322 has enough supply voltage to properly control the MOSFET

gate. Ensure the MOSFET has a sufficient continuous body diode current rating IS to handle the current at startup.

The LT4322’s GATE drive ranges from 10V to 14V when powered from VDDC input supply voltages greater than or equal

to 15V. When powered directly from VDDA, the GATE drive voltage equals the VDDA voltage. VGS(TH) must be a minimum

of 2V or higher. A gate threshold voltage lower than 2V is not recommended because too much time is needed to

discharge the gate below the threshold and halt reverse current conduction when the input AC voltage falls below

the CATHODE output voltage.

GATE Capacitor Selection

The GM amplifier is optimally compensated with 10nF capacitance between the gate and source of the external

power MOSFET. For applications using MOSFETs with CISS less than 10nF and/or when voltage peaking is observed in

the GATE waveform, adding a 10nF ceramic capacitor CGATE directly across the gate and source of the external power

MOSFET can greatly improve the stability of the forward regulation voltage. This comes at the expense of the added

charge needed to enhance the gate of the external MOSFET. For high output current and/or high frequency

applications where the circuit is rarely in forward regulation, this capacitor can be omitted.

Output Capacitor COUT Selection

Capacitance COUT is needed across the application output voltage and system ground to provide the output load

current for the majority of the AC period. Downstream power needs and voltage ripple tolerance determine how

much capacitance is required. COUT in the hundreds to thousands of microfarads is common. A combination of

electrolytic and ceramic capacitors is used to optimize capacitance ESR while minimizing cost. Add a 1µF ceramic

capacitor from the output to the ground near the LT4322 if the bulk electrolytic capacitors are physically remote

from the LT4322.

For full-wave rectifier applications, a good starting point is selecting COUT such that:

COUT ≥ IAVG/(VRIPPLE × 2 × Freq)

(4)

Where IAVG is the average output load current, VRIPPLE is the maximum tolerable output ripple voltage, and Freq is the

frequency of the AC input source. For example, in a 60Hz, 24V application where the load current is 1A and the

tolerable ripple is 15V, choose:

COUT ≥ 1A/(15V × 2 × 60Hz) = 556µF

(5)

For half-wave rectifier applications, use:

COUT ≥ IAVG/(VRIPPLE × Freq)

(6)

Users must also ensure that the RMS current in the electrolytic capacitors does not exceed the maximum ripple

current rating so that the capacitor’s lifetime is not compromised. An electrolytic capacitor’s ripple current rating is a

function of RMS current, frequency, and ambient temperature. Consult the manufacturer’s specifications and ensure