Data Sheet

LT4322

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Rev. 0

10 of 29

BLOCK DIAGRAM

Figure 18. Block Diagram

THEORY OF OPERATION

The LT4322 is a floating-ideal diode controller that operates from input power supply frequencies ranging from DC

to 100kHz. Due to its floating architecture, the maximum operating voltage is limited only by the external MOSFETs

in the application circuit. The operation is best understood by referring to the block diagram in Figure 18.

The LT4322 controls an external N-Channel MOSFET to form an ideal diode. The GM AMP senses the ANODE-to-

CATHODE voltage and drives the gate of the MOSFET through the GATE pin to regulate the forward voltage to 25mV

(ΔVSD). As the load current increases, the GATE voltage is driven higher to maintain a drop of 25mV. When the MOSFET

gate is fully turned on for very large load currents, the forward drop rises linearly with the current according to RDS(ON)

x ILOAD. If the forward drop is less than 25mV, or if the ANODE-to-CATHODE voltage reverses, the amplifier drives the

GATE voltage low to turn off the MOSFET.

During fast ANODE-to-CATHODE excursions, such as fast varying AC input signals where the GM AMP regulating loop

is too slow, high-speed GATE drivers provide fast pull-up and pull-down currents capable of driving the external

MOSFET on and off quickly. This rectifies the input signal similarly to a diode but with significantly less power

dissipation.

Powering the LT4322

The GM AMP and fast pull-up currents are powered from the VDDA pin. VDDC powers an internal LDO, whose output is

VDDA. The LT4322 draws power from VDDC and regulates 12V on VDDA.

For AC applications, the LDO draws power from VDDC during the reverse mode of operation when the VDDC voltage

exceeds the VDDA voltage. An external hold-up capacitor is required between VDDA and VSSA to power the LT4322 while

in the forward mode of operation, where the VDDC voltage is very low.

25mV

D1

N1

LDO

VDDC

CATHODE

ANODE

VSSA

VDDA

GATE

2

3

1

8

4

5

GM AMP

+

–