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LT8494

8494fa

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OPERATION

The LT8494 is a constant-frequency, current mode

SEPIC/boost/flyback regulator. Operation can be best

understood by referring to the Block Diagram. In the Block

Diagram, the adjustable oscillator, with frequency set by

the external RT resistor, enables an RS latch, turning on

the internal power switch. An amplifier and comparator

monitor the switch current flowing through an internal

sense resistor, turning the switch off when this current

reaches a level determined by the voltage at VC. An error

amplifier adjusts the VC voltage by measuring the output

voltage through an external resistor divider tied to the FB

pin. If the error amplifier’s output voltage (VC) increases,

more current is delivered to the output; if the VC voltage

decreases, less current is delivered. An active clamp on

the VC voltage provides current limit. An internal regulator

provides power to the control circuitry.

In order to improve efficiency, the NPN power switch

driver (see Block Diagram) supplies NPN base current

from whichever of VIN and BIAS has the lower supply

voltage. However, if either of them is below 2.4V or above

34V (typical values), the power switch draws current from

the other pin. If both supply pins are below 2.4V or above

34V then switching activity is stopped.

To further optimize efficiency, the LT8494 automatically

enters Burst Mode operation in light load situations.

Between bursts, all circuitry associated with controlling

the output switch is shut down, reducing the VIN/BIAS

pin supply currents to be less than 3µA typically (see

Electrical Characteristics).

The LT8494 contains a power good comparator which

trips when the FB pin is above 92% of its regulated value.

The PG output is an open-drain transistor that is off when

the output is in regulation, allowing an external resistor to

pull the PG pin high (See Applications Information section

for details).

Several functions are provided to enable a very clean

start-up for the LT8494.

• First, the SWEN pin voltage is monitored by an internal

voltage reference to give a precise turn-on threshold.

An external resistor divider can be connected from

the input power supply to the SWEN pin to provide a

user-programmable undervoltage lockout function.

• Second, the soft-start circuitry provides for a grad-

ual ramp-up of the switch current. When the part is

brought out of shutdown, the external SS capacitor is

first discharged, and then an integrated 256k resistor

pulls the SS pin up to ~2.1V. By connecting an exter-

nal capacitor to the SS pin, the voltage ramp rate on

the pin can be set. Typical values for the soft-start

capacitor range from 100nF to 1µF.

• Finally, the frequency foldback circuit reduces the

maximum switching frequency when the FB pin is

below 1V. This feature reduces the minimum duty

cycle that the part can achieve thus allowing better

control of the switch current during start-up.

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