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LT8494

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APPLICATIONS INFORMATION

regulator tries to charge the output capacitor as quickly as

possible, which results in large peak currents. The start-up

current can be limited by connecting an external capacitor

(typically 100nF to 1µF) to the SS pin. This capacitor is

slowly charged to ~2.1V by an internal 256k resistor once

the part is activated. SS pin voltages below ~0.8V reduce

the internal current limit. Thus, the gradual ramping of

the SS voltage also gradually increases the current limit

as the capacitor charges. This, in turn, allows the output

capacitor to charge gradually toward its final value while

limiting the start-up current. When the switching regula-

tor shuts down, the soft-start capacitor is automatically

discharged to ~100mV or less before charging resumes,

thus assuring that the soft-start occurs after every reac-

tivation of the switching regulation.

Power Supplies and Operating Limits

The LT8494 draws supply current from the VIN and BIAS

pins. The largest supply current draw occurs when the

switching regulator is enabled (SWEN is high) and the

power switch is toggling on and off. Under light load con-

ditions the switching regulator enters Burst Mode opera-

tion where the power switch toggles infrequently and the

input current is significantly reduced (see the Low Ripple

Burst Mode Operation section).

Power Switch Driver (PSD) Operating Range: The NPN

power switch is driven by a power switch driver (PSD) as

shown in the Block Diagram. The driver must be powered

by a supply (VIN or BIAS) that is above the minimum oper-

ating voltage and below the PSD overvoltage threshold.

These voltages are typically 2.4V and 34V respectively

(see Electrical Characteristics).

If neither VIN nor BIAS is within this operating range, the

PSD and the switching regulator are automatically dis-

abled. Voltages up to 60V are not harmful to the PSD,

however, as discussed, switching regulation is automati-

cally disabled when neither VIN nor BIAS is in the valid

operating range.

When both VIN and BIAS are too low for proper LT8494

operation (typically < 2.4V), the chip will enter shutdown

and draw minimal current from both supplies.

Automatic Power Supply Selection: In order to minimize

power loss, the LT8494 draws as much of its required

current as possible from the lowest suitable voltage sup-

ply (VIN or BIAS) in accordance with the requirements

described in the previous two sections. This selection is

automatic and can change as VIN and/or BIAS voltages

change.

The LT8494 compares the VIN and BIAS voltages to

determine which is lower. The comparator has an offset

and hysteresis as shown in the Electrical Characteristics

section. The voltage comparison happens continuously

when the power switch is toggling. The result of the latest

comparison is latched inside the LT8494 when switching

stops. If the power switch is not toggling, the LT8494

uses the last VIN vs BIAS comparison to determine which

supply is lower. After initial power up or any thermal lock-

out the LT8494 always concludes that VIN is the lower

supply voltage until subsequent voltage comparisons can

be made while the power switch is toggling.

BIAS Connection for SEPIC Converters: For SEPIC con-

verters, where VIN can be above or below VOUT, BIAS is

typically connected to VOUT which improves efficiency

when VIN voltage is higher than VOUT. Connecting BIAS to

VOUT in a SEPIC topology also allows the switching regu-

lator to operate with VIN above 34V (typical switch driver

overvoltage threshold) in cases where VOUT is regulated

below the PSD overvoltage threshold. Finally, connecting

BIAS to VOUT also allows the converter to operate from

VIN voltages less than 2.4V after VOUT rises within the

PSD operating range. This can be very useful in battery

powered applications since the battery voltage drops as

it discharges.

BIAS Connection for Boost Converters: For boost con-

verters, BIAS is typically connected to VOUT or to ground.

Connecting BIAS to VOUT allows the converter to operate

with VIN < 2.5V after VOUT has risen within the PSD oper-

ating range. However, during no load conditions on VOUT,

despite VIN being selected as the primary input supply,

the overall power loss will be slightly elevated due to the

small amount of current still being drawn from the higher

voltage BIAS pin. To minimize boost converter power

loss during no load conditions, connect BIAS instead to

ground.

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