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LTC4366

436612fe

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APPLICATIONS INFORMATION

Layout Considerations

Due to the high impedances on the SD, VDD, and GATE

pins, these pins are susceptible to leakages to ground.

For example, a leakage to ground on SD will activate the

shutdown state if greater than 1.6µA. Providing adequate

spacingawayfromgroundedtracesandaddingconformal

coating on exposed pins lowers the risk that leakage cur-

rent will interrupt system operation.

It is important to put the bypass capacitor, C1, as close as

possible to the OUT and VSS pins. Place the 10Ω resistor

as close as possible to the MOSFET gate pin. This will

limit the parasitic trace capacitance that leads to MOSFET

self-oscillation.

The FB pin is sensitive to parasitic capacitance when the

regulation loop is closed. One result from this capacitive

loading is output oscillations during overvoltage regula-

tion. It is suggested that the resistors RFB1 and RFB2 be

placed close to the pin and that the FB trace itself be

minimized in size.

DESIGN EXAMPLE

Overview

The design process starts with minimum input voltage

start-up equations to calculate values for RSS and RIN.

These values need further refinement to meet two other

conditions: the maximum input voltage start-up condi-

tions and proper current for the charging of C1. The the

remaining element values are calculated based on the

input parameters.

Following are the input parameters for this example:

VSUPPLY(MIN) = 18V, VREG = 43V, VIN(MAX) = 250V, ILOAD

= 1.5A at start-up, ILOAD = 3A after start-up, VTH = 5V

ImportantElectricalCharacteristicstableparametersused

in this example are summarized in Table 1.

Step 1: Maximum RSS

In this design example (Figure 6.) the component sizing

first considers the start-up phase after the charge pump

is active. The goal is to maximize the resistance of RSS

which still allows operation when the input voltage is at

the minimum value.

VDD

470k

100k

OUT

10nF

GATE

FQA62N25C

8.2nF

0.47µF

RFB1

12.4k

VOUT

1.5A

(43V CLAMP)

VIN

28V

(18V DC TO 250V DC)

RFB2

422k

436612 F06

RSS

46.4k

RIN

324k

MMBT3904

LTC4366-2

TIMER

BASE

VSS

Figure 6. Overvoltage Protected 28V, 1.5A Supply

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