LTC4366 datasheet(15/24 Pages) LINER

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LTC4366

436612fe

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APPLICATIONS INFORMATION

has exceeded VREG. Since the OUT pin voltage is at least

2.55V larger than VSS it exceeds the specified maximum.

Choosing the match point (with supply at the maximum)

sufficiently below VREG (by at least 2.55V), allows C1 to

charge up in time to protect the load from overvoltage.

In reality having VSS pin voltage 7V below VREG provides

required margin for charging C1.

VSS(MATCH)(MAX) = VREG – 7V

Increasing RSS increases the match point, so determin-

ing the maximum RSS value while still protecting from

overvoltage is useful. Using IRIN = IRSS:

RSS =RIN •

VRSS

VRIN

Using:

VRSS = VSS(MATCH)(MAX) = VREG – 7V

VRIN = VIN – VZ(VDD) – VRSS

Substituting:

RSS(MAX) =

RIN • VREG – 7V

(

)

VIN(MAX) – 12V – VREG – 7V

(

)

RSS(MAX) =

RIN • VREG – 7V

(

)

VIN(MAX) – 5V – VREG

If we guarantee that RSS < RSS(MAX) then the following

is true:

VSS(MATCH) < VSS(MATCH)(MAX)

C1 bypasses the charge pump, and requires at least a

0.22µF. The size of C1 needs limits also. The gate capaci-

tor (CG) dictates the maximum output capacitor C1(MAX)

that will charge to the 2.55V UVLO1 threshold (VUVLO1)

beforetheOUTvoltageexceedstheovervoltagethreshold.

C1(MAX) =

–CG • RSS +RIN

(

) VREG – VSS(MATCH)

(

)

IG •RSS •RIN •In 1–

2 • VUVLO1

VREG – VSS(MATCH)













In most cases:

C1(MAX) = 10 • CG to 100 • CG

GATE Capacitor, CG

The gate capacitor is used for three functions. First, CG

absorbs charge from the gate-to-drain capacitance of

the MOSFET during overvoltage transients. Second, the

capacitor also acts as a compensation element for the

overvoltage regulation amplifier. The minimum value for

CG to guarantee stability is 2nF. Finally, CG sets the slew

rate of the GATE and OUT pins. The voltage at the GATE pin

rises at a slope equal to 20µA/CG. This slope determines

the charging current into the load capacitor.

IINRUSH =

CLOAD

•IG

The voltage rating for CG must be greater than the regula-

tion voltage (VREG).

MOSFET Selection

The LTC4366 drives an N-channel MOSFET to conduct

the load current. The important features of the MOSFET

are on-resistance, RDS(ON), the maximum drain-source

voltage, V(BR)DSS, the threshold voltage, and the SOA.

The maximum allowable drain-source voltage must be

higher than the supply voltage. If the output is shorted

to ground or during an overvoltage event, the full supply

voltage will appear across the MOSFET.

The threshold voltage of the MOSFET is used in the mini-

mum supply start-up calculation. For applications with

supplies less than 12V, a logic-level MOSFET is required.

Above 12V a standard threshold N-channel MOSFET is

sufficient.

The SOA of the MOSFET must encompass all fault condi-

tions. In normal operation the pass transistor is fully on,

dissipating very little power. But during overvoltage faults,

the GATE pin is servoed to regulate the output voltage

through the MOSFET. Large current and high voltage drop

across the MOSFET can coexist in these cases. The SOA

curves of the MOSFET must be considered carefully along

with the selection of the fault timer capacitor.

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