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LTC4366

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Step 5: Determine CG, C1(MAX), Check RSS

The gate capacitor (CG) determines the gate slew rate and

therefore the slew rate of the OUT pin since the output

voltage follows the GATE pin. The voltage at the GATE pin

riseswithaslopeequalto 7.5µA/CGatstartupand20µA/CG

when the charge pump is on. Limiting this slope will limit

the inrush current charging the load capacitance where:

IINRUSH =

CLOAD

•IG

In this example we choose CG to be 10nF which limits the

inrush current to be 660mA for a 330µF CLOAD.

C1 is used as a bypass capacitor for the circuitry between

the OUT and VSS pins. C1 also stabilizes the shunt regula-

tor that clamps the voltage between these pins where the

minimum value for regulator stability is 0.22µF. An even

greater 0.47µF value is desired for C1 to protect the OUT

to VSS circuitry from transients on the OUT pin.

The startup into an overvoltage creates an upper bound-

ary on the value of C1. The value of CG, RSS and RVIN

determines a maximum C1 that will reach UVLO1 and

power the regulation amplifier before the OUT pin voltage

exceeds the overvoltage threshold. If our desired value

for C1 (0.47µF) exceeds the maximum allowed C1 then

a smaller RSS must be used to iterate a new solution for

C1(MAX). We start with calculating VSS(MATCH):

VSS(MATCH) =

RSS

RSS +RVIN

• VIN – VZ(VDD)

(

)

If we use the worst-case 1% maximum value for RSS

(51.6k) and minimum value for RVIN (291k):

VSS(MATCH) = 35.8V

C1(MAX) =

–CG • RSS +RIN

(

) VREG – VSS(MATCH)

(

)

IG •RSS •RIN •In 1–

2 • VUVLO1

VREG – VSS(MATCH)













Usetheworst-casemaximumgatecurrentof11µAinstead

of the typical 7.5µA and the worst-case minimum UVLO1

APPLICATIONS INFORMATION

threshold, 2.75V:

C1(MAX) =

–10nF • 51.6k + 291k

(

) 43V – 35.8V

(

)

11µA • 51.6k • 291k •In 1– 2 • 2.75V

43V − 35.8V









C1(MAX) = 0.1µF

This limit on C1 does not meet the shunt regulator stability

requirements (C1 > 0.22µF).

If we desire a larger value of C1 then a lower size of RSS

is required. A lower value for RSS is 48.7k, which calls

out an RIN value of 309k and a max C1 value of 0.27µF.

The next lower value of 46.4k with RVIN of 324k, results

in the worst-case maximum C1 value of 0.49µF. A larger

C1 increases circuit immunity to transients in exchange

for slightly higher current. Therefore, a selection of com-

ponents that allow a 0.47µF C1 is recommended.

The lowered RSS value of 46.4k now considers the toler-

ances of all the components that set the C1 ramp rate to

guarantee it charges to the 2.55V UVLO1 threshold before

the OUT voltage exceeds the overvoltage threshold.

Step 6: Determine RFB1, RFB2

The feedback resistors, RFB1 and RFB2, are chosen to

regulatetheovervoltageat43V.Onewaytoquicklychoose

these resistors is to assign 100µA or 1.2V across a 12.4k

RFB1. RFB2 would need to drop the remainder of the regu-

lated voltage. Dividing this remainder by 100µA yields the

value for RFB2. In this example RFB2 drops 41.8V. When

divided by 100µA it results in a 422k value.

Step 7: Determine CT, R1

DuringanovervoltagethepowerdissipatedintheMOSFET

is dependent on the load current and the difference be-

tween the supply and regulated voltages. It is necessary

to keep the device power in a safe range. In the power

MOSFET data sheets there is a maximum safe operating

curve displaying current versus drain to source voltage

for a fixed pulsed time. Other pulsed time data from DC

to 10µs are plotted on the one graph. The different lines

of operation generally follow a constant power squared

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