LTC4366 datasheet(17/24 Pages) LINER

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LTC4366

436612fe

For more information www.linear.com/LTC4366

APPLICATIONS INFORMATION

After the charge pump is active the VSS current increases

to 160µA (worst-case 230µA, see Table 1) current while

the final value OUT voltage is equal to the minimum supply

voltage. The C1 voltage is clamped at 5.7V (worst-case

6.0V):

RSS(MAX) =

VIN(MIN) – VZ(OUT)

IVSS(CP)

RSS(MAX) =

18V – 6V

230µA

= 52.3k

Step 2: Determine RIN

The value for resistor RINiscalculatedusingthecalculated

RSS value. RIN is chosen to provide enough headroom to

sufficiently charge C1 to 4.9V the maximum undervoltage

lockout2threshold(VUVLO2)whichstartsthechargepump.

The parameters that determine RIN include: minimum

supply voltage, the final C1 voltage, MOSFET threshold

voltage, RSS, 72µA maximum VSS pin current (regulation

amplifier on, IVSS(AMP)), and finally the 13µA maximum

start-up current in the VDD pin (IVDD(STHI)):

RIN(MAX) =

VIN(MIN) – VUVLO2 − VD − VTH − ISS(AMP) •RSS

(

)

IVDD(STHI)

RIN(MAX) =

18V − 4.9V − 0.58V − 5V − 72µA • 52.3k

(

)

13µA

RIN(MAX) = 287k

Table 1. Electrical Parameters Used in Design Example

SYMBOL

PARAMETER

CONDITIONS

TYP

MAX

VZ(OUT)

OUT Shunt Reg. Voltage

I = 1mA, BASE = 0V

5.7V

6.0V

VUVLO2

OUT Undervoltage Lockout 2

Rising

4.75V

4.9V

IVSS(CP)

VSS Pin Current – Charge Pump On

–160µA

–230µA

IVSS(AMP)

VSS Pin Current – Regulation Amplifier On

–45µA

–72µA

IVDD(STHI)

VDD Pin Current – Start-Up, Gate High

GATE Open, VDD = 7V, OUT = 0V

9µA

13µA

IGATE(ST)

GATE Pin Current – Start-Up

GATE = OUT = 0V

–7.5µA

–11µA

VUVLO1

OUT Undervoltage Lockout 1

Rising

2.55V

2.75V

Step 3: Find RSS(MAX)

In some cases this value for RSS is too large to charge C1

and power the overvoltage amplifier before the maximum

input voltage passes to the output. The voltage at the VSS

pinwhenIRIN=IRSSiscalledthematchpoint(VSS(MATCH)).

Choosing the match point (with supply at the maximum)

sufficientlybelowVREG(byatleast7V),allowsC1tocharge

up in time to protect the load from overvoltage:

RSS(MAX) =

RIN • VREG – 7V

(

)

VIN(MAX) −5V − VREG

RSS(MAX) =

287k • 43V – 7V

(

)

250V – 5V – 43V

= 51.1k

In this case the RSS value of 52.3k calculated in Step 1

is too large.

Step 4: Iterate Smaller RSS

Using 51.1k (RSS(MAX)) as the next guess for RSS, we can

now calculate RIN and RSS(MAX):

RIN =

18V – 4.9V − 0.58V − 5V − 72µA • 51.1k

(

)

13µA

RIN = 294k

RSS(MAX)

294k • 43V – 7V

(

)

250V – 5V − 43V

= 52.3k

In this case the RSS value of 51.1k is less than RSS(MAX)

and the solution is acceptable.

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