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LT3511

3511fa

Step 11: Calculate new value for RTC.

RTC(NEW) =

RFB

NPS

• 1.85mV / °C

ΔVOUT

ΔTemp

Example:

RTC(NEW) =

237k

•

1.85

1.9

= 118k

Step 12: Place new value for RTC, measure VOUT, and

readjust RFB due to RTC change.

RFB(NEW) =

VOUT

VOUT(MEAS)

• RFB(OLD)

Example:

RFB(NEW) =

15V

• 237k = 237k

Step 13: Verify new values of RFB and RTC over

temperature.

Measure output voltage over temperature with RTC

connected.

Step 14: Optimize compensation.

Now that values for RFB and RTC are fixed, optimize the

compensation. Compensation should be optimized for

transient response to load steps on the output. Check

transient response across the load range.

Example:

The optimal compensation for the application is:

RC = 22.1k, CC = 4.7nF

Step 15: Ensure minimum load.

Check minimum load requirement at maximum input

voltage. The minimum load occurs at the point where the

output voltage begins to climb up as the converter delivers

more energy than what is consumed at the output.

Example:

The minimum load at an input voltage of 72V is:

7mA

Step 16: EN/UVLO resistor values.

Determine amount of hysterysis required.

Voltage hysteresis = 2.6μA • R1

Example:

Choose 2V of hysteresis.

R1=

2.6µA

= 768k

Determine UVLO Threshold.

VIN(UVLO,FALLING) =

1.2V • R1+R2

(

)

R2 =

1.2V • R1

VIN(UVLO,FALLING) – 1.2V

Set UVLO falling threshold to 30V.

R2 =

1.2V • 768k

30V – 1.2V

= 32.4k

VIN(UVLO,FALLING) =

1.2V • R1+R2

(

)

1.2V • 768k + 32.4k

(

)

32.4k

= 30V

VIN(UVLO,RISING) = VIN(UVLO,FALLING) + 2.6μA • R1 = 30V

+ 2.6μA • 768k = 32V

applicaTions inForMaTion

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