LT3511
Applications Information
Step 11: Calculate new value for RTC.
RTC(NEW )
= RFB
NPS
•
1.85mV /
ΔVOUT
°C
ΔTemp
Example:
RTC(NEW )
=
237k
2
•
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
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= 2V = 768k
2.6µA
Determine UVLO Threshold.
VIN(UVLO,FALLING)
=
1.2V
• (R1+
R2
R2)
R2 =
1.2V • R1
VIN(UVLO,FALLING) – 1.2V
Set UVLO falling threshold to 30V.
R2 = 1.2V • 768k = 32.4k
30V – 1.2V
VIN(UVLO,FALLING)
=
1.2V
• (R1+
R2
R2)
= 1.2V • (768k + 32.4k) = 30V
32.4k
VIN(UVLO,RISING) = VIN(UVLO,FALLING) + 2.6μA • R1 = 30V
+ 2.6μA • 768k = 32V
3511fa
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