LT3511
19
3511fc
Calculate the temperature coefficient of V
OUT
:
ΔV
OUT
ΔTemp
=
V
OUT(HOT)
–V
OUT(COLD)
T
HOT(°C)
–T
COLD(°C)
Example:
V
OUT
measured at 100mA and 48V
IN
ΔV
OUT
ΔTemp
=
15.70V – 15.37V
125°C– –50°C
()
= 1.9mV/°C
Step 11: Calculate new value for R
TC
.
R
TC(NEW)
=
R
FB
N
PS
•
1.85mV / °C
ΔV
OUT
ΔTemp
Example:
R
TC(NEW)
=
237k
2
•
1.85
1.9
= 118k
Step 12: Place new value for R
TC
, measure V
OUT
, and
readjust R
FB
due to R
TC
change.
R
FB(NEW)
=
V
OUT
V
OUT(MEAS)
•R
FB(OLD)
Example:
R
FB(NEW)
=
15V
15V
• 237k = 237k
Step 13: Verify new values of R
FB
and R
TC
over
temperature.
Measure output voltage over temperature with R
TC
connected.
Step 14: Optimize compensation.
Now that values for R
FB
and R
TC
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:
R
C
= 22.1k, C
C
= 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 hysteresis required.
Voltage hysteresis = 2.6A • R1
Example:
Choose 2V of hysteresis.
R1=
2V
2.6µA
= 768k
Determine UVLO Threshold.
V
IN(UVLO,FALLING)
=
1.2V • R1+R2
()
R2
R2 =
1.2V • R1
V
IN(UVLO,FALLING)
– 1.2V
Set UVLO falling threshold to 30V.
R2 =
1.2V • 768k
30V – 1.2V
= 32.4k
V
IN(UVLO,FALLING)
=
1.2V • R1+R2
()
R2
=
1.2V • 768k + 32.4k
()
32.4k
= 30V
V
IN(UVLO,RISING)
= V
IN(UVLO,FALLING)
+ 2.6A • R1 = 30V
+ 2.6A • 768k = 32V
APPLICATIONS INFORMATION
