Datasheet

LT3511
18
3511fc
Example:
Design for ripple levels below 50mV.
C =
0.1A • 0.39
0.05V • 256kHz
= 3.1µF
A 10F, 25V output capacitor is chosen. Remember ce-
ramic capacitors lose capacitance with applied voltage.
The capacitance can drop to 40% of quoted capacitance
at the max voltage rating.
Step 6: Design clamp circuit.
The clamp circuit protects the switch from leakage induc-
tance spike. A DZ clamp is the preferred clamp circuit. The
Zener and the diode need to be chosen.
The maximum Zener value is set according to the maxi-
mum V
IN
:
V
ZENER(MAX)
≤ 150V – V
IN(MAX)
Example:
V
ZENER(MAX)
≤ 150V – 72V
V
ZENER(MAX)
≤ 78V
In addition, power loss in the clamp circuit is inversely
related to the clamp voltage as shown previously. Higher
clamp voltages lead to lower power loss.
A 68V Zener with a maximum of 72V will provide optimal
protection and minimize power loss. Half-watt Zeners will
satisfy most clamp applications involving the LT3511.
Power loss can be calculated using the equations presented
in the Leakage Inductance and Clamp Circuit section.
The Zener chosen is a 68V 0.5W Zener from On Semicon-
ductor (MMSZ5266BT1G).
Choose a diode that is fast and has sufficient reverse
voltage breakdown:
V
REVERSE
> V
SW(MAX)
V
SW(MAX) =
V
IN(MAX)
+ V
ZENER(MAX)
Example:
V
REVERSE
> 140V
The diode needs to handle the peak switch current of the
switch which was determined to be 0.24A. A 200V, 0.6A
diode from Diodes Inc. (BAV21W) is chosen.
Step 7: Compensation.
Compensation will be optimized towards the end of the de-
sign procedure. Connect a resistor and capacitor from the
VC node to ground. Use a 20k resistor and a 2.2nF capacitor.
Step 8: Select R
FB
and R
TC
Resistors.
Use the following equations to choose starting values for
R
FB
and R
TC
. Set R
REF
to 10k.
R
FB
=
V
OUT
+ V
F
+ 0.55V
()
•N
PS
•R
REF
1.2V
R
REF
= 10k
R
TC
=
R
FB
N
PS
Example:
R
FB
=
15+ 0.5+ 0.55V
()
•2•10k
1.2V
= 267k
R
TC
=
267k
2
= 133k
Step 9: Adjust RFB based on output voltage.
Power up the application with application components
connected and measure the regulated output voltage.
Readjust R
FB
based on the measured output voltage.
R
FB(NEW)
=
V
OUT
V
OUT(MEAS)
•R
FB(OLD)
Example:
R
FB(NEW)
=
15V
16.8V
• 267k = 237k
Step 10: Remove R
TC
and measure output voltage over
temperature.
Measure output voltage in a controlled temperature envi-
ronment like an oven to determine the output temperature
coefficient. Measure output voltage at a consistent load
current and input voltage, across the temperature range
of operation. This procedure will optimize line and load
regulation over temperature.
APPLICATIONS INFORMATION