Datasheet

LT3512
18
3512fb
applicaTions inForMaTion
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 LT3512.
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 0.45A. A 200V, 1.0A
diode from Diodes Inc. (DFLS1200) is chosen.
Step 7: Compensation.
Compensation will be optimized towards the end of the
design procedure. Connect a resistor and capacitor from
the VC node to ground. Use a 15k resistor and a 4.7nF
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 R
FB
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
16.7V
• 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.
Calculate the temperature coefficient of V
OUT
:
∆V
OUT
∆Temp
=
V
OUT(HOT)
– V
OUT(COLD)
T
HOT(°C)
– T
COLD(°C)