lt3663 Linear Technology Corporation, lt3663 Datasheet - Page 7

• Manufacturer Part Number: lt3663
• Description: 1.2a, 1.5mhz Step-down Switching Regulator With Output Current Limit
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Inductor Selection
A good fi rst choice for the inductor value is:
where V
and L is in μH. With this value there is no sub-harmonic
oscillation for applications with 50% or greater duty
cycle. The inductor’s RMS current rating must be greater
than the maximum load current. Size the inductor so the
saturation current rating is about 30% higher than the
maximum load current. The output current limit circuit
tightly controls the maximum average inductor current,
therefore the inductor RMS current rating does not have
to be overrated to handle short circuit or overload condi-
tions. For high effi ciency, keep the series resistance (DCR)
less than 0.1Ω. A higher value inductor is larger in size
and slows transient response but reduces output voltage
ripple. A lower value inductor has higher ripple currents
but is physically smaller or, for the same size, it has lower
DCR typically resulting in higher effi ciency.
Catch Diode
The catch diode conducts current only during switch-off
time. Average forward current in normal operation is
calculated from:
where I
grammed by the I
L = V
I
D AVG
AC-COUPLED
(
200mV/DIV
20V/DIV
OUT
1A/DIV
OUT
D
)
V
V
OUT
=
is the voltage drop of the catch diode (~0.4V)
SW
I
L
+ V
I
is the maximum output load current pro-
OUT
D
Figure 2. Pulse-Skip Mode
LIM
(μH)
V
IN
resistor. Peak reverse voltage is equal
–
V
IN
V
OUT
2.5μs/DIV
3663 F02
to the regulator input voltage. Use a Schottky diode with a
reverse voltage rating greater than the input voltage. The
overvoltage protection feature in the LT3663 keeps the
switch off when V
45V rated Schottky even when V
Input Capacitor
Bypass the input of the LT3663 circuit with a 1μF or higher
value ceramic capacitor of X7R or X5R type. Y5V types have
poor performance over temperature and applied voltage
and are not recommended. If the input power source has
high impedance, or there is signifi cant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor. Step-down regulators draw current
from the input supply in pulses with very fast rise and
fall times. The input capacitor is required to reduce the
resulting voltage ripple at the LT3663 and to force this very
high frequency switching current into a tight local loop,
minimizing EMI. Place the capacitor in close proximity
to the LT3663 and the catch diode; (see the PCB Layout
section).
Output Capacitor
A 10μF or greater ceramic capacitor is recommended to
provide low output ripple and good transient response.
Ceramic capacitors have very low equivalent series re-
sistance (ESR) and provide the best ripple performance.
Use X5R or X7R types and keep in mind that a ceramic
capacitor biased with V
nominal capacitance.
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose
one that is intended for use in switching regulators. Keep
the ESR less than 0.1Ω.
I
The LT3663 output current limit controls the maximum
current delivered from the LT3663 regulator. This allows
tighter control of the system power dissipation and also
protects the inductor and diode from overheating during
an overload or short circuit condition. A resistor con-
LIM
Resistor
IN
> 39V (typical) allowing the use of a
OUT
has less than its specifi ed
IN
ranges up to 60V.
LT3663
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