Datasheet

LT3663
8
3663fc
For more information www.linear.com/LT3663
2.5µs/DIV
3663 F02
V
SW
20V/DIV
I
L
1A/DIV
V
OUT
200mV/DIV
AC-COUPLED
Figure 2. Pulse-Skip Mode
V
SW
20V/DIV
I
L
1A/DIV
V
OUT
200mV/DIV
AC-COUPLED
2.5µs/DIV
3663 F01
applicaTions inForMaTion
FB Resistor Network
The output voltage of the LT3663 is programmed with a
resistor divider. Choose the 1% resistors according to:
R1 = R2 (V
OUT
/0.8 – 1)
Reference designators refer to the Block Diagram.
The resistor divider is internal on the fixed output voltage
versions, LT3663-X.
Minimum Duty Cycle
As the input voltage increases the duty cycle decreases. At
some point, the minimum duty cycle of the IC is reached
and pulse skipping begins. The output voltage continues to
regulate but the output voltage ripple increases. The input
voltage at which this occurs is calculated as follows:
V
VV
DC
VV
IN
OUT F
MIN
FS
W
=
+
()
+–
Where V
F
is the forward voltage drop of the catch diode,
D1, and V
SW
is the voltage drop on the internal switch,
Q1. LT3663 DC
MIN
is typically 0.12.
For V
F
= 0.4V, V
SW
= 0.3V, and V
OUT
= 3.3V pulse skipping
occurs at about 24.5V.
Figure 1 illustrates switching waveforms in normal mode
for a 3.3V output application with V
IN
= 20V.
Figure 2. illustrates switching waveforms in pulse skip
mode a 3.3V output application with V
IN
= 30V.
Figure 1. Normal Operation
Inductor Selection
A good first choice for the inductor value is:
L = V
OUT
+ V
D
(µH)
where V
D
is the voltage drop of the catch diode (~0.4V)
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 efficiency, 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 efficiency.