LT3502 LINER [Linear Technology], LT3502 Datasheet - Page 11

• Manufacturer Part Number: LT3502
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
off until the output voltage dips below the programmed
value before it begins switching again (Figure 2).
Provided that the load can tolerate the increased output
voltage ripple and that the components have been properly
selected, operation above V
damage the part.
As the input voltage increases, the inductor current ramps
up quicker, the number of skipped pulses increases and
the output voltage ripple increases. For operation above
V
components be adequately rated for operation at the
intended voltage levels.
Inductor current may reach current limit when operating
in pulse skip mode with small valued inductors. In this
case, the LT3502/LT3502A will periodically reduce its
frequency to keep the inductor valley current to 650mA
(Figure 3). Peak inductor current is therefore peak current
plus minimum switch delay:
The part is robust enough to survive prolonged operation
under these conditions as long as the peak inductor cur-
rent does not exceed 1.2A. Inductor current saturation
and junction temperature may further limit performance
during this operating regime.
IN(MAX)
500mA/DIV
100mV/DIV
900mA +
20V/DIV
V
V
OUT
SW
I
L
the only component requirement is that the
V
V
L = 6.8μH
C
I
OUT
IN
OUT
OUT
= 40V
Figure 2. Pulse Skip Occurs when
Required On Time is Below 60ns
V
= 250mA
= 3.3V
= 10μF
IN
– V
L
OUT
• 60ns
1μs/DIV
IN(MAX)
is safe and will not
3502 F02
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
where V
L is in μH. With this value there will be no subharmonic
oscillation for applications with 50% or greater duty cycle.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should
be about 30% higher. For robust operation during fault
conditions, the saturation current should be above 1.2A.
To keep effi ciency high, the series resistance (DCR) should
be less than 0.1Ω. Table 1 lists several vendors and types
that are suitable.
There are several graphs in the Typical Performance
Characteristics section of this data sheet that show the
maximum load current as a function of input voltage and
inductor value for several popular output voltages. Low
inductance may result in discontinuous mode opera-
tion, which is okay, but further reduces maximum load
current. For details of the maximum output current and
discontinuous mode operation, see Linear Technology
Application Note 44.
Figure 3. Pulse Skip with Large Load Current Will be Limited
by the DA Valley Current Limit. Notice the Flat Inductor Valley
Current and Reduced Switching Frequency
500mA/DIV
100mV/DIV
L = 1.6(V
L = 4.6(V
20V/DIV
V
V
OUT
SW
I
D
L
V
V
L = 6.8μH
C
I
is the voltage drop of the catch diode (~0.4V) and
OUT
IN
OUT
OUT
= 40V
OUT
OUT
= 500mA
= 3.3V
= 10μF
+ V
+ V
D
D
) for the LT3502A
) for the LT3502
LT3502/LT3502A
1μs/DIV
3502 F03
11
3502fc