Datasheet

LT3502/LT3502A
11
3502fd
Figure 2. Pulse-Skipping Occurs when
Required On-Time is Below 60ns
V
SW
20V/DIV
V
OUT
100mV/DIV
1µs/DIV
V
IN
= 40V, V
OUT
= 3.3V
L = 6.8µH, C
OUT
= 10µF, I
OUT
= 250mA
3502 F02
I
L
500mA/DIV
applications inForMation
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 1.6(V
OUT
+ V
D
) for the LT3502A
L = 4.6(V
OUT
+ V
D
) for the LT3502
where V
D
is the voltage drop of the catch diode (~0.4V) and
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 efficiency 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
-
Figure 3. Pulse-Skipping with Large Load Current Will be
Limited by the DA Valley Current Limit. Notice the Flat Inductor
Valley Current and Reduced Switching Frequency
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
IN(MAX)
the only component requirement is that the
components be adequately rated for operation at the
intended voltage levels.
Inductor current may reach current limit when operating
in
pulse-skipping
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:
900mA +
V
IN
– V
OUT
L
• 60ns
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 per
formance
during this operating regime.
V
SW
20V/DIV
V
OUT
100mV/DIV
1µs/DIV
V
IN
= 40V, V
OUT
= 3.3V
L = 6.8µH, C
OUT
= 10µF, I
OUT
= 500mA
3502 F03
I
L
500mA/DIV
Table 1
VENDOR URL PART SERIES INDUCTANCE RATE (µH) SIZE (mm)
Sumida www.sumida.com CDRH4D28
CDRH5D28
CDRH8D28
1.2 to 4.7
2.5 to 10
2.5 to 33
4.5 ×
4.5
5.5 × 5.5
8.3 × 8.3
Toko www.toko.com A916CY
D585LC
2 to 12
1.1 to 39
6.3 ×
6.2
8.1 × 8
Würth Elektronik www.we-online.com WE-TPC(M)
WE-PD2(M)
WE-PD(S)
1 to 10
2.2 to 22
1 to 27
4.8 ×
4.8
5.2 × 5.8
7.3 × 7.3