Datasheet
LT3845
14
3845fd
has a maximum voltage threshold of, typically, 100mV.
Therefore, the peak inductor current is 100mV/R
SENSE
.
The maximum output load current, I
OUT(MAX)
, is the peak
inductor current minus half the peak-to-peak ripple cur-
rent, ΔI
L
.
Allowing adequate margin for ripple current and exter-
nal component tolerances, R
SENSE
can be calculated as
follows:
R
SENSE
=
70mV
I
OUT
(
MAX
)
Typical values for R
SENSE
are in the range of 0.005Ω
to 0.05Ω.
Operating Frequency
The choice of operating frequency is a trade off between
effi ciency and component size. Low frequency operation
improves effi ciency by reducing MOSFET switching losses
and gate charge losses. However, lower frequency opera-
tion requires more inductance for a given amount of ripple
current, resulting in a larger inductor size and higher cost.
If the ripple current is allowed to increase, larger output
capacitors may be required to maintain the same output
ripple. For converters with high step-down V
IN
to V
OUT
ratios, another consideration is the minimum on-time of
the LT3845 (see the Minimum On-time Considerations
section). A fi nal consideration for operating frequency is
that in noise-sensitive communications systems, it is often
desirable to keep the switching noise out of a sensitive
frequency band. The LT3845 uses a constant frequency
APPLICATIONS INFORMATION
architecture that can be programmed over a 100kHz to
500kHz range with a single resistor from the f
SET
pin to
ground, as shown in Figure 2. The nominal voltage on the
f
SET
pin is 1V and the current that fl ows from this pin is
used to charge an internal oscillator capacitor. The value
of R
SET
for a given operating frequency can be chosen
from Figure 2 or from the following equation:
R
SET(kΩ)
= 8.4 • 10
4
• f
SW
(–1.31)
Table 1 lists typical resistor values for common operating
frequencies.
Table 1. Recommended 1% Standard Values
R
SET
(k) f
SW
(kHz)
191 100
118 150
80.6 200
63.4 250
49.9 300
40.2 350
33.2 400
27.4 450
23.2 500
Inductor Selection
The critical parameters for selection of an inductor are
minimum inductance value, volt-second product, satura-
tion current and/or RMS current.
For a given ΔI
L
, The minimum inductance value is calcu-
lated as follows:
L ≥ V
OUT
•
V
IN(MAX)
–V
OUT
f
SW
•V
IN(MAX)
• ΔI
L
f
SW
is the switch frequency.
The typical range of values for ΔI
L
is (0.2 • I
OUT(MAX)
) to
(0.5 • I
OUT(MAX)
), where I
OUT(MAX)
is the maximum load
current of the supply. Using ΔI
L
= 0.3 • I
OUT(MAX)
yields a
good design compromise between inductor performance
versus inductor size and cost. A value of ΔI
L
= 0.3 • I
OUT(MAX)
produces a ±15% of I
OUT(MAX)
ripple current around the DC
output current of the supply. Lower values of ΔI
L
require
larger and more costly magnetics. Higher values of ΔI
L
FREQUENCY (kHz)
0
20
R
SET
(kΩ)
40
80
100
120
400
200
3845 F2
60
200
100
500
300 60
0
140
160
180
Figure 2. Timing Resistor (R
SET
) Value