Datasheet

LT3844
11
3844fb
APPLICATIONS INFORMATION
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
mV
I
SENSE
OUT MAX
=
70
()
Typical values for R
SENSE
are in the range of 0.005Ω to
0.05Ω.
Operating Frequency
The choice of operating frequency and inductor value 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 operation requires more inductance for a
given amount of ripple current, resulting in a larger induc-
tor 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 LT3844 (see the Minimum
On-time Considerations section). A fi nal consideration
for operating frequency is that in noise-sensitive com-
munications systems, it is often desirable to keep the
switching noise out of a sensitive frequency band. The
LT3844 uses a constant frequency 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 1. 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 4 or from
the following equation:
Rk f
SET SW
() .• •
(– . )
Ω=84 10
4131
Table 1 lists typical resistor values for common operating
frequencies:
Table 1. Recommended 1% Standard Values
R
SET
f
SW
191kΩ 100kHz
118kΩ 150kHz
80.6kΩ 200kHz
63.4kΩ 250kHz
49.9kΩ 300kHz
40.2kΩ 350kHz
33.2kΩ 400kHz
27.4kΩ 450kHz
23.2kΩ 500kHz
Step-Down Converter: 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, The minimum inductance value is calculated
as follows:
LV
VV
fV I
OUT
IN MAX OUT
SW IN MAX L
≥
Δ
•
–
••
()
()
f
SW
is the switch frequency.
FREQUENCY (kHz)
0
20
R
SET
(kΩ)
40
80
100
120
400
200
3844 G19
60
200
100
500
300 600
140
160
180
Figure 1. Timing Resistor (R
SET
) Value