Datasheet

LT3845A
14
3845afa
APPLICATIONS INFORMATION
ground, as shown in Figure 2. The nominal voltage on the
f
SET
pin is 1V and the current that flows 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)
FREQUENCY (kHz)
0
20
R
SET
(kΩ)
40
80
100
120
400
200
3845A F2
60
200
100
500
300
600
140
160
180
Figure 2. Timing Resistor (R
SET
) Value
R
SENSE
Selection
The current sense resistor, R
SENSE
, monitors the inductor
current of the supply (See Typical Application on front
page). Its value is chosen based on the maximum required
output load current. The LT3845A current sense amplifier
has a maximum voltage threshold dependent on duty
cycle. The maximum peak current is calculated as follows:
I
PEAK(MAX)
=
100mV – 45mV
V
OUT
V
IN






R
SENSE
The maximum output load current, I
OUT(MAX)
, is the peak
inductor current minus half the peak-to-peak ripple cur-
rent, DI.
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
efficiency and component size. Low frequency operation
improves efficiency 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 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 LT3845A (see the Minimum On-time Considerations
section). A final 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 LT3845A 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
Table 1 lists typical resistor values for common operating
frequencies.
Table 1. Recommended 1% Standard Values
R
SET
(kΩ) 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