Datasheet

LTM8001
14
8001fb
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APPLICATIONS INFORMATION
Programming Switching Frequency
The LTM8001 has an operational switching frequency
range between 200kHz and 1MHz. This frequency is
programmed with an external resistor from the RT pin to
ground. Do not leave this pin open under any condition.
See Table 2 for resistor values and the corresponding
switching frequencies.
Table 2. R
T
Resistor Values and Their Resultant Switching
Frequencies
SWITCHING FREQUENCY (MHz) R
T
(kΩ)
1 39.2
0.75 53.6
0.5 82.5
0.3 140
0.2 200
Switching Frequency Trade-Offs
It is recommended that the user apply the optimal R
T
resis-
tor value given in Table 1 for the input and output operating
condition. System level or other considerations, however,
may necessitate another operating frequency
. While the
LTM8001 is flexible enough to accommodate a wide range
of operating frequencies, a haphazardly chosen one may
result in undesirable operation under certain operating or
fault conditions. A frequency that is too high can reduce
efficiency, generate excessive heat or even damage the
LTM8001 in some fault conditions. A frequency that is too
low can result in a final design that has too much output
ripple or too large of an output capacitor.
Switching Frequency Synchronization
The nominal switching frequency of the LTM8001 is
determined by the resistor from the RT pin to GND and
may be set from 200kHz to 1MHz. The internal oscillator
may also be synchronized to an external clock through
the SYNC pin. The external clock applied to the SYNC pin
must have a logic low below 0.8V and a logic high greater
than 1.2V. The input frequency must be 20% higher than
the frequency determined by the resistor at the RT pin.
The SYNC pin must be tied to GND if the synchroniza
-
tion to an external clock is not required. When SYNC is
grounded, the switching frequency is determined by the
resistor at the RT pin.
Soft-Start
The soft-start function controls the slew rate of the power
supply output V
OUT0
voltage during start-up. A controlled
output voltage ramp minimizes output voltage overshoot,
reduces inrush current from the V
IN0
supply, and facili-
tates supply sequencing. A capacitor connected from the
SS pin to GND programs the slew rate. The capacitor is
charged from an internal 11μA current source to produce
a ramped output voltage.
Maximum Output Current Adjust
The L
TM8001 features an adjustable accurate current
limit. To adjust the load current limit, an analog voltage
is applied to the ILIM pin. Varying the voltage between
0V and 1.5V adjusts the maximum current between the
minimum and the maximum current, 5.6A typical. Above
1.5V, the control voltage has no effect on the regulated
inductor current. Graphs of the output current vs ILIM volt
-
ages are given in the Typical Performance Characteristics
section. The LTM8001 provides a 2V reference voltage for
conveniently applying resistive dividers to set the current
limit. The current limit can be set as shown in Figure 1
with the following equation:
I
MAX
= 7.47
R2
R1+R2
A convenient value of R1 may be 10k. In that case,
R2=
10 •I
MAX
7.47 –I
MAX
kΩ
LTM8001
V
REF
R1
2V
R2
8001 F01
ILIM
Figure 1. Setting the Output Current Limit, I
MAX