Datasheet

LTM8052/LTM8052A
15
8052fc
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applicaTions inForMaTion
Programming Switching Frequency
The LTM8052/LTM8052A has an operational switch-
ing frequency
range between 100kHz and 1MHz. This
frequency is programmed with an external resistor from
the RT pin to ground. Do not leave this pin open under
any circumstance. 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
0.2 210
0.1 453
In addition, the Typical Performance Characteristics sec-
tion contains
a graph that shows the switching frequency
versus R
T
value.
Switching Frequency Trade-Offs
It is recommended that the user apply the optimal R
T
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
LTM8052/LTM8052A 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 LTM8052/LTM8052A 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 LTM8052/
LTM8052A is determined by the resistor from the RT pin
to GND and may be set from 100kHz to 1MHz. The internal
oscillator may also be synchronized to an external clock
through the
SYNC pin. T
he external clock applied to the
SYNC pin must have a logic low below 0.6V, a logic high
greater than 1.2V and frequency between 100kHz and
1MHz. The input frequency must be 20% higher than the
frequency determined by the resistor at the RT pin. Input
signals outside of these specified parameters may cause
erratic switching behavior and subharmonic oscillations.
The SYNC pin must be tied to GND if synchronization to
an external clock is not required. When SYNC is grounded,
the switching frequency is determined by the resistor at
the RT pin.
Switching Mode
The LTM8052/LTM8052A operates in forced continuous
mode. This means that the part will not skip cycles when
the load approaches zero amps. This may be particularly
useful in applications where the synchronization function
is used, or any time discontinuous switching is undesir
-
able. T
he LT
M8052/LTM8052A will not operate in forced
continuous mode when an input UVLO, output OVLO or
minimum duty cycle violation occurs.
Soft-Start
The soft-start function controls the slew rate of the power
supply output voltage during start-up. A controlled output
voltage ramp minimizes output voltage overshoot, reduces
inrush current from the V
IN
supply, and facilitates 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.