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LTM8027
APPLICATIONS INFORMATION
C SS =
t FAULT =
Forexample,supposetheLTM8027needstoprovide5V
from an input voltage source that is nominally 12V. From
Table 2, the recommended R T value is 162k, which cor-
responds to an operating frequency of 210kHz. From the
graphs in the Typical Performance Characteristics, the
typical internal regulator (INTV CC ) current at 12V IN and
210kHz is 15mA. The power dissipated by the internal
linear regulator at 12V IN is given by the equation:
P INTVCC = (V IN – 8) ? I INTVCC
or only 60mW. This has a small but probably acceptable
effect on the operating temperature of the LTM8027.
If the input rises to 60V, however, the power dissipation is
a lot higher, over 780mW. This can cause unnecessarily
high junction temperatures if the INTV CC regulator must
dissipate this amount of power for very long.
Connect BIAS2 to AUX (if 24V or less) or a voltage source
above 3V.
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 2μA current source producing a ramped voltage
that overrides the command reference to the controller,
resulting in a smooth output voltage ramp. The soft-start
circuit is disabled once the SS pin voltage has been charged
to 200mV above the internal reference of 1.231V.
During a V IN UVLO, RUN event, or undervoltage on internal
bias, the SS pin voltage is discharged with a 50μA current.
Therefore, the value of the SS capacitor determines how
long one of these events must be in order to completely
discharge the soft-start capacitor. In the case of an output
overload or short circuit, the SS pin voltage is clamped to
a diode drop above the ADJ pin. Once the short has been
removed the V ADJ pin voltage starts to recover. The soft-
start circuit takes control of the output voltage slew rate
once the V ADJ pin voltage has exceeded the slowly ramp-
ing SS pin voltage, reducing the output voltage overshoot
during a short-circuit recovery.
The desired soft-start time (t SS ) is programmed via the
C SS capacitor as follows:
2μA ? t SS
1.231V
The amount of time in which the power supply must be
under a V IN , internal regulator (INTV CC ) or V SHDN UVLO
fault condition (t FAULT ) before the SS pin voltage enters
its active region is approximated by the following formula:
C SS ? 0.65V
50μA
Operating Frequency Trade-offs
The LTM8027 uses a constant frequency architecture that
can be programmed over a 100kHz to 500kHz range with
a single resistor from the RT pin to ground. The nominal
voltage on the RT pin is 1V and the current that flows from
this pin is used to charge an internal oscillator capacitor.
The value of R T for a given operating frequency can be
chosen from Figure 1 or Table 1.
600
500
400
300
200
100
0
0 50 100 150 200
R T (k )
8027 F01
Figure 1. Timing Resistor (R T ) Value
8027fb
12
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