LTM8027
applicaTions inForMaTion
For example, suppose the LTM8027 needs to provide 5V
ing SS pin voltage, reducing the output voltage overshoot
during a short-circuit recovery.
from an input voltage source that is nominally 12V. From
Table 2, the recommended R value is 162k, which cor-
T
The desired soft-start time (t ) is programmed via the
SS
responds to an operating frequency of 210kHz. From the
C
capacitor as follows:
SS
graphs in the Typical Performance Characteristics, the
2µA • tSS
1.231V
typical internal regulator (INTV ) current at 12V and
CC
IN
CSS
=
210kHz is 15mA. The power dissipated by the internal
linear regulator at 12V is given by the equation:
IN
The amount of time in which the power supply must be
P
= (V – 8) • I
IN INTVCC
INTVCC
under a V , internal regulator (INTV ) or V
UVLO
IN
CC
SHDN
fault condition (t
) before the SS pin voltage enters
or only 60mW. This has a small but probably acceptable
effect on the operating temperature of the LTM8027.
FAULT
its active region is approximated by the following formula:
CSS • 0.65V
If the input rises to 60V, however, the power dissipation is
a lot higher, over 780mW. This can cause unnecessarily
tFAULT
=
50µA
high junction temperatures if the INTV regulator must
CC
dissipate this amount of power for very long.
Operating Frequency Trade-offs
Connect BIAS2 to AUX (if 24V or less) or a voltage source
above 3V.
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.
Soft-Start
The soft-start function controls the slew rate of the power
supply output voltage during start-up. A controlled output
voltagerampminimizesoutputvoltageovershoot,reduces
The value of R for a given operating frequency can be
T
chosen from Figure 1 or Table 1.
inrush current from the V supply, and facilitates supply
IN
sequencing.AcapacitorconnectedfromtheSSpintoGND
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
circuitisdisabledoncetheSSpinvoltagehasbeencharged
to 200mV above the internal reference of 1.231V.
600
500
400
300
200
100
0
DuringaV UVLO,RUNevent,orundervoltageoninternal
IN
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
0
50
100
(kΩ)
150
200
R
T
8027 F01
Figure 1. Timing Resistor (RT) Value
removed the V
pin voltage starts to recover. The soft-
ADJ
start circuit takes control of the output voltage slew rate
once the V
pin voltage has exceeded the slowly ramp-
ADJ
8027fb
12
Linear [ Linear ]
