ltm8027 Linear Technology Corporation, ltm8027 Datasheet - Page 12

• Manufacturer Part Number: ltm8027
• Description: 60v, 4a Dc/dc ?module Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTM8027
For example, suppose the LTM8027 needs to provide 5V
from an input voltage source that is nominally 12V. From
Table 2, the recommended R
responds to an operating frequency of 210kHz. From the
graphs in the Typical Performance Characteristics, the
typical INTV
power dissipated by the internal linear regulator at 12V
is given by the equation:
or only 54mW. 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 750mW. This can cause unnecessarily
high junction temperatures if the INTV
dissipate this amount of power for very long.
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
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
the SS pin voltage is discharged with a 50μA. 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
takes control of the output voltage slew rate once the
V
voltage, reducing the output voltage overshoot during a
short-circuit recovery.
12
ADJ
P
INTVCC
ADJ
pin voltage has exceeded the slowly ramping SS pin
pin voltage starts to recover. The soft-start circuit
IN
= (V
CC
UVLO, INTV
current at 12V
IN
– 8.5) • I
IN
CC
INTVCC
supply, and facilitates supply
undervoltage or RUN event,
IN
T
value is 162k, which cor-
and 210kHz is 15mA. The
CC
regulator must
IN
Operating Frequency Tradeoffs
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 fl ows from
this pin is used to charge an internal oscillator capacitor.
The value of R
chosen from Figure 1 or Table 1.
Table 1 lists typical resistor values for common operating
frequencies.
Table 1. R
T
Resistor Values vs Frequency
600
500
400
300
200
100
R
63.4k
48.7k
40.2k
31.6k
27.4k
23.7k
T
82.5
0
187
118
Figure 1. Timing Resistor (R
(kΩ)
0
T
for a given operating frequency can be
50
R
T
100
(kΩ)
150
T
) Value
f
SW
8027 F01
100
150
200
250
300
350
400
450
500
(kHz)
200
8027f