LTM8027EVPBF datasheet(12/20 Pages) LINER

12 / 20 page

LTM8027

8027f

APPLICATIONS INFORMATION

For example, suppose the LTM8027 needs to provide 5V

from an input voltage source that is nominally 12V. From

Table 2, the recommended RT value is 162k, which cor-

responds to an operating frequency of 210kHz. From the

graphs in the Typical Performance Characteristics, the

typical INTVCC current at 12VIN and 210kHz is 15mA. The

power dissipated by the internal linear regulator at 12VIN

is given by the equation:

PINTVCC = (VIN – 8.5) • IINTVCC

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 INTVCC regulator must

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 VIN 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 VIN UVLO, INTVCC undervoltage or RUN event,

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 VADJ pin voltage starts to recover. The soft-start circuit

takes control of the output voltage slew rate once the

VADJ pin voltage has exceeded the slowly ramping SS pin

voltage, reducing the output voltage overshoot during a

short-circuit recovery.

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 ﬂows from

this pin is used to charge an internal oscillator capacitor.

The value of RT for a given operating frequency can be

chosen from Figure 1 or Table 1.

Figure 1. Timing Resistor (RT) Value

RT (kΩ)

100

200

600

8027 F01

100

150

200

300

400

500

Table 1 lists typical resistor values for common operating

frequencies.

Table 1. RT Resistor Values vs Frequency

RT (kΩ)

fSW (kHz)

187

100

118

150

82.5

200

63.4k

250

48.7k

300

40.2k

350

31.6k

400

27.4k

450

23.7k

500

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