ISL6420EVAL3【PDF 2/10 页】EVALUATION BOARD 3 ISL6420

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Application Note 1159

Start-up

The Power On Reset (POR) function initiates the soft-start

sequence. An internal 10μA current source charges an

external capacitor connected to the ENSS (P9) pin from 0V

to 3.3V. When the ENSS pin reaches 1V the error amplifier

reference voltage ramps from 0V to 0.6V following the slope

of the ENSS pin voltage.

There are two distinct start-up methods for the ISL6420. The

first method is invoked through the application of power to

the IC. The soft-start feature allows for a controlled turn on of

the output once the POR threshold of the input voltage has

been reached. Figure 1 shows the start-up profile of the

regulator in relation to the start-up of the 12V input supply.

FIGURE 1. SOFT-START

The second method of start-up is through the use of the

enable feature. Holding the ENSS (P9) pin on the ISL6420

below 1V will disable the regulator by forcing both the upper

and lower MOSFETs off. Releasing the pin allows the

regulator to start-up.

Shutdown

As discussed in the previous section, if the ENSS pin is

pulled down and held below 1V the regulator will be turned

off. Figure 2 shows the shutdown profile of the regulator with

the ENSS pin pulled low. Figure 3 shows the shutdown of the

regulator when powering down of the input supply.

2

FIGURE 2. SHUTDOWN USING ENSS

FIGURE 3. POWER DOWN OF THE INPUT SUPPLY

Output Performance

Switching Frequency

The switching frequency of the ISL6420 can be adjusted

from 100kHz to 1.4MHz by connecting a resistor from the RT

pin to GND. The free running frequency of the IC is 300kHz

when the RT pin is tied to VCC5. The evaluation board has a

0 ? resistor (R9) connecting RT to VCC5. By removing this

0 ? resistor and placing a resistor (R4) from this pin to GND,

the nominal 300kHz switching frequency can be increased

or decreased as per Figure 4.

AN1159.0

November 10, 2004

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