LED6000PHR datasheet(15/49 Pages) STMICROELECTRONICS

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LED6000

Functional description

4.2

Soft-start

The soft-start is essential to assure a correct and safe startup of the step-down converter. It

avoids an inrush current surge and makes the output voltage to increase monotonically.

The soft-start is performed as soon as the EN and DIM pin are asserted high; when this

occurs, an external capacitor, connected between the SS pin and ground, is charged by

a constant current (5 µA typ.). The SS voltage is used as reference of the switching

regulator and the output voltage of the converter tracks the ramp of the SS voltage. When

the SS pin voltage reaches the 0.25 V level, the error amplifier switches to the internal

0.25 V reference to regulate the output voltage.

Figure 7. Soft-start

During the soft-start period the current limit is set to the nominal value.

The dVSS/dt slope is programmed in agreement with Equation 2:

Equation 2

Before starting the CSS capacitor charge, the soft-start circuitry turns-on the discharge

switch shown in Figure 7 for TSSDISCH minimum time, in order to completely discharge the

CSS capacitor.

As a consequence, the maximum value for the soft-start capacitor, which assures an almost

complete discharge in case of the EN signal toggle, is provided by:

Equation 3

given TSSDISCH = 530 µs and RSSDISCH = 380  typical values.

The enable feature allows to put the device into the standby mode. With the EN pin lower

than VENL the device is disabled and the power consumption is reduced to 10 A (typ.). If

the EN pin is higher than VENH, the device is enabled. If the EN pin is left floating, an internal

pull-down current ensures that the voltage at the pin reaches the inhibit threshold and the

device is disabled. The pin is also VCC compatible.

REF











SSDISCH

MAX

270







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