Figure 7. Soft-start

COMP

FB

+

+

-

Rf

Cf

Cp

VLED+

R

Iss

SS

Css

Vref

250 mV

ERROR

AMPLIFIER

SNS

Ru

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

The dVSS/dt slope is programmed in agreement with the following equation:

CSS=ISS⋅TSS

VREF =5μA⋅TSS

0.25V

(2)

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

Figure 7. Soft-start 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 EN signal toggle, is provided by:

CSS_MAX≤ TSSDISCH

5⋅RSSDISCH≅270nF

(3)

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

The enable feature allows the device to be put in standby mode. With the EN pin lower than VENL the device is

disabled and the power consumption is reduced to less than 10 μA (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.

5.3

Digital dimming

The switching activity is inhibited as long as DIM pin is kept below VDIML threshold.

When DIM is asserted low, the HS MOS is turned off as soon as the minimum on-time is expired and the COMP

pin is parked close to the maximum ramp peak value, in order to limit the input inrush current when the IC restart

the switching activity. The internal oscillator and, consequently, the IC quiescent current are reduced only if DIM is

kept low for more than TDIMTO timeout.

When DIM is forced above the VDIMH threshold after TDIMTO has elapsed, a new soft-start sequence is performed.

The inductor current dynamic performance, when dimming input goes high, depends on the

designed system response. The best dimming performance is obtained by maximizing the bandwidth and phase

margin, when possible.

As a general rule, the output capacitor minimization improves dimming performance in terms of shorter LEDs

current rising time and reduced inductor peak current .

An oversized output capacitor value requires extra current for fast charge so generating an inductor current

overshoot and oscillations.

ALED6000

Digital dimming

DS13018 - Rev 4

page 12/45