Figure 12. ISKIP typical current and RILIM value

100

150

200

250

300

350

400

450

500

550

20

30

40

50

60

70

80

90

100

RILIM [kΩ]

However, due to current sensing comparator delay, the actual inductor charge current is slightly impacted by VIN

and selected inductor value.

In order to let the bootstrap capacitor recharge, in case of extremely light load the ALED6000 is able to pull-down

the LX net through an integrated small LS MOS. In this way the bootstrap recharge current can flow from VIN

through CBOOT, LX and the LS MOS.

This mechanism is activated if the HS MOS has been kept turned-off for more than 3 ms (typ.).

5.5

Low VIN operation

In normal operation (i.e. VOUT programmed lower than input voltage) when the HS MOS is turned off, a minimum

off-time (TOFFMIN) interval is performed.

In case the input voltage falls close or below the programmed output voltage (low-dropout, LDO) the ALED6000

control loop is able to increase the duty cycle up to 100%. However, in order to keep the boot capacitor properly

recharged, a maximum HS MOS on-time is limited (TONMAX). When this limit is reached the HS MOS is turned

off and an integrated switch working as pull-down resistor between LX and GND is turned on, until one of the

following conditions is met:

•

A negative current limit (300 mA typ.) is reached

•

timeout (1 μs typ.) is reached

So doing the ALED6000 device is able to work in low-dropout operation, due to the advanced boot capacitor

management, and the effective maximum duty cycle is about 12 μs / 13 μs = 92%.

5.6

Overcurrent protection

The ALED6000 implements overcurrent protection by sensing the current flowing through the power MOSFET.

Due to the noise created by the switching activity of the power MOSFET, the current sensing circuitry is disabled

during the initial phase of the conduction time. This avoids an erroneous detection of a fault condition. This

interval is generally known as “masking time” or “blanking time”. The masking time is about 120 ns.

If the overcurrent limit is reached, the power MOSFET is turned off implementing pulse-by-pulse overcurrent

protection. In the overcurrent condition, the device can skip turn-on pulses in order to keep the output current

constant and equal to the current limit. If, at the end of the “masking time”, the current is higher than the

overcurrent threshold, the power MOSFET is turned off and one pulse is skipped. If, at the following switching

on, when the “masking time” ends, the current is still higher than the overcurrent threshold, the device skips two

pulses. This mechanism is repeated and the device can skip up to seven pulses (refer to Figure 13. OCP and

frequency scaling).

ALED6000

Low VIN operation

DS13018 - Rev 4

page 16/45