SSL21081T datasheet(7/24 Pages) PHILIPS

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SSL21081T_2T_3T_4T

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Preliminary data sheet

Rev. 2 — 6 December 2011

7 of 24

NXP Semiconductors

SSL21081T/2T/3T/4T

GreenChip drivers for LED lighting

• Internal OverTemperature Protection (OTP)

• Brownout protection

• Short-Winding Protection (SWP)

• Output Short Protection (OSP)

• LED overtemperature control and protection

The SWP and the OSP are latched protections. These protections cause the IC to halt

until a reset (a result of power cycling) is executed. When Vcc drops lower than Vcc(rst), the

IC resets the latch protection mode. The internal OTP and LED over temperature

protections are safe-restart protections. The IC halts, causing VCC to fall lower than

VCC(stop), and instigates start-up. Switching starts only when no fault condition exists.

8.4.1 UnderVoltage LockOut (UVLO)

When the voltage on the VCC pin drops lower than Vcc(stop), the IC stops switching. An

attempt is then made to restart by supplying VCC from the HV pin voltage.

8.4.2 Leading-Edge Blanking (LEB)

To prevent false detection of the short-winding or overcurrent, a blanking time following

switch-on is implemented. When the internal MOSFET switch turns on there can be a

short current spike due to capacitive discharge of voltage over the drain and source.

During the LEB time (tleb), the spike is disregarded.

8.4.3 OverCurrent Protection (OCP)

The SSL2108X contains a highly accurate peak current detector. It triggers when the

voltage at the SOURCE pin reaches the peak-level Vth(ocp)SOURCE. The current through

the switch is sensed using a resistor connected to the SOURCE pin. The sense circuit is

activated following LEB time tleb. As the LED current is half the peak current (by design), it

automatically provides protection for maximum LED current during operation. There is a

propagation delay between overcurrent detection and the actual closure of the switch

td(ocp-swoff). Due to the delay, the actual peak current is slightly higher than the OCP level

set by the resistor in series to the SOURCE pin.

8.4.4 OverTemperature Protection (OTP)

When the internal OTP function is triggered at a certain IC temperature (Tth(act)otp), the

converter stops operating. The OTP safe-restart protection and the IC restarts again with

switching resuming when the IC temperature drops lower than Tth(rel)otp.

8.4.5 Brownout protection

Brownout protection is designed to limit the lamp power when the input voltage drops

close to the output voltage level. Since the input power has to remain constant, the input

current would otherwise increase to a level that is too large for the input circuitry. For the

SSL2108X, there is a maximum limit on the on-time of the switch ton(high). The rate of

current rise in the coil during the on-phase is proportional to the difference between input

voltage and output voltage. Therefore, the peak current cannot be reached before ton(high)

and as a result the average output current to the LEDs is reduced.

Using the SO12 package, the ton(high) can be lowered by connecting a capacitor to the

TONMAX pin. The external capacitor is charged during the primary stroke with ITONMAX. If

VTONMAX level is reached before the ton(high) time, the switch is turned off and the

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