SSL2129AT datasheet(7/21 Pages) NXP

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SSL2129AT

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Product data sheet

Rev. 2 — 23 October 2012

7 of 21

NXP Semiconductors

SSL2129AT

Dimmable LED controller IC

8.5 Protective features

The IC has the following protective features:

• UnderVoltage LockOut (UVLO)

• Leading-Edge Blanking (LEB)

• OverCurrent Protection (OCP)

• Internal OverTemperature Protection (OTP)

• Brownout protection

• LED overtemperature control and protection

• An optional output OverVoltage Protection circuit is implemented using external

components and the NTC pin.

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. When VCC drops

lower than VCC(rst), the IC resets the latch protection mode. Switching starts only when no

fault condition exists.

8.5.1 UnderVoltage LockOut (UVLO)

When the voltage on the VCC pin < VCC(stop), the IC stops switching. An attempt is then

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

8.5.2 Leading-Edge Blanking (LEB)

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

switch-on is implemented. When the 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.

8.5.3 OverCurrent Protection (OCP)

The SSL2129AT contains a highly accurate peak current detector. It triggers when the

voltage on 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 td(ocp-swoff) between overcurrent detection and the actual closure of the

switch. 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.

8.5.4 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 below Tth(rel)otp.

8.5.5 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. In the

SSL2129AT, there is a maximum limit on the on-time of switch ton(high).

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