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SSL21084T/1,118

GreenChip drivers for LED lighting

NXP Semiconductors. 

NXP Semiconductors

SSL2108X

Drivers for LED lighting

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 internal MOSFET switch opens there can be a short

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

LEB time tleb is disregarded.

8.5.3 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),

this state 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.

8.5.4

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 lower than 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. In constant power mode, the input current would

otherwise increase to a level that is too large for the input circuitry. In the SSL2108X, there

is a maximum limit on the on-time of switch ton(high). Since the rate of current rise in the coil

during the on-phase is proportional to the difference between input voltage and output

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

secondary stroke starts. When no capacitor is connected to the pin, VTONMAX is reached

quickly, shorter than the minimum limit of microsecond. In this case, or in case the

TONMAX pin is grounded, the internal time constant, ton(high) determines the maximum

on-time.

8.5.6 Short-Winding Protection (SWP)

SWP activates if there is a steep rising MOSFET current through the external resistor

connected to the SOURCE pin. This current can occur when there is a short from the

freewheel diode. Additionally, it occurs due to a small/shorted inductor between the input

voltage and the DRAIN pin. If the voltage on the SOURCE pin is greater than 1.5 V,

latched protection is triggered following LEB time tleb. In addition, if Vcc drops lower than

VCC(rst) the IC resets the latched protection mode.

8.5.7 Output Short Protection (OSP)

During the second stroke (switch-of time), if a valley is not detected within the off-time limit

(toff(high)), it is likely that the output voltage is less than the minimum limit allowed in the

application. This condition can occur either during starting up or due to a short. A timer is

SSL2108X

Preliminary data sheet

All information provided in this document is subject to legal disclaimers.

Rev. 1 — 9 September 2011

© NXP B.V. 2011. All rights reserved.

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