SSL21081AT datasheet(9/22 Pages) PHILIPS

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SSL21081AT

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Product data sheet

Rev. 6 — 3 October 2013

9 of 21

NXP Semiconductors

SSL21081AT

Compact non-dimmable LED driver IC

SSL21081AT, there is a maximum limit on the on-time 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). As a result

the average output current to the LEDs is reduced.

8.3.6 Output Short-circuit Protection (OSP)

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

limit (toff(high)), then typically the output voltage is less than the minimum limit allowed in

the application. This condition can occur either during start-up or due to a short-circuit. A

timer tdet(sc) is started when toff(high) is detected. Timer tdet(sc) is reset when a valid valley

detection occurs in one of the subsequent cycles or when VCC drops to below VCC(stop).

The timer can also be reset if the maximum limit on the on-time of the switch (ton(high)) is

reached, which is usually the case at start-up (brownout protection). If no valley is

detected and (ton(high)) is not reached before tdet(sc), then it is concluded that a real

short-circuit exists. The IC enters latched protection. If VCC drops to below VCC(rst), the IC

resets the latched protection mode (see Figure 5).

8.4 VCC supply

The SSL21081AT is supplied using three methods:

• Under normal operation, the voltage swing on the DVDT pin is rectified within the IC

providing current towards the VCC pin

• At start-up, there is an internal current source connected to the HV pin. The current

source provides internal power until either the dV/dt supply or an external current on

the VCC pin provides the supply

• Using an auxiliary winding, the voltage can be rectified and connected to the VCC pin

via a series resistor.

The IC starts up when the voltage at the VCC pin exceeds VCC(startup). The IC locks out

(stops switching) when the voltage at the VCC pin is < VCC(stop). The hysteresis between

the start and stop levels allows the IC to be supplied by a buffer capacitor until the dV/dt

supply is settled. The SSL21081AT has an internal VCC clamp, which is an internal active

Zener (or shunt regulator). This internal active Zener limits the voltage on the supply VCC

pin to the maximum value of VCC. If the maximum current of the dV/dt supply minus the

current consumption of the IC (determined by the load on the gate drivers) is lower than

the maximum value of IDD, no external Zener diode is required in the dV/dt supply circuit.

Fig 5.

OSP logic diagram

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