SSL21084T/1,118(2013) 查看數據表（PDF） - NXP Semiconductors.

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SSL21084T/1,118
(Rev.:2013)

Mains driver LED driver IC

NXP Semiconductors. 

NXP Semiconductors

SSL21084T

Mains dimmable LED driver IC

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. The input power must remain constant. The input current

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

SSL21084T, there is a maximum limit on the on-time of switch ton(high).

In buck mode, 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.

8.4.6

ton control

The ton(high) can be lowered by connecting a capacitor to the TONMOD pin. The external

capacitor is charged during the primary stroke with Ioffset(TONMOD). If the Vth(TONMOD) level

is reached before the ton(high) time, the switch is turned off and the secondary stroke starts.

When a capacitor is not connected to the pin, Vth(TONMOD) is reached quickly, shorter than

the minimum limit of 1 s. In this case or when the TONMOD pin is grounded, the internal

time constant, ton(high) determines the maximum on-time. This function is used to control

the converter operation over the mains cycle which enables the design of a mains

dimmable driver.

8.4.7 Short-Winding Protection (SWP)

This protection activates when there is a steep rising current on the SOURCE pin. This

current occurs when there is very small inductor or no inductor is present at all between

the input voltage and DRAIN pin. When the voltage on the SOURCE pin is > 1.5 V after

the leading-edge blanking time, the latched protection is triggered.

8.4.8 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). During PWM dimming, the OSP timer

is paused during the off cycle.

SSL21084T

Product data sheet

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Fig 5. OSP logic diagram

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

Rev. 6 — 3 October 2013

4

263

DDD

© NXP B.V. 2013. All rights reserved.

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