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SSL21081AT Datasheet, PDF (9/22 Pages) NXP Semiconductors – Compact non-dimmable LED driver IC
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NXP Semiconductors
SSL21082AT
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
SSL21082AT, 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 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 starting 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 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.
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Fig 5. OSP logic diagram
The value of ton(high) depends on the capacitor connected to the TONMOD pin. An open or
shorted TONMOD pin sets ton(high) to 15 s (see Section 8.4.6 and Table 6).
SSL21082AT
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 6 — 3 October 2013
© NXP B.V. 2013. All rights reserved.
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