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SSL21081T/1,118

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SSL21081T All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 6 — 3 October 2013 7 of 22
NXP Semiconductors
SSL21081T
Compact non-dimmable LED driver IC
8.2 Valley detection
A new cycle is started when the primary switch is switched on (see Figure 4). Following
time t
1
, when the peak current is detected on the SOURCE pin, the switch is turned off
and the secondary stroke starts (3). When the secondary stroke is complete and the coil
current at t
3
equals zero, the drain voltage starts to oscillate around the V
IN
V
OUT
level.
The amplitude equals V
OUT
. A special feature, called valley detection is an integrated part
of the SSL21081T circuitry. Dedicated built-in circuitry connected to the DRAIN pin,
senses when the voltage on the drain of the switch has reached its lowest value. The next
cycle is then started and as a result the capacitive switching losses are reduced.
If both the frequency of the oscillations and the volt
age swing are within the range
specified (f
ring
and ∆V
vrec(min)
) for detection, a valley is detected and accepted. If a valid
valley is not detected, the secondary stroke is continued until the maximum off-time
(t
off(high)
) is reached. Then the next cycle is started.
Fig 4. Buck waveforms and valley detection
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SSL21081T All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 6 — 3 October 2013 8 of 22
NXP Semiconductors
SSL21081T
Compact non-dimmable LED driver IC
8.3 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
• Short-Winding Protection (SWP)
• Output Short Protection (OSP)
• LED overtemperature control and protection
The SWP and the OSP are latched protections. These pr
otections cause the IC to halt
until a reset (a result of power cycling) is executed. When V
CC
drops to below V
CC(rst)
, the
IC resets the latch protection mode. The internal OTP and LED over temperature
protections are safe-restart protections. The IC halts, causing V
CC
to drop to below
V
CC(stop)
, triggering a start-up. When V
CC
drops to below V
CC(rst)
, the IC resets the latch
protection mode. Switching starts only when a no fault condition exists.
8.3.1 UnderVoltage LockOut (UVLO)
When the voltage on the VCC pin drops to below V
CC(stop)
, the IC stops switching. An
attempt is made to restart by supplying V
CC
from the HV pin voltage.
8.3.2 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 switches on, there can be a
short current spike due to capacitive discharge of voltage over the drain and source and
the charging of the gate to source capacitance. During the LEB time (t
leb
), the spike is
disregarded.
8.3.3 OverCurrent Protection (OCP)
The SSL21081T contain a highly accurate built-in peak current detector. It triggers when
the voltage at the SOURCE pin reaches the peak-level V
th(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 t
leb
. 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 (t
d(ocp-swoff)
) between the overcurrent detection and the
actual switching off 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.
8.3.4 OverTemperature Protection (OTP)
When the internal OTP function is triggered at a certain IC temperature (T
th(act)otp
), the
converter stops operating. The safe-restart protection is triggered and the IC restarts with
switching resuming when the IC temperature drops lower than T
th(rel)otp
.
SSL21081T All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 6 — 3 October 2013 9 of 22
NXP Semiconductors
SSL21081T
Compact non-dimmable LED driver IC
8.3.5 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 has to remain constant. The input
current would otherwise increase to a level that is too high for the input circuitry. For the
SSL21081T, there is a maximum limit on the on-time t
on(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 t
on(high)
. As a result
the average output current to the LEDs is reduced.
8.3.6 Short-Winding Protection (SWP)
SWP activates if there is a steep rising current through the MOSFET and thus 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
> 1.5 V, latched protection is triggered following LEB time t
leb
. In addition, if V
CC
drops to
below V
CC(rst)
the IC resets the latched protection mode.
8.3.7 Output Short-circuit Protection (OSP)
During the secondary stroke (switch-off time), if a valley is not detected within the off-time
limit (t
off(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 t
det(sc)
is started when t
off(high)
is detected. Timer t
det(sc)
is reset when a valid valley
detection occurs in one of the subsequent cycles or when V
CC
drops to below V
CC(stop)
.
The timer can also be reset if the maximum limit on the on-time of
the switch (t
on(high)
) is
reached, which is usually the case at start-up (brownout protection). If no valley is
detected and (t
on(high)
) is not reached before t
det(sc)
, then it is concluded that a real
short-circuit exists. The IC enters latched protection. If V
CC
drops to below V
CC(rst)
, the IC
resets the latched protection mode (see Figure
5).
Fig 5. OSP logic diagram
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SSL21081T/1,118

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Buy SSL21081T/1,118
Manufacturer:
NXP Semiconductors
Description:
LED Lighting Drivers Compact non-dimmable LED drive
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