CS1610 Datasheet, PDF (10/17 Pages) Cirrus Logic – TRIAC Dimmable LED Driver IC

CS1610/11/12/13
5.4.4 Boost Overvoltage Protection
The CS1610/11/12/13 supports boost overvoltage protection
(BOP) to protect the bulk capacitor C8 (see Figure 12. Flyback
Model). If the boost output voltage exceeds the overvoltage
protection thresholds of 249V for a 120V system, or 448V for
a 230V system, a BOP fault signal is generated. The control
logic continuously averages this BOP fault signal, and if at any
point in time the average exceeds a set event threshold, the
boost stage is disabled. The BOP fault averaging algorithm
sets the event threshold such that the boost output voltage is
never allowed to stay above the BOP threshold for more than
1.6 ms.
During a boost overvoltage protection event, the second stage
is kept enabled, and its dim input is railed to full scale. This
allows the second stage to dissipate the stored energy on the
bulk capacitor (C8) quickly, bringing down the boost output
voltage to a safe value. A visible flash on the LED might
appear, indicating that an overvoltage event has occurred.
When the boost output voltage drops to 195V for a 120V
application or 368V for a 230V application, the boost stage is
enabled, and the system returns to normal operation.
5.5 Voltage Clamp Circuit
To keep dimmers conducting and prevent them from misfiring,
a minimum power needs to be delivered from the dimmer to
the load. This power is nominally around 2W for 230V and
120 V TRIAC dimmers. At low dim angles (< 90°), this excess
power cannot be converted into light by the second output
stage due to the dim mapping at light loads. The output
voltage of the boost stage (V_BST) can rise above the safe
operating voltage of the primary-side bulk capacitor (C6).
The CS1610/11/12/13 provides active clamp circuitry on the
CLAMP pin, as shown in Figure 11.
V_BST
VDD
I_{CLAM P}
R10
CLAMP ₃
Q3
S1
V_BE
CS1610 /11/12/13
Figure 11. CLAMP Pin Model
A PWM control loop ensures that the voltage on V_BST (the
boost output) does not exceed 227 V for 120VAC applications
or 424 V for 230VAC applications. This control turns on the
BJT of the voltage clamp circuit, allowing the clamp circuit to
sink current through the load resistor, preventing V_BST from
exceeding the maximum safe voltage.
5.5.1 Clamp Overpower Protection
The CS1610/11/12/13 clamp overpower protection (COP)
control logic averages the ‘ON’ time of the clamp circuit. If the
output of the averaging logic exceeds 49%, a COP event is
actuated, disabling the boost and second stages. The clamp
circuitry is turned off during the fault event. The ‘ON’ time
averaging algorithm sets the COP threshold such that the
clamp circuit cannot be continuously ‘ON’ for more than
13.8 ms.
5.6 Dimming Signal Extraction and the Dim
Mapping Algorithm
When operating with a dimmer, the dimming signal is
extracted in the time domain and is proportional to the
conduction angle of the dimmer. A control variable is passed
to the quasi-resonant second stage to achieve 2% to 100%
output currents.
5.7 Quasi-resonant Second Stage
The second stage is a quasi-resonant current-regulated DC-
DC Converter capable of flyback or buck operation, delivering
the highest possible efficiency at a constant current while
minimizing line frequency ripple. Primary-side control is used
to simplify system design and reduce system cost and
complexity.
VB S T
T1
D8
LED +
C8 Z2
D7
C9
LED -
CS1610/11
GD ¹³
Q4
R12
FBAUX ¹⁵
R13
FBSENSE ¹¹
GND
12
FBGAIN
9
RFB GA IN
R11
Figure 12. Flyback Model
The digital algorithm ensures monotonic dimming from 2% to
100% of the dimming range with a linear relationship between
the dimming signal and the LED current. The flyback stage is
controlled by sensing current in the transformer primary.
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DS929F5