SM8205 Datasheet, PDF (11/13 Pages) M.S. Kennedy Corporation – High-Power LED Driver with Integrated PWM Dimming MOSFET Driver

LED Short-Circuit Protection
The LED short-circuit condition is indicated by FAULT.
At start-up, a monoshot circuit (triggered by the POR
circuit), reset an internal SR flip-flop, which causes
FAULT to go high, and remains high during normal
operation. This also allows the GATE driver to function
normally. The steady state current is reflected in the
reference voltage connected to the transconductance
amplifier. The instantaneous output current is sensed
from FDBK terminal of the op-amp. The short-circuit
threshold current is internally set to 200% of the steady
state current. During short-circuit condition, when the
current exceeds the internally set threshold, the SR flip-
flop is set and FAULT goes low. Meanwhile, the GATE
driver of the power MOSFET is inhibited, providing a
latching protection. The converter can be reset by
cycling the input voltage for operating again.
Current Sense Filter
Parasitic circuit capacitance, inductance and gate driver
current create a spike in the current sense voltage at the
point where Q1 turns on. In order to prevent this spike
from terminal the on-time prematurely, every circuit
should have a low-pass filter that consists of RF and CF,
shown in figure 4. The time constant of this filter should
be long enough to reduce the parasitic spike without
significantly affecting the shape of the actual current
sense voltage. The recommended range for RF is
between 10Ω and 500Ω, and CF is between 100pF and
2.2nF.
SM8205
Thermal Shutdown
Internal thermal shutdown circuitry is provided to protect
the SM8205 in the event that the maximum junction
temperature is exceeded. When activated, typically at
140℃, the controller is forced to disable the output
GATE and FAULT drivers in low-state. After the
temperature is reduced (typical hysteresis is 25℃) the
gate driver will be re-enabled.
VIN
CIN
SM8205
GATE
CS
RF
CF
D1
Q1
RCS
Figure 4. Adding a low-pass filter to prevent
premature triggering
www.samhop.com.tw
11
Rev. 2.0 @ 2011/10