SM8205 datasheet(11/13 Pages) MSK

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SM8205

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

RCS

GATE

SM8205

CIN

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.

Figure 4. Adding a low-pass filter to prevent

premature triggering

VIN

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.

www.samhop.com.tw

Rev. 2.0 @ 2011/10

SM8205 Datasheet(PDF) 11 Page - M.S. Kennedy Corporation

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