SM8205 Datasheet, PDF (9/13 Pages) M.S. Kennedy Corporation – High-Power LED Driver with Integrated PWM Dimming MOSFET Driver

SM8205
Over-Voltage Protection (OVP)
Over voltage protection is achieved by connecting the
output voltage to the OVP pin through a resistor divider.
The voltage at the OVP pin is constantly compared to
the internal 1.2V. When the output voltage at LED load
terminal is exceeded 1.2V, the IC is turned off,
immediately, at the same time the GATE and FAULT
pins goes low. Power on again to resume this situation.
The output voltage can be set by selecting the values of
R1 and R2 (see figure 1) according to the following
equation:
R1+R2
VOUT = 1.2 •
R2
SM8205
1.2V
VOUT
R1
OVP
R2
Figure 1. Over-voltage protection resistor connection
PWM Dimming (PWMD)
PWM dimming can be achieved by driving the PWMD
pin with a TTL compatible source. The PWM signal is
connected internally to the three different node – the
transconductance amplifier, the FAULT output, and the
GATE output.
When the PWMD signal is high, the GATE and FAULT
pins are enabled, and the output of the EA amplifier is
connected to the external compensation network. Thus,
the internal amplifier controls the LED current. When the
PWMD signal goes low, the output of the EA amplifier is
disconnected from the compensation network. So, the
integrating capacitor maintains the voltage across it. The
GATE is disabled, the converter stops switching and the
FAULT pin goes low, turning off the disconnect switch.
By turning off the disconnect switch action, the output
capacitor is prevented from being discharged, and thus
the PWM dimming response of the boost converter
improvs dramatically.
When the voltage at PWMD is greater than 2.0V, the
PWM dimming MOSFET turns on and when the voltage
on PWMD is below 0.8V, the PWM dimming MOSFET
turns off.
LED Current Reference (IREF)
The LED current is propotional to the voltage at IREF.
Applying an external DC voltage at IREF or using a
potentionmeter from IREF to GND allow analog dimming
of the LED current.
Gate driver (GATE)
External MOSFETs are driven by the SM8205's internal
low impedance gate driver. These driver are biased from
the VDD and have a source current of 150mA and a sink
current of 300mA, to switch a ground-referenced N-
channel MOSFET in high-power applications. The
average current demanded from the supply to drive the
external MOSFET depends on the total gate charge (Qg)
and the operating frequency of the converter, FSW. Use
the following equation to calculate the driver supply
current IGATE required for the switching MOSFET:
IGATE = Qg x FSW
Dimming MOSFET Driver (FAULT)
The SM8205 requires an external N-channel MOSFET
for PWM dimming. Connect the gate of the MOSFET to
the output of the dimming driver, FAULT, for normal
operation. The dimming driver is capable of sourcing or
sinking up to 50mA of current.
LED Current-Sense Input (FDBK)
The current through the LED string is set via the value
chosen for the current sense resistor, RSN. This value
can be calculated using equation of below:
ILED =
VIREF
RSN
COUT
SM8205
FAULT
FDBK
RSN
ILED
Figure 2. LED Forward Current Controls Path
Another important parameter to be aware of in the boost
controller design, is the ripple current. The amount of
ripple current through the LED string is equal to the
output ripple voltage divided by LED AC resistance (RLED
is provided by the LED manufacturer) plus the current
sense and RON of the MOSFET resistor. The amount of
allowable ripple current through the LED string is
dependent upon the application and design’s discretion.
The equation is shown as below:
ΔILED =
VOUT(RIPPLE)
RLED + RON + RSN
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Rev. 2.0 @ 2011/10