SM8205SL datasheet(9/13 Pages) SAMHOP

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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:

VOUT

OVP

SM8205

R1+R2

1.2 •

VOUT =

Figure 1. Over-voltage protection resistor connection

1.2V

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:

Dimming MOSFET Driver (FAULT)

IGATE = Qg x FSW

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:

VIREF

RSN

ILED =

FDBK

RSN

FAULT

SM8205

ILED

COUT

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:

VOUT(RIPPLE)

RLED + RON + RSN

ΔILED =

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Rev. 2.0 @ 2011/10

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