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MAX16822AASA Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX16822AASA Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 10 page Undervoltage Lockout (UVLO) The MAX16822A/MAX16822B include UVLO with 500mV hysteresis. The internal MOSFET turns off when VIN falls below 5.5V to 6.0V. DIM Input LED dimming is achieved by applying a PWM signal at DIM. A logic level below 0.6V at DIM forces the MAX16822A/MAX16822B’s output low, thus turning off the LED current. To turn the LED current on, the logic level at DIM must be greater than 2.8V. Thermal Shutdown The MAX16822A/MAX16822B thermal-shutdown fea- ture turns off the LX driver when the junction tempera- ture exceeds +165°C. The LX driver turns back on when the junction temperature drops 10°C below the shutdown-temperature threshold. Analog Dimming The MAX16822A/MAX16822B offer an analog dimming feature that reduces the output current when the volt- age at TEMP_I is below the internal 2V threshold volt- age. The MAX16822A/MAX16822B achieve analog dimming by either an external DC voltage source con- nected between TEMP_I and ground or by a voltage on a resistor connected across TEMP_I and ground induced by an internal current source of 25µA. When the voltage at TEMP_I is below the internal 2V threshold limit, the MAX16822A/MAX16822B reduce the LED cur- rent. Use the following formula to set the analog dim- ming current. where VTFB_ON = 2V and FBSLOPE = 0.75 are obtained from the Electrical Characteristics table and VAD is the voltage at TEMP_I. Thermal Foldback The MAX16822A/MAX16822B also include a thermal- foldback feature that reduces the output current when the temperature of the LED string exceeds a specified temperature point. These devices enter thermal-fold- back mode when the voltage drop on the NTC thermis- tor, thermally attached to the LEDs and electrically connected between TEMP_I and ground, drops below the internal 2V threshold limit. Applications Information Selecting RSENSE to Set LED Current The LED current is programmed with a current-sense resistor connected between IN and CS. Use the equa- tion below to calculate the value of this resistor: where VSNSHI is the sense voltage threshold high and VSNSLO is sense voltage threshold low (see the Electrical Characteristics table for values). Current-Regulator Operation The MAX16822A/MAX16822B regulate the LED current using a comparator with hysteresis (see Figure 2). As the current through the inductor ramps up and the volt- age across the sense resistor reaches the upper threshold, the internal MOSFET turns off. The internal MOSFET turns on again when the inductor current ramps down through the freewheeling diode until the voltage across the sense resistor equals the lower threshold. Use the following equation to determine the operating frequency: where n is the number of LEDs, VLED is the forward voltage drop of 1 LED, and ΔV = (VSNSHI - VSNSLO). Inductor Selection The MAX16822A/MAX16822B operate up to a switching frequency of 2MHz. For space-sensitive applications, the high switching frequency allows the size of the inductor to be reduced. Use the following formula to calculate an approximate inductor value and use the closest standard value: For component selection, use the MAX16822A/B Design Tool available at:www.maxim-ic.com/MAX16822- software. L approx VnV nV R VV f IN LED LED SENSE IN (.) () = ×× ×× − Δ S SW f VnV nV R VV L SW IN LED LED SENSE IN = ×× ×× − () Δ R VV V IA SENSE SNSHI SNSLO LED () ()( ) () Ω= + 1 2 IA I A FB V VV TF LED SLOPE TFB ON A () () _ =× ⎛ ⎝⎜ ⎞ ⎠⎟ × −− 1 1 D D V () ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ () 2MHz, High-Brightness LED Drivers with Integrated MOSFET and High-Side Current Sense _______________________________________________________________________________________ 7 |
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