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AME5144 Datasheet(PDF) 8 Page - AMETHERM Circuit Protection Thermistors |
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AME5144 Datasheet(HTML) 8 Page - AMETHERM Circuit Protection Thermistors |
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8 / 15 page  AME 8 AME5144 Rev. D.01 High Efficiency, 37V Step-Up Converter for 2 to 10 White LEDs n Detailed Description The high efficiency and small size of the AME5144 make them ideally suited to driver up to ten series-connected LEDs. They operate as a boost DC-DC converter that con- trols output current rather than voltage. A low 300mV cur- rent-sense threshold minimized the losses. The AME5144 provide even illumination by sourcing the same output cur- rent through each LED, eliminating the need for expensive factory calibration. The fast 1MHz internal oscillator al- lows for small inductor and small input and output capaci- tors while minimizing input and output ripple. The single analog control input (CTRL) allows easy ad- justment of LED brightness and on/off control. This allows simple logic-level on/off control, analog voltage control, or PWM duty-cycle control of both brightness and shutdown. In shutdown, supply current is reduced to a low 0.3 µA (Typ.). A soft-start gradually illuminates the LEDs, elimi- nating the inrush current during startup. Shutdown The AME5144 enter shutdown when VCTRL is less than 100mV for more than 8.2ms. When shutdown, the supply current is reduced to 0.3 µA (Typ.) approximately. The 0.3uA supply current is applied for the voltage-de- tection circuitry. C COMP is discharge during shutdown, al- lowing the device to reinitiate soft-start when enabled. Al- though the internal n-channel MOSFET does not switch in shutdown, there is still a DC current path between the input and the LEDs through the inductor and Schottky di- ode. The minimum forward voltage of the LED array must exceed the maximum input voltage to ensure that the LEDs remain off in shutdown. However, with two or more LEDs, the forward voltage is enough to keep leakage current low, less than 1 µA (Typ.). Soft-Start The AME5144 attain soft-start by charging C COMP gradu- ally with a current source. When V COMP rises above 1.25V, the internal MOSFET begins switching at a reduced duty cycle. When V COMP rises above 2.25V, the duty cycle is at its maximum. See the Typical Operation Characteris- tics for an example of soft-start operation. c Over Voltage Protection OVP is designed to prevent the output voltage from ex- ceeding the maximum switch voltage rating of 38.5V (Typ.). The protection circuitry stops the internal MOSFET from switching. There is a 1.5V hysteresis associated with this circuitry which will cause the output to fluctuate be- tween 38.5V and 37V. Adjusting LED Current Adjusting the output current of the AME5144 changes the brightness of the LEDs. An analog input (CTRL) and the sense-resistor values set the output current. Output current is given by: I LED=VCTRL/(5xRSENSE) The V CTRL voltage range for adjusting output current is 0.24V to 1.65V. To set maximum current, calculate RSENSE when VCTRL is at its maximum as follows: R SENSE=1.65V / (5 x ILED) Power dissipation in R SENSE is typical less than 10mW, allowing the use of small surface-mount resistor. PWM Dimming Control A. Using DC Signal for CTRL Pin For CTRL, applying a DC signal in the range of 0.24V to 1.65V control the LED current. CTRL can be overdriven; however, applying a CTRL greater than 1.65V does not increased the LED current above the level at 1.65V. B. Using PWM signal for CTRL pin The CTRL input is used as a digital input allowing LED brightness control with a logic-level PWM signal applied directly to CTRL. The frequency range is from 200Hz to 30KHz, while 0% duty cycle corresponds to minimum cur- rent, and 100% duty cycle corresponds to full current. The error amplifier and compensation capacitor form a low pass filter so PWM dimming results in DC current to the LEDs without the need for any additional RC filters. |
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