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MAX8901A Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX8901A Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 15 page Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package 10 ______________________________________________________________________________________ Startup and Soft-Start The MAX8901_ use a single input (ON) to enable the IC and to control WLED intensity. After ON is driven high, the MAX8901_ enter soft-start and gradually increase the WLED current to full brightness. When the MAX8901_ is first turned on, CCOMP is charged to 1.25V with a 60µA current source. When VCOMP rises above 1.25V, CCOMP is charged with a 4µA current source, and the internal MOSFET begins switching at a reduced duty cycle. When VCOMP rises above 2.25V, the duty cycle is at its maximum. The VCOMP where the IC exits soft-start depends on the final duty cycle required by the load. Typical shutdown timing characteristics are shown in the Typical Operating Characteristics section. A PWM signal (MAX8901A) or serial pulses (MAX8901B) are applied to dim the WLEDs. See the PWM Dimming Control (MAX8901A) and Serial-Pulse Dimming Control (MAX8901B) sections for more details. Shutdown The MAX8901_ enter shutdown when VON is held low for more than 1.33ms (typ), or when an output overvolt- age condition is engaged. In shutdown, the supply cur- rent is reduced to 0.01µA (typ) by powering down the entire IC except for the ON voltage-detection circuitry. CCOMP is discharged to GND during shutdown, allowing the device to reinitiate soft-start when it is re-enabled. Although the internal n-channel MOSFET does not switch in shutdown, there is still a DC current path between the input and the WLEDs through the inductor and Schottky diode. In shutdown, the minimum forward voltage of the WLED string must exceed the maximum input voltage to ensure that the WLEDs remain off. Typically, with two or more WLEDs, the for- ward voltage is large enough to keep leakage current low, less than 1µA (typ). Shutdown timing characteris- tics are shown in the Typical Operating Characteristics. Thermal Shutdown The MAX8901 includes a thermal shutdown feature that protects the IC by turning it off when the die temperature reaches +160°C (typ). After thermal shutdown occurs, the MAX8901_ must be restarted by toggling the ON pin low, then high, or by cycling the input voltage. PWM Dimming Control (MAX8901A) After VIN is above UVLO, apply a PWM signal to ON for a WLED current that is proportional to the PWM signal’s duty cycle (0% duty cycle corresponds to zero LED current and 100% duty cycle corresponds to full LED current). Restrict PWM frequency to between 30kHz to 500kHz for a maximum ILED current accuracy. If dim- ming control is not required, ON works as a simple on/off control. Drive ON high to enable the IC, or drive ON low for shutdown. A resistor connected from CS to GND pro- grams maximum ILED. See the Current-Sense Resistor (MAX8901A Only) section for more details. Serial-Pulse Dimming Control (MAX8901B) After the MAX8901B is enabled by driving ON high for the minimum initial ON high pulse width (40µs, min), soft-start is engaged and brings WLED current to maxi- mum brightness. After soft-start is completed, dim the MAX8901B by pulsing ON low (500ns to 500µs pulse width). Each pulse reduces the WLED current by 0.75mA. The maximum WLED current is 24.75mA for the MAX8901B (0.75mA/step). The overall dimming range for serial-pulse dimming control is from maximum WLED current down to 1/33 of maximum WLED current. The WLEDs are turned off by holding ON low for at least 1.33ms (typ). See Figure 1 for the serial-pulse dimming control timing diagram. Applications Information Inductor Selection Recommended inductor values range from 10µH to 47µH. A 22µH inductor optimizes the efficiency for most applications while maintaining low input voltage ripple. With input voltages near 5V, a larger value of inductance may be more efficient. To prevent core saturation, ensure that the inductor saturation current rating exceeds the peak inductor current for the application. Calculate the peak inductor current with the following formula: where ILED(MAX) is in mA and L is in µH. I V V V x L PEAK OUT(MAX) IN(MIN) IN(MIN) x .2 = × + I x LED MAX () 900 2 1 |
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Similar Description - MAX8901A |
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