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MP3394ES Datasheet(PDF) 9 Page - Monolithic Power Systems |
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MP3394ES Datasheet(HTML) 9 Page - Monolithic Power Systems |
9 / 17 page MP3394—4-STRING, MAX 200mA/STRING WHITE LED DRIVER MP3394 Rev. 1.07 www.MonolithicPower.com 9 3/21/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2013 MPS. All Rights Reserved. OPERATION The MP3394 employs a programmable constant frequency, peak current mode step-up converter and 4-channels regulated current sources to drive the array of 4 strings white LEDs. The operation of the MP3394 can be understood by referring to the block diagram of Figure 1. Internal 6V Regulator The MP3394 includes an internal linear regulator (VCC). When VIN is greater than 6.5V, this regulator offers a 6V power supply for the external MOSFET switch gate driver and the internal control circuitry. The VCC voltage drops to 0V when the chip shuts down. The MP3394 features Under Voltage Lockout. The chip is disabled until VCC exceeds the UVLO threshold. And the hysteresis of UVLO is approximately 200mV. System Startup When the MP3394 is enabled, the chip checks the topology connection first. The chip monitors the OVP pin to see if the Schottky diode is not connected or the boost output is short to GND. If the OVP voltage is lower than 70mV, the chip will be disabled. The MP3394 will also check other safety limits, including UVLO and OTP after the OVP test is passed. If they are all in function, it then starts boosting the step-up converter with an internal soft-start. It is recommended on the start up sequence that the enable signal comes after input voltage and PWM dimming signal established. Step-up Converter The converter operation frequency is programmable (from 150kHz to 500kHz) with a external set resistor on OSC pin, which is helpful for optimizing the external components sizes and improving the efficiency. At the beginning of each cycle, the external MOSFET is turned on by the internal clock. To prevent sub-harmonic oscillations at duty cycles greater than 50 percent, a stabilizing ramp is added to the output of the current sense amplifier and the result is fed into the PWM comparator. When this result voltage reaches the output voltage of the error amplifier (VCOMP) the external MOSFET is turned off. The voltage at the output of the internal error amplifier is an amplified signal of the difference between the 350mV reference voltage and the feedback voltage. The converter automatically chooses the lowest active LEDX pin voltage for providing enough bus voltage to power all the LED arrays. If the feedback voltage drops below the 350mV reference, the output of the error amplifier increases. It results in more current flowing through the power FET, thus increasing the power delivered to the output. In this way it forms a close loop to make the output voltage in regulation. At light-load or Vout near to Vin operation, the converter runs into the pulse-skipping mode, the FET is turned on for a minimum on-time of approximately 100ns, and then the converter discharges the power to the output in the remain period. The external MOSFET will keep off until the output voltage needs to be boosted again. Dimming Control The MP3394 provides two PWM dimming methods: external PWM signal or DC signal input PWM Dimming mode (see Figure 2). + - DPWM Oscillator DPWM Comparator DBRT BOSC Ex-PWM Input CBOSC DPWM Output Figure 2—PWM Dimming Method |
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