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MP3397 Datasheet(PDF) 9 Page - Monolithic Power Systems |
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MP3397 Datasheet(HTML) 9 Page - Monolithic Power Systems |
9 / 16 page MP3397—4-STRING, MAX 350mA/STRING WHITE LED DRIVER MP3397 Rev. 1.01 www.MonolithicPower.com 9 9/4/2012 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. OPERATION The MP3397 employs a programmable constant- frequency, peak-current–mode step-up converter with 4 channels or regulated current sources to drive an array of up to 4 strings of white LEDs. Internal 6V Regulator The MP3397 includes an internal linear regulator (VCC). When VIN is greater than 6.5V, this regulator outputs a 6V power supply to the external MOSFET switch gate driver and the internal control circuitry. The VCC voltage drops to 0V when the chip shuts down. The MP3397 features under-voltage lockout (UVLO). The chip is disabled until VCC exceeds the UVLO threshold. The UVLO hysteresis is approximately 200mV. System Startup When enabled, the MP3397 checks the topology connection first. The chip monitors the over- voltage protection (OVP) pin to see if the Schottky diode is not connected or if the boost output is shorted to GND. An OVP voltage of less than 70mV will disable the chip. The MP3397 also checks other safety limits, including UVLO and over-temperature protection (OTP) after passing the OVP test. If all the protection tests pass, the chip then starts boosting the step-up converter with an internal soft-start. The enable signal must occur after the establishment of the input voltage and PWM dimming signal during the start-up sequence. Step-Up Converter The converter operating frequency is programmable (from 150kHz to 500kHz) with an external set resistor on the OSC pin. This flexibility helps to optimize the size of external components and improve the efficiency. At the beginning of each cycle, the internal clock turns on the external MOSFET. A stabilizing ramp added to the output of the current sense amplifier prevents sub-harmonic oscillations for duty cycles greater than 50 percent. This result is fed into the PWM comparator. When this resulting voltage rises to the level of the error amplifier output voltage (VCOMP), the external MOSFET turns off. The output voltage of the internal error amplifier is an amplified signal of the difference between the reference voltage and the feedback voltage. The converter automatically chooses the lowest active LEDX pin voltage to provide a high- enough bus voltage to power all the LED arrays. If the feedback voltage drops below the reference, the output of the error amplifier increases. This result in more current flowing through the MOSFET, thus increasing the power delivered to the output. This forms a closed loop that regulates the output voltage. Under light-load operation—where VOUT ≈ VIN— the converter runs in pulse-skipping mode where the MOSFET turns on for a minimum on-time of approximately 100ns, and then the converter discharges the power to the output for the remaining period. The external MOSFET remains off until the output voltage needs to be boosted again. Dimming Control The MP3397 provides two PWM dimming methods: external PWM signal or DC-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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