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T6309B Datasheet(PDF) 5 Page - Taiwan Memory Technology |
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T6309B Datasheet(HTML) 5 Page - Taiwan Memory Technology |
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5 / 11 page  TE CH tm T6309B TM Technology Inc. reserves the right P. 5 Publication Date: SEP. 2005 to change products or specifications without notice. Revision: A Functional Description The T6309B is like a standard current-limit PFM step-up DC-DC converter, but regulates the voltage across the sense resistor Rs instead of the output voltage. This results in a constant LED current regardless the input voltage and number of LEDs connected. With integrated overvoltage protection (OVP), the T6309B can be used as a current source with overvoltage protection to drive LEDs. This allows up to 3 LEDS to be connected in series to the output. The low 400mV feedback reference voltage reduces power loss and improves efficiency. Step-Up Converter The step-up DC-DC converter operation can be understood by referring to the block diagram in Figure 1. PFM comparator monitors the output voltage via the sense resistor. When the feedback voltage is higher than the reference voltage, the MOSFET switch is turned off. As the feedback voltage is lower than reference voltage and the MOSFET switch has been off for at least a period of minimum off-time decided by the minimum off-time one-shot, the MOSFET switch is then turned on one-shot, or until the current limit signal is asserted. During the internal MOSFET switch is turned on, current ramps up in the inductor and store energy in a magnetic field. When the MOSFET is turned off, the voltage across the inductor reverses and forces current through the diode to the output filter capacitor and load, so the energy in the inductor is transferred to output filter capacitor and the load. As the energy stored in the inductor is depleted, the current ramps down and the output diode turns off. The output filter capacitor stores the charge while the inductor current is higher than the output current, then sustains the output voltage until the next switching cycle. Current Limit The T6309B utilizes cycle-by-cycle current limiting by means of protecting the output MOSFET switch from overstress and preventing the small value inductor from saturation. Current limiting is implemented by monitoring the output MOSFET current build-up during conduction, and upon sensing an over-current conduction immediately turning off the switch for the duration of the oscillator cycle. The current through the output MOSFET is monitored and compared against a reference signal. When the threshold is reached, a signal is sent to the PFM controller block to terminate the power switch conduction. The current limit threshold is typically set at 250mA. Overvoltage Protection Since T6309B is configured as current source, the output voltage rises as the output impedance increases or output is open-circuit (e.g. fault LED). The output voltage may exceed the 14V maximum voltage rating of the internal main switch. An overvoltage protection circuit is integrated to prevent the |
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