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TPS53124 Datasheet(PDF) 9 Page - Texas Instruments |
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TPS53124 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 22 page DETAILED DESCRIPTION PWM Operation Low-Side Driver High-Side Driver PWM Frequency and Adaptive On-Time Control Soft Start Output Discharge Control TPS53124 www.ti.com ..................................................................................................................................................... SLUS825B – FEBRUARY 2008 – REVISED MAY 2008 The main control loop of the switching mode power supply (SMPS) is designed as an adaptive on-time pulse width modulation (PWM) controller. It supports a proprietary D-CAP™ Mode. D-CAP™ Mode uses internal compensation circuit and is suitable for low external component count configuration with appropriate amount of ESR at the output capacitor(s). The output ripple bottom voltage is monitored at a feedback point voltage. At the beginning of each cycle, the synchronous high-side MOSFET is turned on, or becomes ON state. This MOSFET is turned off, or becomes OFF state, after internal one-shot timer expires. This one shot is determined by the converter’s input voltage ,VIN, and the output voltage ,VOUT, to keep frequency fairly constant over the input voltage range, hence it is called adaptive on-time control. The high-side MOSFET is turned on again when feedback information indicates insufficient output voltage. Repeating operation in this manner, the controller regulates the output voltage. The low-side driver is designed to drive high current low RDS(on) N-channel MOSFET(s). The drive capability is represented by its internal resistance. A dead time to prevent shoot through is internally generated between high-side MOSFET off to low-side MOSFET on, and low-side MOSFET off to high-side MOSFET on. 5-V bias voltage is delivered from internal regulator VREG5 output. The instantaneous drive current is supplied by an input capacitor connected between VREG5 and GND. The average drive current is equal to the gate charge at VGS = 5 V times switching frequency. This gate drive current as well as the high-side gate drive current times 5 V makes the driving power which need to be dissipated from TPS53124 package. The high-side driver is designed to drive high current, low RDS(on) N-channel MOSFET(s). When configured as a floating driver, 5-V bias voltage is delivered from VREG5 supply. The average drive current is also calculated by the gate charge at VGS = 5 V times switching frequency. The instantaneous drive current is supplied by the flying capacitor between VBSTx and LLx pins. The drive capability is represented by its internal resistance. TPS53124 employs adaptive on-time control scheme and does not have a dedicated oscillator on board. However, the part runs with pseudo-constant frequency by feed-forwarding the input and output voltage into the on-time one-shot timer. The on-time is controlled inverse proportional to the input voltage and proportional to the output voltage so that the duty ratio will be kept as VOUT/VIN technically with the same cycle time. The TPS53124 has an internal, 1.2 ms, voltage servo softstart for each channel. When the ENx pin becomes high, an internal DAC begins ramping up the reference voltage to the PWM comparator. Smooth control of the output voltage is maintained during start up. As TPS53124 shares one DAC with both channels, if ENx pin is set to high while another channel is starting up, soft start is postponed until another channel soft start has completed. If both of EN1 and EN2 are set high at a same time, both channels start up at same time. TPS53124 discharges the output when ENx is low, or the controller is turned off by the protection functions (OVP, UVP, UVLO, and thermal shutdown). TPS53124 discharges outputs using an internal 40- Ω MOSFET which is connected to VOx and PGNDx. The external low-side MOSFET is not turned on for the output discharge operation to avoid the possibility of causing negative voltage at the output. This discharge ensures that, on start, the regulated voltage always start from zero volts. Copyright © 2008, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Link(s): TPS53124 |
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