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TPS56428 Datasheet(PDF) 9 Page - Texas Instruments |
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TPS56428 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 28 page ( ) IN OUT OUT OUT(LL) SW IN V V V 1 I = 2 L V - ´ ´ ´ ´ f TPS56428 www.ti.com SLVSBV4B – APRIL 2013 – REVISED OCTOBER 2015 Feature Description (continued) At the beginning of each cycle, the high-side MOSFET is turned on. This MOSFET is turned off after internal one shot timer expires. This one shot is set by the converter input voltage, VIN, and the output voltage, VO, to maintain a pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control. The one-shot timer is reset and the high-side MOSFET is turned on again when the feedback voltage falls below the reference voltage. An internal ramp is added to reference voltage to simulate output ripple, eliminating the need for ESR induced output ripple from D-CAP2™ mode control. 8.3.2 PWM Frequency and Adaptive On-Time Control TPS56428 uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The TPS56428 runs with a pseudo-constant frequency of 650 kHz by using the input voltage and output voltage to set the on-time one-shot timer. The on-time is inversely proportional to the input voltage and proportional to the output voltage, therefore, when the duty ratio is VOUT/VIN, the frequency is constant. 8.3.3 Advanced Auto-Skip Eco-mode™ Control The TPS56428 is designed with advanced auto-skip Eco-mode™ to increase higher light load efficiency. As the output current decreases from heavy load condition, the inductor current is also reduced. If the output current is reduced enough, the inductor current ripple valley reaches the zero level, which is the boundary between continuous conduction and discontinuous conduction modes. The rectifying low-side MOSFET is turned off when its zero inductor current is detected. As the load current further decreases the converter run into discontinuous conduction mode. The on-time is kept approximately the same as is in continuous conduction mode. The off-time increases as it takes more time to discharge the output capacitor to the level of the reference voltage with smaller load current. The transition point to the light load operation IOUT(LL) current can be calculated in Equation 1. (1) 8.3.4 Soft Start and Pre-Biased Soft Start The TPS56428 has an internal 1.0ms soft-start. When the EN pin becomes high, internal soft-start function begins ramping up the reference voltage to the PWM comparator. The TPS56428 contains a unique circuit to prevent current from being pulled from the output during startup if the output is pre-biased. When the soft-start commands a voltage higher than the pre-bias level (internal soft start becomes greater than feedback voltage VFB), the controller slowly activates synchronous rectification by starting the first low side FET gate driver pulses with a narrow on-time. It then increments that on-time on a cycle-by- cycle basis until it coincides with the time dictated by (1-D), where D is the duty cycle of the converter. This scheme prevents the initial sinking of the pre-bias output, and ensure that the out voltage (VO) starts and ramps up smoothly into regulation and the control loop is given time to transition from pre-biased start-up to normal mode operation. 8.4 Device Functional Modes 8.4.1 Power Good The power-good function is activated after soft start has finished. The power good function becomes active after 1.7 times soft-start time. When the output voltage becomes within –10% of the target value, internal comparators detect power good state and the power good signal becomes high. The power good output, PG is an open drain output. If the feedback voltage goes under 15% of the target value, the power good signal becomes low. 8.4.2 Output Discharge Control TPS56428 discharges the output via SW pin when EN is low, or the controller is turned off by the protection functions(UVP, UVLO and thermal shutdown). The internal regular low-side MOSFET is not turned on during the output discharge operation to avoid the possibility of causing negative voltage at the output Copyright © 2013–2015, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: TPS56428 |
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