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TPS54625PWPR Datasheet(PDF) 7 Page - Texas Instruments |
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TPS54625PWPR Datasheet(HTML) 7 Page - Texas Instruments |
7 / 22 page (nF) x V (nF) x 0.765 1.1 REF SS SS t (ms) = = SS I ( A) SS C 1.1 C 6 ´ m ´ TPS54625 www.ti.com SLVSC33 – AUGUST 2013 OVERVIEW The TPS54625 is a 6.5A synchronous step-down (buck) converter with two integrated N-channel MOSFETs with Auto-Skip mode to improve light load efficiency. It operates using D-CAP2™ mode control. The fast transient response of D-CAP2™ control reduces the output capacitance required to meet a specific level of performance. Proprietary internal circuitry allows the use of low ESR output capacitors including ceramic and special polymer types. DETAILED DESCRIPTION PWM Operation The main control loop of the TPS54625 is an adaptive on-time pulse width modulation (PWM) controller that supports a proprietary D-CAP2™ mode control. D-CAP2™ mode control combines constant on-time control with an internal compensation circuit for pseudo-fixed frequency and low external component count configuration with both low ESR and ceramic output capacitors. It is stable even with virtually no ripple at the output. 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. PWM Frequency and Adaptive On-Time Control TPS54625 uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The TPS54625 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. The actual frequency may vary from 650kHz depending on the off time, which is ended when the feed back portion of the output voltage falls to the VFB threshold voltage. Soft Start and Pre-Biased Soft Start The soft start function is adjustable. When the EN pin becomes high, 6- μA current begins charging the capacitor which is connected from the SS pin to GND. Smooth control of the output voltage is maintained during start up. The equation for the slow start time is shown in Equation 1. VFB voltage is 0.765 V and SS pin source current is 6 μA. (1) TPS54625 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 . Power Good TPS54625 has power-good open drain output. 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. Rpg resistor value, which is connected between PG and VREG5, is required from 25 k Ω to 150 kΩ. If the feedback voltage goes under 15% of the target value, the power good signal becomes low. Copyright © 2013, Texas Instruments Incorporated Submit Documentation Feedback 7 Product Folder Links: TPS54625 |
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