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TPS54325PWP Fiches technique(PDF) 6 Page - Texas Instruments |
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TPS54325PWP Fiches technique(HTML) 6 Page - Texas Instruments |
6 / 20 page OVERVIEW DETAILED DESCRIPTION PWM Operation PWM Frequency and Adaptive On-Time Control Soft Start and Pre-Biased Soft Start Tss(ms)= C6(nF) Vref • Iss( A) µ − = C6(nF) • 0.765 2 − (1) Power Good Output Discharge Control TPS54325 SLVS932A – MAY 2009 – REVISED JULY 2009 ................................................................................................................................................................ www.ti.com The TPS54325 is a 3-A synchronous step-down (buck) converter with two integrated N-channel MOSFETs. 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. The main control loop of the TPS54325 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 timer 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 the reference voltage to simulate output ripple, eliminating the need for ESR induced output ripple from D-CAP2™ mode control. TPS54325 uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The TPS54325 runs with a pseudo-constant frequency of 700 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. The TPS54325 has an adjustable soft start . When the EN pin becomes high, 2.0- 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 2 µA. The TPS54325 contains a unique circuit to prevent current from being pulled from the output during startup in the condition 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. The TPS54325 has power-good output. The power-good function is activated after soft start has finished. If the output voltage becomes within -10% of the target value, internal comparators detect power good state and the power good signal becomes high. During start up, power good start after 1.7 times soft-start time to avoid a glitch of power-good signal. If the feedback voltage goes under 15% of the target value, the power good signal becomes low after 10 µs internal delay. The TPS54325 discharges the output when EN is low, or the controller is turned off by the protection functions (OVP, UVP, UVLO and thermal shutdown). The device discharges outputs using an internal 50- Ω MOSFET which is connected to VO and PGND. The internal low-side MOSFET is not turned on during the output discharge operation to avoid the possibility of causing negative voltage at the output. 6 Submit Documentation Feedback Copyright © 2009, Texas Instruments Incorporated Product Folder Link(s): TPS54325 |
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