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TPS51275CRUKT Datasheet(PDF) 10 Page - Texas Instruments |
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TPS51275CRUKT Datasheet(HTML) 10 Page - Texas Instruments |
10 / 31 page ( ) ( ) - ´ = ´ ´ ´ IN OUT OUT OUT LL SW IN V V V 1 I 2 L f V TPS51275, TPS51275B, TPS51275C SLUSB45B – JUNE 2012 – REVISED MARCH 2013 www.ti.com DETAILED DESCRIPTION PWM Operations The main control loop of the switch 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 does not require external conpensation circuit and is suitable for low external component count configuration when used with appropriate amount of ESR at the output capacitor(s). At the beginning of each cycle, the synchronous high-side MOSFET is turned on, or enters the ON state. This MOSFET is turned off, or enters the ‘OFF state, after the internal, one-shot timer expires. The MOSFET is turned on again when the feedback point voltage, VVFB, decreased to match the internal 2-V reference. The inductor current information is also monitored and should be below the overcurrent threshold to initiate this new cycle. By repeating the operation in this manner, the controller regulates the output voltage. The synchronous low-side (rectifying) MOSFET is turned on at the beginning of each OFF state to maintain a minimum of conduction loss. The low-side MOSFET is turned off before the high-side MOSFET turns on at next switching cycle or when inductor current information detects zero level. This enables seamless transition to the reduced frequency operation during light-load conditions so that high efficiency is maintained over a broad range of load current. Adaptive On-Time/ PWM Frequency Control Because the TPS51275/B/C does not have a dedicated oscillator for control loop on board, switching cycle is controlled by the adaptive on-time circuit. The on-time is controlled to meet the target switching frequency by feed-forwarding the input and output voltage into the on-time one-shot timer. The target switching frequency is varied according to the input voltage to achieve higher duty operation for lower input voltage application. The switching frequency of CH1 (5-V output) is 300 kHz during continuous conduction mode (CCM) operation when VIN = 20 V. The CH2 (3.3-V output) is 355 kHz during CCM when VIN = 20 V. (See Figure 27 and Figure 28). To improve load transient performance and load regulation in lower input voltage conditions, TPS51275/B/C can extend the on-time. The maximum on-time extension of CH1 is 4 times for CH2 is 3 times. To maintain a reasonable inductor ripple current during on-time extension, the inductor ripple current should be set to less than half of the OCL (valley) threshold. (See Step 2. Choose the Inductor). The on-time extension function provides high duty cycle operation and shows better DC (static) performance. AC performance is determined mostly by the output LC filter and resistive factor in the loop. Light Load Condition in Auto-Skip Operation (TPS51275/C) The TPS51275/C automatically reduces switching frequency during light-load conditions to maintain high efficiency. This reduction of frequency is achieved smoothly and without an increase in output voltage ripple. A more detailed description of this operation is as follows. As the output current decreases from heavy-load condition, the inductor current is also reduced and eventually approaches valley zero current, which is the boundary between continuous conduction mode and discontinuous conduction mode. The rectifying MOSFET is turned off when this zero inductor current is detected. As the load current further decreases, the converter runs in discontinuous conduction mode and it takes longer and longer to discharge the output capacitor to the level that requires the next ON cycle. The ON time is maintained the same as that in the heavy-load condition. In reverse, when the output current increase from light load to heavy load, the switching frequency increases to the preset value as the inductor current reaches to the continuous conduction. The transition load point to the light load operation IOUT(LL) (i.e. the threshold between continuous and discontinuous conduction mode) can be calculated as shown in Equation 1. where • fSW is the PWM switching frequency (1) Switching frequency versus output current during light-load conditions is a function of inductance (L), input voltage (VIN) and output voltage (VOUT), but it decreases almost proportional to the output current from the IOUT(LL). 10 Submit Documentation Feedback Copyright © 2012–2013, Texas Instruments Incorporated |
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