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LM25085QMYE bảng dữ liệu(PDF) 9 Page - Texas Instruments |
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LM25085QMYE bảng dữ liệu(HTML) 9 Page - Texas Instruments |
9 / 30 page Duty Cycle = tON tON + tOFF = = tON x FS VOUT VIN - + Q R + S Negative Bias Regulator Thermal Shutdown ON Time One-Shot Soft-Start Gate Driver Control Logic VCC UVLO Gate Driver REGULATION COMPARATOR OVER-VOLTAGE COMPARATOR CURRENT LIMIT COMPARATOR OFF Time One-Shot FB LM25085 PGATE ISEN GND VCC ADJ 4.5V to 42V Input VIN GND SHUTDOWN RT CVCC CADJ RADJ Q1 D1 L1 1.25V RFB1 RFB2 CBYP RSEN VCC VOUT COUT R3 C1 C2 1.6V + - + - 7.7V 40 PA + - 0.73V + - RT CIN VIN VIN VIN LM25085, LM25085-Q1 www.ti.com SNVS593H – OCTOBER 2008 – REVISED MARCH 2013 Block Diagram Sense resistor method shown for current limit detection. Minimum output ripple configuration shown. FUNCTIONAL DESCRIPTION OVERVIEW The LM25085 is a PFET buck (step-down) DC-DC controller using the constant on-time (COT) control principle. The input operating voltage range of the LM25085 is 4.5V to 42V. The use of a PFET in a buck regulator greatly simplifies the gate drive requirements and allows for 100% duty cycle operation to extend the regulation range when operating at low input voltage. However, PFET transistors typically have higher on-resistance and gate charge when compared to similarly rated NFET transistors. Consideration of available PFETs, input voltage range, gate drive capability of the LM25085, and thermal resistances indicate an upper limit of 10A for the load current for LM25085 applications. Constant on-time control is implemented using an on-time one-shot that is triggered by the feedback signal. During the off-time, when the PFET (Q1) is off, the load current is supplied by the inductor and the output capacitor. As the output voltage falls, the voltage at the feedback comparator input (FB) falls below the regulation threshold. When this occurs Q1 is turned on for the one-shot period which is determined by the input voltage (VIN) and the RT resistor. During the on-time the increasing inductor current increases the voltage at FB above the feedback comparator threshold. For a buck regulator the basic relationship between the on-time, off-time, input voltage and output voltage is: where • Fs is the switching frequency (1) Equation 1 is valid only in continuous conduction mode (inductor current does not reach zero). Since the LM25085 controls the on-time inversely proportional to VIN, the switching frequency remains relatively constant as VIN is varied. If the input voltage falls to a level that is equal to or less than the regulated output voltage Q1 is held on continuously (100% duty cycle) and VOUT is approximately equal to VIN. The COT control scheme, with the feedback signal applied to a comparator rather than an error amplifier, requires no loop compensation, resulting in very fast load transient response. Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM25085 LM25085-Q1 |
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