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LM5018 Datasheet(PDF) 8 Page - Texas Instruments |
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LM5018 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 25 page = 1.225V RFB1 VOUT - 1.225V RFB2 RFB2 + RFB1 VOUT = 1.225V x RFB1 VOUT Csw = 10 -10 x R ON LM5018 SNVS787E – JANUARY 2012 – REVISED DECEMBER 2013 www.ti.com Functional Description The LM5018 step-down switching regulator features all the functions needed to implement a low cost, efficient, buck converter capable of supplying up to 300 mA to the load. This high voltage regulator contains 100 V, N- channel buck and synchronous switches, is easy to implement, and is provided in thermally enhanced SO PowerPAD-8 and WSON-8 packages. The regulator operation is based on a constant on-time control scheme using an on-time inversely proportional to VIN. This control scheme does not require loop compensation. The current limit is implemented with a forced off-time inversely proportional to VOUT. This scheme ensures short circuit protection while providing minimum foldback. The simplified block diagram of the LM5018 is shown in Figure 12. The LM5018 can be applied in numerous applications to efficiently regulate down higher voltages. This regulator is well suited for 48 V telecom and 42 V automotive power bus ranges. Protection features include: thermal shutdown, undervoltage lockout, minimum forced off-time, and an intelligent current limit. Control Overview The LM5018 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with the output voltage feedback (FB) compared to an internal reference (1.225 V). If the FB voltage is below the reference the internal buck switch is turned on for the one-shot timer period, which is a function of the input voltage and the programming resistor (RON). Following the on-time the switch remains off until the FB voltage falls below the reference, but never before the minimum off-time forced by the minimum off-time one-shot timer. When the FB pin voltage falls below the reference and the minimum off-time one-shot period expires, the buck switch is turned on for another on-time one-shot period. This will continue until regulation is achieved and the FB voltage is approximately equal to 1.225 V (typ). In a synchronous buck converter, the low side (sync) FET is 'on' when the high side (buck) FET is 'off.' The inductor current ramps up when the high side switch is ‘on’ and ramps down when the high side switch is ‘off’. There is no diode emulation feature in this IC, and therefore, the inductor current may ramp in the negative direction at light load. This causes the converter to operate in continuous conduction mode (CCM) regardless of the output loading. The operating frequency remains relatively constant with load and line variations. The operating frequency can be calculated as: The output voltage (VOUT) is set by two external resistors (RFB1, RFB2). The regulated output voltage is calculated as: This regulator regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum amount of ESR for the output capacitor (COUT). A minimum of 25 mV of ripple voltage at the feedback pin (FB) is required for the LM5018. In cases where the capacitor ESR is too small, additional series resistance may be required (RC in Figure 13 ). For applications where lower output voltage ripple is required the output can be taken directly from a low ESR output capacitor, as shown in Figure 13. However, RC slightly degrades the load regulation. VCC Regulator The LM5018 contains an internal high voltage linear regulator with a nominal output of 7.6 V. The input pin (VIN) can be connected directly to the line voltages up to 100 V. The VCC regulator is internally current limited to 30mA. The regulator sources current into the external capacitor at VCC. This regulator supplies current to internal circuit blocks including the synchronous MOSFET driver and the logic circuits. When the voltage on the VCC pin reaches the undervoltage lockout threshold of 4.5 V, the IC is enabled. The VCC regulator contains an internal diode connection to the BST pin to replenish the charge in the gate drive boot capacitor when SW pin is low. 8 Submit Documentation Feedback Copyright © 2012–2013, Texas Instruments Incorporated Product Folder Links: LM5018 |
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