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LM25017 数据表(PDF) 9 Page - Texas Instruments |
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LM25017 数据表(HTML) 9 Page - Texas Instruments |
9 / 19 page FB SW L1 COUT RFB2 VOUT RC LM25017 + RFB1 VOUT (low ripple) FB SW L1 COUT RFB2 VOUT RC LM25017 + RFB1 VOUT (low ripple) RFB2 + RFB1 VOUT = 1.225V x RFB1 VOUT gsw = 10 -10 x R ON LM25017 www.ti.com SNVS951 – DECEMBER 2012 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 follows: (1) The output voltage (VOUT) is set by two external resistors (RFB1, RFB2). The regulated output voltage is calculated as follows: (2) (3) 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 25mV of ripple voltage at the feedback pin (FB) is required for the LM25017. In cases where the capacitor ESR is too small, additional series resistance may be required (RC in Figure 13 Low Ripple Output Configuration). 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 Low Ripple Output Configuration. However, RC slightly degrades the load regulation. VCC Regulator The LM25017 contains an internal high voltage linear regulator with a nominal output of 7.6V. The input pin (VIN) can be connected directly to the line voltages up to 48V. 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 (VCC UVLO) threshold of 4.5V, 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. At high input voltages, the power dissipated in the high voltage regulator is significant and can limit the overall achievable output power. As an example, with the input at 48V and switching at high frequency, the VCC regulator may supply up to 7mA of current resulting in 48V x 7mA = 336mW of power dissipation. If the VCC voltage is driven externally by an alternate voltage source, between 8V and 13V, the internal regulator is disabled. This reduces the power dissipation in the IC. Figure 13. Low Ripple Output Configuration Copyright © 2012, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM25017 |
类似零件编号 - LM25017 |
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类似说明 - LM25017 |
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