Datenblatt-Suchmaschine für elektronische Bauteile |
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LM34927 Datenblatt(PDF) 10 Page - Texas Instruments |
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LM34927 Datenblatt(HTML) 10 Page - Texas Instruments |
10 / 28 page FB SW L1 COUT RFB2 VOUT RC LM34927 + RFB1 VOUT (low ripple) RFB2 + RFB1 VOUT = 1.225V x RFB1 VOUT1 f SW = .x RON LM34927 SNVS799G – APRIL 2012 – REVISED DECEMBER 2014 www.ti.com 7.3 Feature Description 7.3.1 Control Overview The LM34927 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 switched on for the one-shot timer period, which is a function of the input voltage and the programming resistor (RT). Following the on-time the switch remains off until the FB voltage falls below the reference, and the forced minimum off-time has expired. When the FB pin voltage falls below the reference and the off-time one-shot period expires, the buck switch is then turned on for another on-time one- shot period. This continues 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 shown in Equation 1. (1) Where K = 9 × 10–11 The output voltage (VOUT) is set by two external resistors (RFB1, RFB2). The regulated output voltage is calculated as shown in Equation 2. (2) 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 LM34927 device. In cases where the capacitor ESR is too small, additional series resistance may be required (RC in Figure 16). 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 16. However, RC slightly degrades the load regulation. Figure 16. Low Ripple Output Configuration 7.3.2 VCC Regulator The LM34927 device 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 30 mA. 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 UVLO 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. 10 Submit Documentation Feedback Copyright © 2012–2014, Texas Instruments Incorporated Product Folder Links: LM34927 |
Ähnliche Teilenummer - LM34927_15 |
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Ähnliche Beschreibung - LM34927_15 |
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