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LM34910 Folha de dados(PDF) 7 Page - Texas Instruments |
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LM34910 Folha de dados(HTML) 7 Page - Texas Instruments |
7 / 17 page FB SW L1 C2 R1 R2 R3 LM34910 BST VCC D2 C3 C4 D1 ISEN SGND VOUT1 VOUT2 FB SW L1 C2 R1 R2 VOUT2 R3 LM34910 LM34910 www.ti.com SNVS297A – OCTOBER 2004 – REVISED FEBRUARY 2005 The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as follows: VOUT = 2.5 x (R1 + R2) / R2 (4) Output voltage regulation is based on ripple voltage at the feedback input, requiring a minimum amount of ESR for the output capacitor C2. The LM34910 requires a minimum of 25 mV of ripple voltage at the FB pin. In cases where the capacitor’s ESR is insufficient additional series resistance may be required (R3 in Typical Application Circuit and Block Diagram). 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 4. However, R3 slightly degrades the load regulation. Figure 4. Low Ripple Output Configuration Start-up Regulator, VCC The start-up regulator is integral to the LM34910. The input pin (VIN) can be connected directly to line voltage up to 36V, with transient capability to 40V. The VCC output regulates at 7.0V, and is current limited to 9 mA. Upon power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage on the VCC pin reaches the under-voltage lockout threshold of 5.8V, the buck switch is enabled and the Softstart pin is released to allow the Softstart capacitor (C6) to charge up. The minimum input voltage is determined by the regulator’s dropout voltage, the VCC UVLO falling threshold ( ≊5.7V), and the frequency. When VCC falls below the falling threshold the VCC UVLO activates to shut off the output. If VCC is externally loaded, the minimum input voltage increases since the output impedance at VCC is ≊140Ω. See Figure 2. To reduce power dissipation in the start-up regulator, an auxiliary voltage can be diode connected to the VCC pin. Setting the auxiliary voltage to between 8V and 14V shuts off the internal regulator, reducing internal power dissipation. The sum of the auxiliary voltage and the input voltage (VCC + VIN) cannot exceed 50V. Internally, a diode connects VCC to VIN. See Figure 5. Figure 5. Self Biased Configuration Copyright © 2004–2005, Texas Instruments Incorporated Submit Documentation Feedback 7 Product Folder Links: LM34910 |
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