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LM3405AXMKX Datasheet(PDF) 10 Page - Texas Instruments |
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LM3405AXMKX Datasheet(HTML) 10 Page - Texas Instruments |
10 / 32 page LM3405A SNVS508C – OCTOBER 2007 – REVISED MAY 2013 www.ti.com The third method can be used in the applications where both VIN and VOUT are greater than 5.5V. In these cases, C3 cannot be charged directly from these voltages; instead C3 can be charged from VIN or VOUT minus a zener voltage (VD3) by placing a zener diode D3 in series with D2 as shown in Figure 16. When using a series zener diode from the input, the gate drive voltage is VIN - VD3 - VD2 + VD1. Figure 16. VBOOST Derived from VIN through a Series Zener An alternate method is to place the zener diode D3 in a shunt configuration as shown in Figure 17. A small 350mW to 500mW, 5.1V zener in a SOT or SOD package can be used for this purpose. A small ceramic capacitor such as a 6.3V, 0.1µF capacitor (C5) should be placed in parallel with the zener diode. When the internal NMOS switch turns on, a pulse of current is drawn to charge the internal NMOS gate capacitance. The 0.1µF parallel shunt capacitor ensures that the VBOOST voltage is maintained during this time. Resistor R2 should be chosen to provide enough RMS current to the zener diode and to the BOOST pin. A recommended choice for the zener current (IZENER) is 1mA. The current IBOOST into the BOOST pin supplies the gate current of the NMOS power switch. It reaches a maximum of around 3.6mA at the highest gate drive voltage of 5.5V over the LM3405A operating range. For the worst case IBOOST, increase the current by 50%. In that case, the maximum boost current will be: IBOOST-MAX = 1.5 x 3.6mA = 5.4mA (6) R2 will then be given by: R2 = (VIN - VZENER) / (IBOOST_MAX + IZENER) (7) For example, let VIN = 12V, VZENER = 5V, IZENER = 1mA, then: R2 = (12V - 5V) / (5.4mA + 1mA) = 1.09k Ω (8) Figure 17. VBOOST derived from VIN through a Shunt Zener The fourth method can be used in an application which has an external low voltage rail, VEXT. C3 can be charged through D2 from VEXT, independent of VIN and VOUT voltage levels. Again for best performance, ensure that the gate drive voltage, VEXT - VD2 + VD1, falls in the range of 2.5V to 5.5V. 10 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated Product Folder Links: LM3405A |
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