Q1 SI1470DN N沟道MOSFET用30V+20V(zener二极管D1电压)漏源极电压(VDS)提供了50V的保护，在2.5V门源极电压处有95mΩ的阻抗，来自高热效的SC70-6封装。对一般应用，稳压器的输出电压不足以完全打开挑选的保护MOSFET，所以可以增加带隔离zener 参考的自举电压，LM2734Z的数据手册见参考文献1。
Automotive switching regulators get input-transient-voltage protection
A simple scheme protects switching regulators against 50V voltage transients in automotive applications.
Kevin Daugherty, National Semiconductor, Novi, MI; Edited by Charles H Small and Fran Granville -- EDN, 1/24/2008
Engineers often face difficult trade-offs when voltage regulators can encounter high-voltage transients that are well above normal input-supply operating ranges. This situation is common in automotive applications in which high-voltage transients from an alternator load dump can produce transients of 36 to 75V for durations as long as 400 msec. Designers must choose between a regulator that can withstand such maximum input voltage or use an input-protection scheme. The simple circuit in this Design Idea provides a highly cost-effective method for clamping an input voltage from a battery input with transients as high as 50V to take advantage of a 20V, 3-MHz regulator. With this circuit, your design can achieve a small total footprint with relatively low cost because of the 3-MHz operation along with lower voltage components than might otherwise be necessary to withstand 50V. #p#副标题#e#
Input-protection components consist of Q1, R1, D1, C5, and one-half of D2 (Figure 1). At start-up, N-channel MOSFET Q1’s source is at ground potential and turns on when R1 applies the battery voltage to the gate. Once the input voltage is above the minimum of 2.74V on IC1, the LM2734Z regulator starts switching, which charges the bootstrap circuit comprising D3, D4, and CB. This bootstrap voltage of approximately VOUT–VFD (forward
Under normal operating conditions, for example, the battery voltage is 8 to 18V, D1 does not limit conduction of Q1, and the gate voltage tracks approximately 2.5V above the input-supply voltage for a low voltage drop from the battery voltage to the input voltage of the LM2734Z. However, when the input voltage increases above the threshold that D1 sets, the input voltage to the LM2734Z regulates to the zener voltage (VZ) of D1 minus the threshold voltage of Q1, or approximately 20–2V=18V, well below the 24V absolute maximum of the LM2734Z. Selecting Q1 requires careful consideration of maximum input voltage, gate-to-source-voltage threshold, and power dissipation under both steady-state and thermal-transient conditions.
Q1, the SI1470DN N-channel MOSFET, provides 50V protection with a drain-to-source voltage (VDS) of 30V+20V (zener diode D1 voltage), has an on-resistance of 95 mΩ at a gate-to-source voltage of 2.5V, and comes in a thermally efficient SC70-6 package. For some applications, the regulator’s output voltage may be insufficient to fully turn on the selected protection MOSFET, so you can increase the bootstrap voltage with a separate zener reference, as the LM2734Z’s data sheet shows (Reference 1).
“LM2734Z Thin SOT23 1A Load Step-Down DC-DC Regulator,” National Semiconductor.