被指定为12 V车用铅酸电池的正常电压水平可能在9 V和16 V之间变化。但是，在极端低温的情况下，启动器接合时的电池电压水平可低至3.0 V，
这种情况可能持续长达19 mS。
传统的车辆电源架构可以适于通过这种条件保证车辆系统的运行。
那么冷车发生的情况会导致电压如此之低？
电池端电压是其卸载电压和内部电阻两端的电压降的函数。
从电池汲取电流，并且电压在其内部电阻上下降，其随电流增加。
电池内阻不是常数而是温度的函数，并且随着温度下降，负载的电池端电压进一步下降。
因此，我们现在在系统输入电压下降到3.0 V，但当然这不是车辆电源会看到的输入电压;
由于反极性保护和输入滤波器电路的串联电阻，这实际上会更低。
系统电源的输入电压实际上可能低至2.0 V.大多数系统电源下降将正常电池输入电压转换为5.0 V或3.3 V.无论系统电源是线性稳压器还是降压器
开关模式电源（SMPS），其输入端为2.0 V，要么在内部电压轨崩溃之前将其置于电压跌落状态，要么发生欠压锁定（UVLO）。
所以我们的电源在冷启动时不起作用！
在这种情况下，系统设计人员希望将2.0 V输入电压增加或提升到防止系统电源进入掉电或UVLO的水平。
传统的升压拓扑可以做到这一点，只要它能够承受它可能遇到的更高的电压瞬变，例如双电池和负载突降。
图1 - 使用NCV8871的常规Boost拓扑
然而，在设计这个要求时，有许多考虑因素，其中一些考虑因素可能会使传统的助推器低估。
任何增压都具有自然占空比限制，这是由高占空比下的损耗增加引起的，导致输出电压（Vout）在给定的占空比下崩溃。
这通常被称为闩锁，并且对于高占空比，大多数升压设计将具有介于输入电压（Vin）的三到五倍之间的Vout限制，这取决于设计的效率。
在连续导通模式（CCM）中，与非连续导通模式（DCM）相比，该传递函数具有较小的负载依赖性。
因此，对于给定占空比的大多数设计，DCM升压设计将提供更高的Vin至Vout传递函数，从而允许更低的输入电压。
如前所述，输入滤波器和反极性保护二极管的任何串联损耗都会降低升压级的输入电压。
输入滤波器本身为升压输入提供一定的输出阻抗。
如果此输出阻抗大于升压级的输出阻抗，则可能由于负输入阻抗而发生振荡，从而使输入电感和电容选择变得至关重要。
当输入电压发生变化时，还需要一定的最小输入电容值来转换输入电压。
由于需要在非常低的输入电压下工作，因此增压器的带宽通常小于10 KHz。
这种有限的带宽导致环路延迟，通常不足以快速跟踪输入电压的变化。
输入电容可以通过转换输入电压的变化率来弥补这一不足，从而使循环时间赶上来。
对于需要低静态电流（Iq）的永久供电系统，需要在输入电压低于某个水平时激活升压。
这通常可能需要施密特触发器来监控电阻分压器上的输入电压。
汽车设计通常会对最大电阻值施加限制，迫使在分压器中使用相对较低值的电阻，并因此导致显着的漏电流。
反馈电压电阻分压器，比较器和升压集成电路（IC）都有助于Iq。
安森美半导体NCV8876和NCV8877非同步升压控制器提供“智能”低频升压控制器，专门用于低输入电压汽车系统。
图2 - NCV8876框图
通过专用引脚监控输入电压和为IC供电，监控功能采用集成电阻分压器，在睡眠模式下断开，以实现极低的Iq。
这将输入电压与三个内部编程阈值进行比较，如果输入电压下降超过这些阈值，则首先唤醒IC（唤醒阈值），然后再调节输出电压（调节）。
当输入电压上升时，第三个阈值触发IC进入低Iq睡眠模式（睡眠阈值）。
传统的升压控制器IC具有软启动（其中Vout通常在启动时呈指数上升），在电池电压冷启动电压下降期间禁止快速响应。
NCV8876和NCV8877没有软启动延迟，允许快速响应以实现输出调节，补偿放大器在唤醒时为补偿电容预充电（STATUS为低电平）。
图3 - NCV8876功能波形
以上来自于谷歌翻译
以下为原文
The normal voltage level of what is designated as a 12 V vehicle lead acid battery may vary between 9 V and 16 V. However, subject to extreme cold temperatures, the battery voltage level when the starter is engaged can be as low as 3.0 V, a condition that might last for as long as 19 mS. The conventional vehicle power supply architecture can be adapted to guarantee vehicle system operation through such conditions.
So what happens at cold-crank that causes the voltage to be so low? The battery terminal voltage is a function of its off-load voltage and the voltage drop across its internal resistance. Current is drawn from the battery, and a voltage is dropped across its internal resistance, which increases with current. The battery internal resistance is not a constant but a function of temperature, and as the temperature falls, the loaded battery terminal voltage falls further. So, we now have down to 3.0 V at the system input, but of course this is not the input voltage that the vehicle power supply will see; this will in fact be lower due to the series resistance of the reverse polarity protection and input filter circuits. The input voltage to the system power supply could in fact be as low as 2.0 V. Most system power supplies down convert normal battery input voltage to 5.0 V or 3.3 V. Irrespective of whether that system power supply is a linear voltage regulator or a buck switched mode power supply (SMPS), 2.0 V on its input will either put it into drop-out or incur under voltage lock-out (UVLO) before the internal voltage rail collapses. So we have a power supply that doesn’t function at cold-crank!
Subject to such a condition the system designer looks to increase or boost that 2.0 V input voltage to a level that prevents the system power supply from entering drop-out or UVLO. A conventional boost topology could do this so long as it can sustain the higher voltage transients it may encounter such as double-battery and load dump.
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Fig. 1 – Conventional Boost Topology using NCV8871
However, in designing to this requirement there are numerous considerations, some of which may discount a conventional boost. Any boost has a natural duty cycle limitation caused by increasing losses at high duty cycles, causing the output voltage (Vout) to collapse at a given duty-cycle. This is commonly referred to as latch-up and most boost designs will have a Vout limitation of somewhere between three and five times the input voltage (Vin), depending upon the efficiency of the design, for high duty cycles. In continuous conduction mode (CCM) this transfer function has less of a load dependency than in discontinuous conduction mode (DCM). As a result, for most designs at a given duty cycle, the DCM boost design will give a higher Vin to Vout transfer function, allowing lower input voltages to be reached.
As previously mentioned, any series losses of the input filter and reverse polarity protection diode reduce the input voltage level to the boost stage. The input filter itself presents a certain output impedance to the input of the boost. If this output impedance is greater than that of the boost stage then oscillations can occur due to negative input impedance, making the input inductor and capacitor choice critical.
A certain minimum input capacitance value is also required to slew the input voltage as it changes. Due to the need to operate at very low input voltages the boost’s bandwidth is limited to often less than 10 KHz. This limited bandwidth results in loop latency that often is insufficiently fast to track changes to input voltage. The input capacitance can make up for this deficiency by slewing the rate of change in the input voltage, giving the loop time to catch up.
For those permanently powered systems requiring a low quiescent current (Iq), the boost needs to be activated when the input voltage falls below a certain level. This typically might entail a schmitt trigger monitoring the input voltage across a resistor divider. Automotive designs often place limitations on the maximum resistor value, compelling the use of relatively low value resistors in the divider, and a significant leakage current as a result. The feedback voltage resistor divider, comparator and the boost integrated circuit (IC) all contribute to the Iq.
The ON Semiconductor NCV8876 and NCV8877 non-synchronous boost controllers present an ‘intelligent’ low frequency boost controller intended specifically to function at such low input voltage automotive systems.
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Fig. 2 - NCV8876 Block Diagram
With a dedicated pin for both monitoring input voltage and supplying the IC, the monitoring function utilises an integrated resistor divider, which is disconnected in sleep mode to achieve a very low Iq. This compares the input voltage to three internally programmed thresholds, which in the event of a falling input voltage crossing these thresholds firstly to wake the IC (Wakeup Threshold) and then secondly regulate the output voltage (Regulation). The third threshold triggers the IC into its low Iq sleep mode (Sleep Threshold) as the input voltage rises. Conventional boost controller ICs have a soft-start (where Vout typically rises exponentially at start-up), prohibiting a rapid response during battery voltage cold crank voltage sag. The NCV8876 and NCV8877 don’t have a soft-start delay allowing a rapid response to achieve output regulation, the compensation amplifier pre-charging the compensation capacitor at wakeup (STATUS low).
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Fig. 3 - NCV8876 Functional Waveforms
Cranky on a cold day? Cold-cranking repetitively all day? The NCV8876 and NCV8877 hold the keys.
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