100V Non-Isolated Step-Down Converter Simplifies Power Buses in Harsh Environments

　　Harsh and difficult environments exist in industrial, automotive, aerospace, and datacom systems, requiring robust electronic systems. In datacom systems, input voltages may vary from 47V to 53V, and transient voltages may reach 80V. In automotive systems, the DC battery voltage may be 12V, 24V, or 42V, and load dump conditions may cause transient voltages as high as 100V. In aerospace and industrial systems with a nominal value of 28V, transient voltages may reach 55V. In some of these applications, a fully isolated design is required, that is, a power transformer and some method of sensing the feedback voltage, or providing voltage feedback through an optocoupler or a transformer for feedback voltage sensing. However, this high voltage is often a surge that lasts only a few milliseconds, so the DC/DC converter can be non-isolated. Obviously, then, there is a need for a non-isolated DC/DC converter that can operate normally without damage and without shutting down, failing, or latching up at this high peak voltage.

　　There is also a need for non-isolated intermediate bus converters that convert 48V backplane voltage to 12V or lower. Datacom systems typically have a 47-53V distributed bus supply that is isolated from the AC power supply. Most intermediate bus converters are also isolated, so when used in datacom applications, double isolation is required, which adds complexity and can reduce the overall efficiency of the system while potentially increasing costs.

　　Engineers are often challenged to design products that operate from high input voltages in the 48V range while producing outputs as low as 1.8V. High input voltage rails above 12V often require intermediate regulation stages, which reduces overall efficiency and increases overall cost. However, synchronous buck controllers with a minimum on-time of only 100ns can be used in high step-down ratio applications to generate low voltages directly from high input voltage rails.

　　Fortunately, there are current mode synchronous DC/DC switching regulator controllers that can directly step down input voltages up to 100 V, such as Linear Technology 's LTC3810. It is ideal for harsh input voltage environments and non-isolated intermediate bus converters, and its ability to directly step down high input voltages allows the use of simple single-inductor topologies, resulting in compact, high-performance power supplies.

　　The LTC3810 uses a synchronizable constant on-time, valley current mode control architecture to drive two external N-channel MOSFETs. A wide bandwidth error amplifier enables fast line and load transient response. A powerful 1Ω gate driver minimizes switching losses, which are often the dominant loss component in high voltage supplies, even when multiple MOSFETs are used in high current applications.

　　Figure 1 shows the schematic and efficiency curve of a non-isolated DC/DC converter based on the LTC3810. The converter has an input voltage of 15 to 100V and can generate 12V and a current of up to 6A.

Figure 1 LTC3810 schematic and efficiency curve

　　Bias voltage control

　　The LTC3810 has an internal linear regulator controller that generates a 10V bias supply voltage from the input voltage using a single external MOSFET, referenced as M3 in Figure 1. This allows the user to use the controller without a separate bias voltage and reduces internal heating by moving the LDO pass elements outside the controller. For continuous operation, the minimum power rating of M3 can be calculated using the following formula:

Power dissipation in M3 = (VIN-10V) × IBIAS

　　Where, the bias current (IBIAS) is approximately 20mA, which depends mainly on the gate drive power required to drive M1 and M2; 10V is the output voltage that powers the LTC3810; VIN is the average input voltage, and the peak voltage is averaged into the total steady-state voltage.

　　If the output voltage is above 6.7 V, but below 15 V, then the internal LDO fed from the output voltage via the EXTVcc pin can be used to power the LTC3810 for maximum efficiency and will disconnect M3 after the output voltage reaches 6.7 V. Since M3 only dissipates power during the brief startup period, careful size selection makes it possible to use a fairly small MOSFET (e.g., SOT-23).

　　Main control loop

　　The LTC3810 is a valley current mode controller. In normal operation, the top MOSFET is turned on at fixed intervals, which are determined by a one-shot timer. When the top MOSFET is turned off, the bottom MOSFET is turned on until the current comparator trips, then the one-shot timer is restarted and the next cycle is started. The inductor current is determined by sensing the voltage between the SENSE pins with a sense resistor or the on-resistance of the bottom MOSFET. The voltage on the ITH pin sets the comparator threshold corresponding to the inductor valley current. The 25MHz fast error amplifier regulates the output voltage by comparing the feedback signal with the internal 0.8V reference voltage. If the load current increases, there is a voltage drop relative to the reference feedback voltage, and the ITH voltage then rises until the average inductor current matches the load current again.

　　In a typical LTC3810 circuit (see Figure 1), the feedback loop consists of a modulator, an output filter, and a feedback amplifier with a compensation network. All of these components affect the behavior of the loop and must be considered in the loop compensation analysis. By moving the inductor inside the loop, current mode control eliminates the effect of the inductor, thus simplifying the loop analysis to a first-order system.

　　The LTC3810 is a current mode control device, so the task of designing the feedback loop is much easier and a Type 2 error amplifier design can be used.

　　Current Sensing

　　The LTC3810 (with programmable current sense threshold) can implement short-circuit and overload protection with or without a current sense resistor. Using a sense resistor in series with the bottom-most MOSFET provides accurate current limit, but increases cost and reduces efficiency. The bottom-most MOSFET can also be used as a current sense element, eliminating the sense resistor.

　　track

　　Typical tracking maintains the feedback voltage to the lower end of the internal reference voltage value or TRACK pin voltage. The LTC3810 goes a step further and combines tracking and soft-start functions on a single pin (TK/SS pin), and determines the switching operation mode according to the state of this TK/SS pin. Figure 2 shows coincident and ratiometric tracking, both of which can be configured with the LTC3810.

Figure 2: Two different output voltage tracking modes using the LTC3810

　　Powerful gate driver

　　The LTC3810 contains a very low impedance gate driver that can deliver amp-level currents to quickly switch large MOSFET gates. This minimizes switching losses and allows paralleling of MOSFETs for applications with higher output currents. A floating high-side driver (up to 100V) drives the top MOSFET, while a low-side driver drives the bottom MOSFET (see Figure 3).

Figure 3 LTC3810 gate drive circuit