■ 4-switch buck-boost controller with DCR current detection technique

A new generation of 4-switch buck-boost controllers utilizing inductor DC resistance (DCR) current sensing techniques enables power system designs that achieve very high efficiency and power density. This article explores the advantages of the inductor DCR current sensing solution over existing solutions.

■ 4-switch buck-boost converter with overcurrent and overvoltage protection

The 4-switch buck-boost converter is a popular and well-known method when you want to generate an output voltage that can be higher, lower, or equal to the input voltage. This converter disconnects the input/output (I/O) under extreme fault conditions such as input short-circuit or output short-circuit. 4-switch buck-boost converters are widely used in battery-powered devices, automotive systems, and general industrial applications with overcurrent and overvoltage protection.

■ New DCR inductor current detection technology

While previous 4-switch buck-boost controllers used an external current sense resistor for current sensing, the LTC7878 is the first 4-switch buck-boost controller designed to use inductor DCR for inductor current sensing.

The device uses a novel peak current mode control scheme that provides cycle-by-cycle peak current limiting regardless of whether the regulator is in buck, boost, or buck-boost operation.

It can regulate the output from 1 V to 70 V with 1% accuracy over a wide input voltage range of 5 V to 70 V.

This new buck-boost converter eliminates the need for a current-sense resistor, eliminating power loss and reducing solution size.

System costs can also be reduced because expensive high-power current sensing resistors are not required.

In addition, inductor DCR current detection provides continuous inductor current information, enabling unified peak current mode control and easy parallel operation in multiphase multi-IC configurations.

■ 4-switch buck-boost converter with inductor current detection


Many 4-switch buck-boost controllers require two or more current sense resistors to sense the I/O current and the inductor current for closed-loop operation.

Analog Devices offers a unique family of buck-boost controllers that require only a single current sense resistor to sense the current used in the current-mode control loop.

Figure 1 shows a ground-referenced current detection technique used in many existing products. This technique is simple and can be easily implemented inside an IC.

On the other hand, this technique can detect the inductor current only when switch B or switch C is turned on.

This is the inductor valley current when in the buck region or the peak current when in the boost region respectively. Since both MOSFETs (B and C) are connected to the current sense resistor and the two MOSFETs must be placed close to each other, the PCB layout options are limited.


Figure 2 shows another buck-boost controller that applies the switching node reference current detection technique.

Since the current sensing resistor is placed in series with the inductor, this resistor can continuously sense the inductor current.

As switch A and switch B turn on/off, the voltage across this sense resistor at the switching node swings up and down between the input voltage and ground.

Therefore, to minimize common-mode noise, the current detection circuit must have a very high common-mode rejection ratio (CMRR).

Compared to the common mode voltage of several tens of volts, the detected inductor current signal is only in the range of 50 mV to 100 mV, so it can be easily distorted during power stage switching.

To avoid this noise, the current comparators are blocked and their inputs are turned off, as shown in Figure 2.

Although the detected signal is continuous, the short blanking time makes the inductor current information unavailable for a short period of time.


Figure 3 shows the inductor DCR current sensing technique applied to the LTC7878.

By matching the time constant of the RC detector network to the inductance and DCR (L/DCR = Rs x Cs), the inductor current is converted to a voltage signal across the detector network (Cs), with the gain being the DCR of the inductor.

A current comparator is designed below the BST1/SW1 circuit, which swings along the VIN-GND switching node during operation.

Because of the same common mode voltage on the current comparator and switching node, the current comparator input does not need to be disconnected from the DCR detection signal when SW1 switches.

In this way the inductor current is regulated and continuously limited cycle-by-cycle.

Compared to the switching node reference current detection technique, only one comparator is required under BST1/SW1.

In addition, it is possible to support various DCR values and use various inductors.

For low DCR inductors, the ISNSD pin can be set to amplify the signal and improve the signal-to-noise ratio (SNR) by up to four times higher than with conventional DCR detection techniques.

This high SNR design significantly improves system reliability and achieves stable switching operation over a wide range of duty cycles.

■ Multi-phase parallel operation

The availability of continuous inductor current information along with inductor DCR current sensing allows a unified peak current mode control circuit to be implemented in the LTC7878.

Like many peak current mode buck or boost DC-DC controllers, it has multiphase operation.Make it possible.

Multiple LTC7878 devices can be paralleled to provide more current to the load by sharing all ITH pins and daisy-chaining all CLKOUT pins.

Evenly distributing the load current across all channels and sharing the current between the inductors ensures thermal balance and high efficiency.

ADI's proprietary cycle-by-cycle inductor current sharing enhances system reliability by reducing overcurrent stress on the inductors during startup and load transients.

■ Other features

The switching frequency can be programmed from 100 kHz to 600 kHz or synchronized to an external clock.

The built-in 7V NMOS gate driver can drive either logic-level or non-logic-level MOSFETs.

Other features include an intelligent external VCC bias pin, a PGOOD indicator pin, and selectable discontinuous conduction mode/continuous conduction mode (DCM/CCM) operation with different current limit settings.

The LTC7878 can be used with inputs up to 70V, has a programmable output from 1V to 70V, and is available in a 5 x 5mm QFN package.

■ Conclusion

The LTC7878 is a high performance 4-switch buck-boost controller with inductor DCR current sensing. It uses peak current mode control in buck, boost and buck-boost regions, always providing cycle-by-cycle peak current limit and protection.

Achieve high efficiency and reduce component costs by using inductor DCR current detection. Multiple devices can be easily paralleled in a multi-phase architecture to provide higher power.

###

※ About the author
Xu Zhang received his BS and MS degrees in Electrical Engineering from Tsinghua University, Beijing, China, in 2000 and 2003, respectively. He received his Ph.D. in Electrical Engineering from the University of Colorado-Boulder, Boulder, Colorado, in 2009. He joined Analog Devices in 2010 and has designed several industry-first power controller ICs, including high-voltage, high-power charge pump controllers, innovative hybrid buck controllers with switched capacitors, bidirectional buck controllers, and 4-switch buck-boost controllers. He currently leads the Power Controller Group, responsible for the development of high-performance silicon-based power regulators and controllers.