Tiny Boost Regulator Eyes Optical Systems

Analog Devices has announced the Power by Linear LTM4661, a low power step-up µModule regulator in a 6.25 mm x 6.25 mm x 2.42 mm BGA package. Only a few capacitors and one resistor are required to complete the design, and the solution occupies less than 1cm² single-sided or 0.5cm² on double- sided PCBs. The LTM4661 incorporates a switching DC/DC controller, MOSFETs, inductors and supporting components. The LTM4661 operates from a 1.8 V to 5.5 V input supply, and continues to operate down to 0.7 V after start-up. The output voltage can be set by a single resistor ranging from 2.5 V to 15 V. The combination of the small, thin package and wide input and output voltage range is ideal for wide range of applications including optical modules, battery-powered equipment, battery-based backup systems, bias voltage for power amps or laser diodes and small DC motors.
The LTM4661 can deliver 4 A continuously under 3.3 VIN to 5 VOUT, and 0.7 A continuously under 3.3 VIN to 12 VOUT. The LTM4661 employs synchronous rectification, which delivers as high as 92% conversion efficiency (3.3 VIN to 5 VOUT). The switching frequency is 1 MHz, and can also be synchronized to an external clock ranging from 500 kHz to 1.5 MHz. The LTM4661 1MHz switching frequency and dual phase single output architecture enable fast transient response to line and load changes and a significant reduction of output ripple voltage. The LTM4661 has three operation modes: Burst Mode operation, forced continuous mode and external sync mode. The quiescent current in Burst Mode operation is only 25 µA, which provides extended battery run time. For applications demanding the lowest possible noise operation, the forced continuous mode or external sync mode minimize possible interference of switching noise.

The LTM4661 features an output disconnect during shutdown and inrush current limit at start-up. Fault protection features include short-circuit, overvoltage and overtemperature protection. It operates from –40℃ to 125℃ operating temperature. Available now, pricing for the LTM4661 starts at $6.98 (1,000s) for  6.25 mm x 6.25 mm x 2.42 mm BGA device.

Analog Devices | www.analog.com

Stepper Motor Back EMF

Supply Voltage vs. Current Control

Continuing with the topic of stepper motors, this time Ed looks at back electromotive force (EMF) and its effects. He examines the relationship between running stepper motors at high speeds and power supply voltage requirements.

By Ed Nisley

Early 3D printers used ATX supplies from desktop PCs for their logic, heater and motor power. This worked well enough—although running high-wattage heaters from the 12 V supply tended to incinerate cheap connectors. More mysteriously, stepper motors tended to run roughly and stall at high printing speeds, even with microstepping controllers connected to the 12 V supply.

In this article, I’ll examine the effect of back EMF on stepper motor current control. I’ll begin with a motor at rest, then show why increasing speeds call for a much higher power supply voltage than you may expect.

Microstepping Current Control

As you saw in my March 2018 article (Circuit Cellar 332), microstepping motor drivers control the winding currents to move the rotor between its full-step positions. Chips similar to the A4988 on the Protoneer CNC Shield in my MPCNC sense each winding’s current through a series resistor, then set the H-bridge MOSFETs to increase, reduce or maintain the current as needed for each step. Photo 1 shows the Z-axis motor current during the first few steps as the motor begins turning, measured with my long-obsolete Tektronix Hall effect current probes, as shown in this article’s lead photo above.

Photo 1 Each pulse in the bottom trace triggers a single Z-axis microstep. The top two traces show the 32 kHz PWM ripple in the A and B winding currents at 200 mA/div. The Z-axis acceleration limit reduces the starting speed to 18 mm/s = 1,100 mm/min.

The upper trace (I’ll call it the “A” winding) comes from the black A6302 probe clamped around the blue wire, with the vertical scale at 200 mA/div. The current starts at 0 mA and increases after each Z-axis step pulse in the bottom trace. Unlike the situation in most scope images, the “ripple” on the trace isn’t noise. It’s a steady series of PWM pulses regulating the winding current.

The middle trace (the “B” winding) increases from -425 mA because it operates in quadrature with the A winding. The hulking pistol-shaped Tektronix A6303 current probe, rated for 100 A, isn’t well-suited to measure such tiny currents, as you can see from the tiny green stepper motor wire lying in the gaping opening through the probe’s ferrite core. Using it with the A6302 probe shows the correct relation between the currents in both windings, even if its absolute calibration isn’t quite right.

Photo 2 zooms in on the A winding current, with the vertical scale at 50 mA/div, to show the first PWM pulse in better detail. The current begins rising from 0 mA, at the rising edge of the step pulse, as the A4988 controller applies +24 V to the motor winding and reaches 110 mA after 18 µs. The controller then applies -24 V to the winding by swapping the H bridge connections. This causes the current to fall to 40 mA, whereupon it turns on both lower MOSFETs in the bridge to let the current circulate through the transistors with very little loss.

Photo 2
Zooming in on the first microstep pulse of Photo 1 shows the A4988 driver raising the stepper winding current from 0 mA as the motor starts turning. The applied voltage and motor inductance determine the slope of the current changes.

The next PWM cycle starts 15 µs later, in the rightmost division of the screen, where it rises from the 40 mA winding current set by the first pulse. It will also end at 110 mA, although that part of the cycle occurs far off-screen. You can read the details of the A4988 control algorithms and current levels in its datasheet, with the two-stage decreasing waveform known as “mixed decay” mode.

Although the H-bridge MOSFETs in the A4988 connect the motor windings directly between the supply voltage and circuit ground, the winding inductance prevents the current from changing instantaneously. The datasheet gives a nominal inductance of 4.8 mH, matching what I measured, but you can also estimate the value from the slope of the current changes.. . …

Read the full article in the May 334 issue of Circuit Cellar

Don’t miss out on upcoming issues of Circuit Cellar. Subscribe today!
Note: We’ve made the October 2017 issue of Circuit Cellar available as a free sample issue. In it, you’ll find a rich variety of the kinds of articles and information that exemplify a typical issue of the current magazine.

Compact Power Regulator Targets FPGAs, GPUs and ASICs

Analog Devices has announced the Power by Linear LTM4646, a dual 10 A or single 20 A output, step-down µModule point-of-load regulator from 5V or 12V input supply rails. The LTM4646 includes the inductors, MOSFETs, a DC/DC controller and supporting components and is housed in a 11.25 mm x 15 mm x 5.01 mm BGA package. Compared to the prior 2 x single 10 A output module solutions, the LTM4646 reduces the solution size of more than 25%.
With its dual regulator design, small package size, and precise voltage regulation, the LTM4646 meets the PCB area constraints of densely populated system boards to power low voltage and high current devices such as FPGAs, ASICs, microprocessors and GPUs. Applications include PCIe boards, communication infrastructure, cloud computing-based systems, as well as medical, industrial, and test and measurement equipment.

Total output voltage DC accuracy is guaranteed at ±1.5% over line, load and temperature (–40°C to 125°C). Moreover, the onboard remote sense amplifiers on both outputs compensate for voltage drop caused by trace impedance of the PC board due to large load currents. The LTM4646 has selectable internal or external feedback loop compensation, enabling users to optimize loop stability and transient performance while minimizing the number of output capacitors. The peak efficiency at 12 VIN to 1.0 VOUT is 86%. With 200LFM air flow, the LTM4646 delivers a full 20A  continuously up to 85°C ambient. The current mode architecture allows multiphase parallel operation to increase output current with very good current sharing.

Standalone, the LTM4646 operates from 4.5 V to 20 V input range. When 5 V external bias is available, the device can operate from 2.375 V. The output voltages are adjustable from 0.6 V to 5.5 V, enabling the device to generate not only low voltage for digital devices but also 2.5 V, 3.3 V and  5V, which are commonly needed in system bus voltages. The switching frequency can be programmed from 250 kHz to 1.3 MHz with one resistor, and can also be synchronized to an external clock ranging from 300 kHz to 1 MHz for noise-sensitive applications. Additionally, it features overvoltage and overcurrent protection. The LTM4646 operates from –40°C to 125°C.

Summary of Features: LTM4646

  •     Dual 10A or Single 20A Output
  •     Wide Input Voltage Range: 4.5V to 20V
  •     2.375VMIN with CPWR Bias
  •     Output Voltage Range: 0.6V to 5.5V
  •     ±1.5% Maximum Total DC Output Error
  •     Multiphase Current Sharing
  •     Differential Remote Sense Amplifier, Each Channel
  •     Internal or External Compensation
  •     11.25mm x 15mm x 5.01mm BGA Package
  •     BGA Ball Finishes Available: SAC305 (RoHS), SnPb (63/37)


Linear Technology | www.linear.com