Handshake Deal: Choosing A Buck Converter for USB 3.1 Type-C



Success with USB 3.1 Type-C powered devices requires low quiescent current and high efficiency.

The new USB 3.1 Type-C standard promises to dramatically simplify the connection of electronic gadgets. The new standard aims at utilizing the same Type-C cable for data and power transfer between any two devices up to 100 watts. This article discusses the characteristics of the always-on buck converter, a key enabler of the handshake required between the source and sink devices to acknowledge connection and negotiate the amount of power delivered.

Figure 1: USB 3.1 Type-C Cable

Figure 1: USB 3.1 Type-C Cable

Negotiations
USB 3.1 Type-C is a new standard that supports high data rates and increased power delivery between electronic products. USB 3.1 can deliver 10Gbps of throughput while delivering up to 3A over standard cables and up to 5A over enhanced cables. The bus voltage can be adjusted up to 20V (60W at 3A with a standard cable or 100W at 5A with an enhanced cable). Many notebook computers today require less than 100W of power, hence new models adopting a Type-C connector can be charged via a USB port the way small devices are charged today.

The complexity of the USB 3.1 Type-C standard requires a device to negotiate as either a power provider (source) or power user (sink) before power transfer takes place. Accordingly, Type-C cables are ‘smart’, requiring additional circuitry to know how to correctly route power and data. The connectors at both ends of a Type-C cable are identical, allowing for reversible plug in. Each connector is also flippable, which allows it to be plugged in with either side facing up. Type-C USB also allows for bi-directional power, hence a peripheral device can be charged, and the same device can also charge a host device. This promises to eliminate many proprietary power adapters and many types of USB cables, ultimately reducing the maze of wires surrounding today’s desktops.

Notebook Computer Example
Figure 2 illustrates a typical notebook computer power management front end equipped to connect to a USB Type-C cable and powered by a stack of three Lithium-Ion batteries (providing a supply voltage between 7.2V and 12.6V).

Figure 2: USB 3.1 Type-C Power Management System

Figure 2: USB 3.1 Type-C Power Management System

The power management integrated circuit (PMIC) is powered directly by the batteries, while the always-on buck converter receives power via the Schottky diodes, either from the batteries or from the USB source. In turn, the always-on buck converter powers the port controller, a device arbitrating the power exchange between the host system microprocessor (µP) and the power source. When the USB is connected the buck charger charges the battery. The sequence below illustrates the steps necessary to recharge a dead battery:

Step 1. Battery dead
Step 2. USB Type-C cable connected
Step 3. USB Type-C cable detects connection and provides initial 5V.
Step 4. Always-on converter wakes up and powers the port controller.
Step 5. Port controller negotiates power requirement (Example: 5A, 20V).
Step 6. System receives required power via USB Type-C cable.
Step 7. Buck charger initiates battery charge.

The always-on buck converter plays a key role in all phases. It provides 3.3V out even with a relatively low input voltage (5V minus the Schottky diode drop) provided initially by the USB. During normal operation, the power source has the ability to change capabilities at any time, forcing the sinking device to renegotiate, hence the necessity for the buck converter to be always ‘on’. Accordingly, the always-on buck converter should be selected to consume minimum power.

Responding to the Demands of Battery-Powered Applications
The MAX77596 is a small, synchronous buck converter with integrated switches (T1 and T2 in Figure 3). The device is designed to deliver up to 300mA with input voltages from 3.5V to 24V, while using only 1.1µA quiescent current at no load (fixed-output version). The device can operate near dropout by running at 98% duty cycle, making it ideal for battery-powered applications.
The device offers fixed 3.3V and 5V output versions, as well as an adjustable version. The adjustable version allows the user to program the output voltage between 1V and 10V by using a resistor-divider. Frequency is fixed at 1.7MHz, which allows for small external passives and reduced output ripple. The on-chip power transistor switches warrant a small bill of materials. The device offers both forced PWM and skip modes of operation.

The MAX77596 is available in a small (2mm x 2.5mm) 10-pin TDFN package and operates across the -40°C to +85°C temperature range.

Very Low Quiescent Current
For an always-on device low quiescent current is critical, because the device is never shut down. The MAX77596 fixed output version allows most of the internal circuitry to be biased from the VOUT pin (IBIAS in Figure 3).

Figure 3: MAX77596 Buck Converter Fixed Output

Figure 3: MAX77596 Buck Converter Fixed Output

This way only a residual current of 1.1µA (ISUP) is drawn from the input pin. Figure 4 shows the temperature behavior of the supply current, ISUP.

Figure 4: No-load Supply Current vs Temperature

Figure 4: No-load Supply Current vs Temperature

This clever bias arrangement makes the MAX77596 best in class for low quiescent current operation.

Buck Converter High Efficiency

Low resistance, integrated MOSFET switches enable high efficiency even at a high 1.7MHz operating frequency. The efficiency comparison in Figure 5 shows that the MAX77596 has an advantage of up to 5% versus a competitive device.

Figure 5: Efficiency Comparison

Figure 5: Efficiency Comparison

More Power to More Devices
The USB 3.1 Type-C standard allows delivery of more power to more devices with a single Type-C cable. One key component in the handshake required between the source and sink devices is the always-on buck converter powering the port controller. The MAX77596, with its best in class low quiescent current and high efficiency, is the ideal choice to support USB Type-C applications.

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RenoRossettiNazzareno (Reno) Rossetti is a seasoned Analog and Power Management professional, a published author, and holds several patents in this field. He holds a doctorate in Electrical Engineering from Politecnico di Torino, Italy.

MehdyKhotanMehdy Khotan is an executive director of business management with the Mobile Group at Maxim Integrated. He has been with the company since 2004 with a primary focus on power management products for battery operated applications. He has been responsible for market introduction of linear and switching Li+ battery chargers, discrete DC-DC converters, and a wide range of highly integrated multi-function power management IC’s for mobile consumer products. He holds a master’s degree in solid state electronics from UC Davis.

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