Although the USB 2.0 specification indicates that a host USB port should supply 2.5W (5V, 500mA) to downstream functions, most USB designs usually supply more power.
But power management has become a challenge in dongles intended for use in GSM/GPRS mobile systems, where the peak pulsed load current in data transmission mode can exceed 2A.
Fig 1 shows the block diagram of a multiband network USB wireless modem.
The front end power block includes a current limit switch, a step down converter and a couple of mF range reservoir capacitors. In the case of a GSM/GPRS modem, extra caution needs to be taken because a 2A peak current is required for one or two slots of the eight available in a GSM/GPRS frame. The 2A peak current has a 577µs pulse width and 4.615ms period (217Hz), as shown in Fig 2.
From a system power standpoint, the challenge is to maintain Vbus at more than 4.4V to meet the USB 2.0 specification, while providing a minimum input of 2.9 to 3V, as required by the GSM/GPRS power amp (PA) for safe operation.
There are two approaches to designing the front end power block (see fig 3).
Topology A uses a USB to over current protection (OCP) to dc/dc arrangement; while topology B implements USB to dc/dc to OCP.
In Topology A, the current limit switch prevents the USB input current from exceeding a preset level and the buck converter provides power to downstream blocks, while topology B gives a dedicated current limiting function to the GSM/GPRS PA before the reservoir capacitor. It is beneficial for designers to understand advantages and disadvantages of each topology.
Topology A
In fig 3, topology A allows designers to limit the input current from the USB host. The current limit switch is placed on the front end and can be programmed to the required current level, resulting in short circuit protection to the system. The input current cannot exceed the preset level, within a tolerance of ±25%, although a 10% tolerance is preferred. However, topology A has disadvantages in a USB modem design. First, a buck converter rated at more than 2A is required to provide downstream power. Second, the reservoir capacitor needs a breakdown voltage of at least 6.3V because of the 5V bus and this could affect solution costs. Lastly, the output voltage of the buck converter can fluctuate, depending on reservoir capacitance and the maximum allowable input current from the USB host.
Topology B
Topology B provides a constant output voltage for the baseband and power management ic, except for the GSM/GPRS PA. This is an advantage, because designers do not have to worry about voltage fluctuation or drop from the main communication chipset. Because the buck converter's Vout is typically regulated to be between 3.6 and 3.8V, a 4V bulk capacitor is suitable.
A relatively lower current rated buck converter can be applied, as long as it provides the preset current limit level in front of the PA and a maximum load current for other power blocks. A buck converter rated at 1A normally works, which is a big advantage compared to topology A.
However, it is comparatively difficult to regulate the input current to meet the USB specification. The current limit is dedicated to the GSM/GPRS PA and designers should take PA load conditions, other power blocks and the USB input current into consideration.
Also, some larger reservoir capacitance is essential, depending on the allowable voltage drop for the GSM/GPRS PA input and current limit threshold. This results in increased BOM costs and a larger form factor.
Meanwhile, Vout for the GSM/GPRS PA drops to 3.1V with 3mF capacitance at the 600mA current limit condition and if a smaller voltage drop is needed at the PA's input, a bigger capacitance is required.
In most system power designs, one of the basics is to check if a power source can supply sufficient power to meet demand efficiently. USB wireless modems are not an exception.
Designers sometimes expect that large tantalum reservoir capacitors can support the rest of the power continuously without exceeding USB power limits. But the main reason for the reservoir capacitors is to support transient peak power during GSM/GPRS transmission.
From fig 2, it can be seen that an average current of 499mA is required for GSM/GPRS Class 10 transmission. If an additional 300mA is needed for the communication chipset, the buck converter's 3.7V output must supply 799mA, or 2.96W.
Assuming 90% system efficiency, the input power should be at least 3.29W. In this case, it is impossible to implement the design with a 2.5W USB power source and tway to solve this is to increase the USB current limit value.
Designers need to know values such as maximum allowable USB power, minimum allowable input voltage for GSM/GPRS PA, GSM/GPRS class and maximum worst case communication chipset load during transmission.
The values for a USB wireless modem based on GSM/GPRS are:
* allowable USB power, VBUS: 5V, max 700mA
* buck converter output voltage: 3.7V
* Vin of GSM/GPRS PA: 3.2V min / Class 10
* baseband/rf/power management ic: 300mA during transmission, worst case.
An appropriate buck converter and current limit switch for this design are Fairchild's FAN5353 and FPF2165R. The FAN5353 is a 3A monolithic synchronous buck converter with an operating switching frequency of 3MHz. An internal compensation feature makes the design simpler. The FPF2165R is a current limit switch adjustable from 0.15A to 1.5A. It keeps the 5VBUS stable during the 2A pulse load of the PA. The device also features a 10% current limit accuracy with a 1% external resistor, which reduces design error. While disabled, the reverse current blocking function is also fit for USB2.0 applications. The FPF2165R's typical on resistance of 95mO at 5V can reduce voltage drop and increase system efficiency.
Discharging and recharging periods during GSM signal transmission should be taken into consideration when sizing the reservoir capacitor. A larger capacitance without a recharging period will result in the system failing, since it discharges and eventually drops to less than the dc/dc converter's undervoltage lockout threshold.
The key concept in topology A is to keep the input at more than 4.1V in order to maintain a 3.7V output, while considering a maximum duty cycle of 90% under a limited current of 700mA by placing the reservoir capacitor in front of the FAN5353 buck converter during transmission.
Topology B has a different approach, with the reservoir capacitor dedicated to the GSM/GPRS PA. It can be shown that, depending upon current, Topology A requires a capacitance ranging from 1.6mF to 2.43mF, while Topology B requires from 3.9mF to 5.9mF.
From the results, even though both topologies meet the design criteria, designers need to consider the tradeoffs between the approaches in order to select the right design.
Author profiles
Jeongil Lee, senior applications engineer, and Ilsoo Yang, principal technical marketing manager, are with Fairchild Semiconductor.
