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AN-8019

更新时间： 2024-09-16 08:30:43

品牌	Logo	应用领域
飞兆/仙童 - FAIRCHILD		转换器调制解调器开关
页数	文件大小	规格书
5页	303K
描述
Reliable USB Modem Design Using the Combination of an Integrated Load Switch and a Buck Converter

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www.fairchildsemi.com

AN-8019

Reliable USB Modem Design Using the Combination of an

Integrated Load Switch and a Buck Converter

GSM / GPRS Power Demands

Introduction

While transmitting signals over the carrier, GSM and GPRS

In the portable electronics market, a wireless network allows

systems require up to 2A of peak current. This requires a

people to communicate effectively anywhere, at any time,

pulsating current for a relatively long period for a normal

without wires. This is one of the critical factors to consider

power management design. A GSM transmitter consumes

when designing portable electronics. A wireless modem is a

2A of peak current for 577µs when it is a single-slot period

modem that connects to a wireless network instead of a

from one time division made up of eight time slots. A GPRS

telephone system. Connecting with a wireless modem,

Class-10 transmitter (two uplink time slots) requires the

attaches directly to a wireless internet services provider.

same peak current amplitude twice for 1.15ms out of eight

Global System for Mobile (GSM) and General Packet Radio

slots. During the other six or seven slots, the consumed

Service (GPRS) wireless modems, powered by a VBUS line

current decreases to approximately 100mA, so the average

of USB, provide a good method to enable a universal

current of the pulsed load is 340mA for GSM and 570mA

portable device environment. This modem design, however,

for GPRS. Figure 1 and Figure 2 show the GSM and GPRS

causes power management issues because the GSM and

transmission models used to evaluate the power management

GPRS transmitters require up to 2A peak current, which

exceeds the maximum current capacity of the USB power

circuit described in this article.

source. Today, most USB ports are only able to supply up to

1A at 5V. For better reliability, designers must consider

several design factors; including input over-current

I[A]

protection (OCP) to avoid overloading the USB power

source, a robust step-down converter to make the most of the

limited input source, and bulk output capacitors to supply

enough power to the pulsed load condition during GSM and

GPRS transmission.

The combination of the FPF2195, IntelliMAX™ load

switch, and FAN8060 DC-to-DC converter provides

GSM/GPRS modem designers with both input OCP and a

maximum output current limit as it satisfies power

requirements in a pulsed load. The FPF2195 IntelliMAX is a

functional load switch with a 0.15 to 1.5A adjustable current

limit and low conductive resistance. The FPF2195 can be

mounted on a small PWB (Printed Wiring Board) while

maintaining healthy thermal performance. The FAN8060 is

a synchronous step-down DC/DC converter operating at

1.2MHz frequency, which allows a low-profile inductor to

charge the large output capacitors safely at 95% efficiency.

The FAN8060’s maximum output current protection

prevents the shutdown of the hub system during output short

conditions.

t(ms)

577µs 4.62ms

Figure 1. GSM Transmission Mode

I[A]

t(ms)

1.15ms

4.62ms

Figure 2. GPRS Transmission Mode

Rev. 1.0.1 • 5/3/10

www.fairchildsemi.com

12 3 4 5

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