AN-8019 Datasheet, PDF (1/5 Pages) Fairchild Semiconductor – Reliable USB Modem Design Using the Combination of an Integrated Load Switch and a Buck Converter

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AN-8019
Reliable USB Modem Design Using the Combination of an
Integrated Load Switch and a Buck Converter
Introduction
In the portable electronics market, a wireless network allows
people to communicate effectively anywhere, at any time,
without wires. This is one of the critical factors to consider
when designing portable electronics. A wireless modem is a
modem that connects to a wireless network instead of a
telephone system. Connecting with a wireless modem,
attaches directly to a wireless internet services provider.
Global System for Mobile (GSM) and General Packet Radio
Service (GPRS) wireless modems, powered by a VBUS line
of USB, provide a good method to enable a universal
portable device environment. This modem design, however,
causes power management issues because the GSM and
GPRS transmitters require up to 2A peak current, which
exceeds the maximum current capacity of the USB power
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
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.
GSM / GPRS Power Demands
While transmitting signals over the carrier, GSM and GPRS
systems require up to 2A of peak current. This requires a
pulsating current for a relatively long period for a normal
power management design. A GSM transmitter consumes
2A of peak current for 577µs when it is a single-slot period
from one time division made up of eight time slots. A GPRS
Class-10 transmitter (two uplink time slots) requires the
same peak current amplitude twice for 1.15ms out of eight
slots. During the other six or seven slots, the consumed
current decreases to approximately 100mA, so the average
current of the pulsed load is 340mA for GSM and 570mA
for GPRS. Figure 1 and Figure 2 show the GSM and GPRS
transmission models used to evaluate the power management
circuit described in this article.
I[A]
2A
577µs 4.62ms
t(ms)
Figure 1. GSM Transmission Mode
I[A]
2A
1.15ms 4.62ms
t(ms)
Figure 2. GPRS Transmission Mode
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 5/3/10
www.fairchildsemi.com