AN-8019 Datasheet, PDF (2/5 Pages) Fairchild Semiconductor – Reliable USB Modem Design Using the Combination of an Integrated Load Switch and a Buck Converter

AN-8019
APPLICATION NOTE
The rectangular current pulses represent a worst-case
condition to ensure the power supply design works well at
the GSM/GPRS modem load. To meet this pulsed load
condition, there are several areas a designer should consider
in GSM/GPRS wireless modem designs based on a USB
input source. First, the power supply should not overload the
self-powered USB hub’s limited power specification.
Although the power specification is 5V and 500mA, there
seems to be some margin at current supplying capacity
because most USB hubs in the market can provide up to 1A
to cover variable portable applications downstream. Second,
a DC/DC step-down converter should cover the periodic 2A
pulse load conditions with limited input current. In addition,
it should regulate the 3.6V output required by most power
amplifiers. Finally, the appropriate reservoir output
capacitor should be placed in front of the power amplifier to
supply pulsed current during transmission.
Table 1. FPF2195 Family
Combination FPF2195 and FAN8060
The FPF2195 provides three different current-limit operations:
auto-restart, shut down after blanking time, and current source
type (see Table 1). The FPF2195 has an adjustable current limit
range that allows the requisite OCP level to be programmed
with an external resistor. This results in a current limit accuracy
of ±25%. In Figure 3, a current limit of 600mA is set using a
900Ω external resistor (see Equation 1). This prevents
overloading on the USB input source.
I_LIM
=
551.6
R_SET
=
551.6
900Ω
= 612mA
(1)
To control the main 5V input line and reduce conduction
losses, a typical conduction resistance of 44mΩ at 5V is
recommended. The 1mmx1.5mmx0.65mm WLCSP package
allows designers to use the PWB space more effectively and
its 140°C thermal shutdown with 10°C hysteresis protects
the FPF2195 from going into thermal runaway.
Part
Number
FPF2193
FPF2194
FPF2195
V_IN
1.8V ~ 5.5V
Package
Switch
Current
Specification Limit
(Typical) (Minimum)
Behavour at
Current Limit
1x1.5mm
WLCSP
44mΩ at 5VIN
0.15 ~ 1.5A
Adjustable
Auto Reset
Shutdown
Current Source
Current
Limit
Blanking
Time
30ms
0
Auto
Restart
Time
450ms
na
USB 5VIN
VIN
VOUT
4.7µF
FPF2195
FLAGB
100kΩ _ON
ISET
0 ^ON
GND
900Ω
4.7µF
EN
0
8.2kΩ
PVIN
SW
AVIN
SYNC
EN _FAN8060
FB
SS
COMP
PGND
AGND
82nF 100pF 1nF
3.3µH
20kΩ
10kΩ
3.6VOUT / PA Input
1500µF x 2
Figure 3. The FPF2195 and FAN8060 in a GSM/GPRS Modem Design
Figure 4. The FPF2195 and FAN8060 Evaluation Board (40mm x 18mm)
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 5/3/10
2
www.fairchildsemi.com