DN307 - Supply 2A Pulses for GSM Transmission from 500mA USB or PCMCIA Ports

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Supply 2A Pulses for GSM Transmission from 500mA USB
or PCMCIA Ports – Design Note 307
Dongyan Zhou
Introduction
GSM modems have become popular as a wireless data
transfer solution; however, they frequently require large
bursts of current that exceed the maximum input current
available to the voltage regulator. Many can be powered
from a USB port (4.5V to 5.5V input) or a PCMCIA port
(3V to 3.6V input), where 3.xV is required to supply the
RF power amplifier that draws current pulses up to 2A.
Since the input from the USB or PCMCIA port is current
limited to 500mA, a high efficiency Buck-Boost converter
with input current limit provides the best power supply
solution. High efficiency maximizes the average output
power, while a bulk output capacitor is used to maintain
the voltage during the high current pulses.
Powering GSM Modems from USB or PCMCIA
Figure 1 shows a Buck-Boost converter powered from
USB or PCMCIA. The input current limit is implemented
by using half of the LT1490A dual micropower op amp.
The op amp is configured as a current source which
limits the input current to 500mA. The other half of
the LT1490A is used as a buffer. Efficiency at the given
pulsing load is shown in Figure 2.
The magnitude and duration of the pulsing current,
together with the ripple voltage specification, determine
the choice of the output capacitor. Both the ESR of the
capacitor and the charge stored in the capacitor each
cycle contribute to the output voltage ripple. The ripple
due to the charge is approximately:
The LTC ®3440 Buck-Boost converter is a compact and
high efficiency solution for GSM modems that can be
powered from USB or PCMCIA. It has four integrated
switches to provide synchronous Buck-Boost operation and requires only one inductor, saving cost and
board space. The IC is available in a tiny 10-pin MSOP
package and operates up to 2MHz, allowing the use
of tiny surface mount components. Quiescent current
is only 25μA in Burst Mode ® operation. The LTC3440
also disconnects the output the from the input during
shutdown, which is required for many USB applications.
VRIPPLE _ BULK =
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
RSENSE
0.1Ω
C1
10μF
4
7
SW1
SW2
LTC3440
6
VIN
VOUT
8
9
2
1
SHDN/SS
FB
MODE/SYNC
RT
VC
GND
R1
392k
R6
100k
1N914
–
5
RT
62k
R5
24.3k
VIN
+
R4
1k
1/2 LT1490A
2N3906
+
95
85
80
75
70
65
DN307 F01
60
3
3.5
4
4.5
INPUT VOLTAGE (V)
5
5.5
DN307 F02
–
ICURRENTLIMIT = t3
3t3SENSE
Figure 1. Converter Powering GSM Modem from USB or PCMCIA
04/03/307_conv
C2
2200μF
w2
R2
200k
C1: TAIYO YUDEN JMK212BJ106MG
C2: SANYO MV-AX SERIES
L1: SUMIDA CDRH4028-100
100
90
+
1/2 LT1490A
C5 10nF
10
VOUT
3.6V
2.1A
(PULSED)
EFFICIENCY (%)
VIN
3V TO 5V
USB/PCMCIA
POWER
500mA MAX
COUT
where IPULSE and tON are the peak current and on time
during transmission burst and ISTANDBY is the current
in standby mode. The above is a worst-case approximation assuming all the pulsing energy comes from the
output capacitor.
L1
10μH
3
(IPULSE –ISTANDBY ) • tON
Figure 2. Efficiency of the Converter in
Figure 1 (Pulsing Load: 1.2A for 1.15ms,
80mA for 3.45ms, within a Period of 4.6ms)
The ripple due to the capacitor ESR is:
INPUT
CURRENT
500mA/DIV
VRIPPLE_ESR = (IPULSE – ISTANDBY ) • ESR
Low ESR and high capacitance are critical to maintain
low output ripple. In this application, two 2200μF
SANYO electrolytic capacitors are used. Each capacitor
has less than 38mΩ ESR. For applications requiring
a very low profile, the BestCap series from AVX and
PowerStor Aerogel Capacitors from COOPER offer very
high capacitance and low ESR in 2mm height packages.
The GSM standard specifies 575μs transmission burst
within a 4.6ms period (1/8 duty factor). The converter in
Figure 1 can provide up to 2.1A during each transmission burst and 100mA in standby for input voltages as
low as 3V with a maximum of 500mA input current.
If the minimum input voltage is higher or the 500mA
input current limit is only required for USB input (4.5V
to 5.5V), more power is available at output. Figure 3
shows the output voltage ripple along with input and
output currents. Other standards (such as GPRS) define
a higher data rate. One popular requirement is 1.15ms
transmission bursts within a 4.6ms period (1/4 duty
factor). The converter can deliver 1.2A pulsing current with 80mA standby current, again assuming 3.0V
minimum input voltage with a 500mA current limit.
The output ripple is similar to Figure 3 except that the
ripple caused by ESR is less due to the lower load step.
5V Converter in USB On-The-Go Devices
As portable devices using USB increase in popularity,
there is a growing need for them to communicate directly
with each other when a PC is not available. The result is
SHDN/SS
FB
9
R6
100k
1N914
Conclusion
The LTC3440 synchronous Buck-Boost converter provides an optimal solution for GSM modems and USB
OTG devices. Compared with the traditional SEPIC or
Boost cascaded with an LDO converter, the LTC3440
converter provides much higher efficiency with fewer
components and smaller size.
VOUT
5V
100mA
100
R1
619k
2N3906
IOUT = 100mA
95
+
90
R2
200k
R5
24.3k
+
–
To optimize light load efficiency, the MODE/SYNC pin
can be pulled high to enable Burst Mode operation
where quiescent current is only 25μA.
–
RT
62k
1/2 LT1490A
USB On-The-Go (OTG). USB OTG is a new supplement
to the USB 2.0 specification that augments the capability by adding a host function for connection to USB
peripherals. These USB OTG dual-role devices need a
5V converter to supply USB peripherals. The LTC3440
provides a compact solution with high efficiency, low
quiescent current and shutdown disconnect. Accurate current limit can again be achieved by using the
LT®1490A dual op amp. Figure 4 shows a converter with
100mA current limit. Efficiency is plotted in Figure 5.
1/2 LT1490A
C5 10nF
C1
10
2
10μF
MODE/SYNC VC
*
6.3V
5
1
X5R
RT
GND
R4
1k
Figure 3. Waveforms Showing Input Current (Top) and
Output Voltage Ripple (Middle) at Pulsing Load (Bottom)
EFFICIENCY (%)
4
SW1
SW2
LTC3440
6
7
VIN
VOUT
DN308 F03
1ms/DIV
C2
22μF
6.3V
X5R
3
8
LOAD
CURRENT
2A/DIV
RSENSE
0.5Ω
L1
10μH
VIN
2.7V TO 5.5V
Li-Ion/WALL PLUG
OUTPUT
VOLTAGE
RIPPLE
200mV/DIV
85
80
75
70
65
C1: TAIYO YUDEN JMK212BJ106MG
C2: TAIYO YUDEN JMK325BJ226MM
L1: SUMIDA CDRH4028-100
*1 = Burst Mode OPERATION
0 = FIXED FREQUENCY
60
2.7
3
3.3 3.6 3.9 4.2 4.5 4.8 5.1 5.4 5.7
INPUT VOLTAGE (V)
DN307 F05
ICURRENTLIMIT = t3
3t3SENSE
DN307 F04
Figure 5. Efficiency of the
Converter in Figure 4
Figure 4. Converter Powering USB Compatible Devices
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