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Supporting legacy 5V SIM cards with the ADP3408 and ADP3522

Posted: 12 Sep 2003     Print Version  Bookmark and Share

Keywords:power 

/ARTICLES/2003SEP/A/2003SEP12_POW_RFD_AN.PDF

AN-678

APPLICATION NOTE

One Technology Way 7 P.O. Box 9106 7 Norwood, MA 02062-9106 7 Tel: 781/329-4700 7 Fax: 781/326-8703 7 www.analog.com

INTRODUCTION

SIM cards for GSM handsets have evolved from using a

5 V supply to a 3 V supply, and currently to a 1.8 V supply.

If the service provider requires the handset to be able to

support legacy SIM cards, it is necessary for the handset

designer to provide a 5 V SIM supply.

The ADP3408 and the ADP3522 contain a SIM LDO that

is used to supply the SIM module and interface circuit-

ry. Designed for newer handsets, they do not have the

capability to generate a 5 V supply. The ADP3408 sup-

ports 3 V SIMs only and the ADP3522 supports 3 V and

1.8 V SIMs. To support legacy SIM cards, it is necessary

to add an external 5 V supply. This application note out-

lines several possible configurations and the problems

with each, and then it describes a circuit that overcomes

these issues.

CONFIGURATION ISSUES

The SIM LDO cannot be directly hooked up to the output

of a 5 V charge pump as shown in Figure 1. Although the

simplest solution, this configuration will cause reverse

current to flow from the 5 V supply to the battery input

of the SIM LDO, regardless of whether or not the SIM

LDO is on.

To prevent the reverse current from flowing, a blocking

diode can be added to the output of the SIM LDO, as

shown in Figure 2. However, this adds a diode drop to the

output of the SIM LDO, and all the inaccuracies that go

along with it such as variation in voltage due to changes

in current and temperature.

Supporting Legacy 5V SIM Cards with the ADP3408 and ADP3522

By Daryl Sugasawara

DIGITAL

BASEBAND

GPIOs

SIMDATA

LEVEL

SHIFT

SIM CARD

CHARGE

PUMP

EN VBAT

SIMPROG

SIMEN

VSIM

VBAT

ADP3408

ADP3522

5V

BATTERY

Figure 1. 5 V SIM Block Diagram

DIGITAL

BASEBAND

GPIOs

SIMDATA

LEVEL

SHIFT

SIM CARD

CHARGE

PUMP

EN VBAT

SIMPROG

SIMEN

VSIM

VBAT

ADP3408

ADP3522

5V

BATTERY

D1

Figure 2. Blocking Diode on SIM Output

REV. 0

-2-

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Another possibility is to add the blocking diode to the

battery side of the SIM LDO, as shown in Figure 3. This

solution eliminates the SIM output accuracy issue, but

introduces a larger dropout voltage. As a result, the bat-

tery voltage will be higher when the SIM LDO begins to

drop out of regulation. Additionally, the blocking diode

does not eliminate the reverse current problem. A cur-

rent path through the BATSNS pin still exists.

DIGITAL

BASEBAND

GPIOs

SIMDATA

LEVEL

SHIFT

SIM CARD

CHARGE

PUMP

EN VBAT

SIMPROG

SIMEN

VSIM

VBAT

ADP3408

ADP3522

5V

BATTERY

D1

Figure 3. Blocking Diode on BAT Input

THE SOLUTION

To add a 5 V SIM capability to the ADP3408 and

ADP3522, it is recommended to add a load switch to the

output of the SIM LDO, as shown in Figure 4. This effec-

tively eliminates the reverse current and output accuracy

issues. The load switch needs to be closed when the

SIM LDO is enabled and open when the 5 V charge pump

is enabled.

The circuit shown in Figure 4 is conceptual, but the cir-

cuit has been tried in the lab as both a discrete solution

as well as with an integrated load switch, as shown in

Figure 5. The load switch is made up of Q1, Q2, and R1.

When the SIMEN signal is high, the SIM LDO and Q2 are

switched on. Q2 will pull the gate of Q1 to ground while

the body diode of Q1 will pull the source up toward the

SIM LDO output voltage. This will exceed Q1's thresh-

old voltage turning on Q1 (VGS > VGS(th)). As a result,

the SIM LDO's output appears at the source side of the

switch, SIMOUT. When the SIMEN signal is low, the SIM

LDO and Q2 are switched off. R1 will pull the gate of Q1

up to the source voltage turning Q1 off (VGS = 0). When

the 5 V charge pump is enabled, SIMEN must be held

low. This will keep Q1 off and prevent any reverse current

from flowing back to the battery.

DIGITAL

BASEBAND

GPIOs

SIMDATA

LEVEL

SHIFT

SIM CARD

CHARGE

PUMP

EN VBAT

SIMPROG

SIMEN

VSIM

VBAT

ADP3408

ADP3522

5V

BATTERY

Figure 4. Load Switch on SIM Output

SHDN

Figure 5. Load Switch Circuit

REV. 0

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It is necessary for the digital baseband to control the

sequencing of the SIM bus voltage. By controlling the

SIMEN, SIMVSEL, and 5VSEL inputs, the processor can

properly sequence the SIM voltage. A logic table for the

different states is given in Table 1.

The dc results are given in Table 2. Note the difference

in the SIM and SIMOUT current. The current in the pull-

up resistor, R1, causes this difference. Also notice the

SIM current for the 5 V SIMOUT case is 0, indicating no

reverse current flow.

The ac results are shown in Figures 6 to 11.

Table 2. DCTests

VSIM (V) ISIM (mA) VSIMOUT (mA) ISIMOUT (V) Q1 Q2 R1() R2()

1.796 20.6 1.793 20.5 FDN340P 2N2222 10 k 1 k

2.836 20.7 2.834 20.5 FDN340P 2N2222 10 k 1 k

0.001 0.0 5.035 20.4 FDN340P 2N2222 10 k 1 k

1.796 20.7 1.792 20.5 FDC6325 10 k 0

2.837 20.8 2.834 20.6 FDC6325 10 k 0

0.000 0.0 5.035 20.5 FDC6325 10 k 0

Table 1. SIM Supply Control Logic

SIMEN SIMVSEL 5VSEL

SIM OFF L X L

SIM = 1.8 V H L L

SIM = 3.0 V H H L

SIM = 5.0 V L X H

X = Don't Care

REV. 0

E04435-0-9/03(0)

-4-

) 2003 Analog Devices, Inc. All rights reserved.Trademarks and registered trademarks are the property of their respective owners.

AN-678

CH1: SIMEN

CH2: SIMOUT

VBAT = 3.6V

VSIMSEL = 0.0V

5VSEL = 0.0V

RSIMOUT = 90

Figure 6. 1.8 V SIM Response

CH1: SIMEN

CH2: SIMOUT

VBAT = 3.6V

VSIMSEL = 0.0V

5VSEL = 0.0V

RSIMOUT = 90

Figure 7. 1.8 V SIM RiseTime

CH1: SIMEN

CH2: SIMOUT

VBAT = 3.6V

VSIMSEL = VMEM

5VSEL = 0.0V

RSIMOUT = 140

Figure 8. 2.8 V SIM Response

CH1: SIMEN

CH2: SIMOUT

VBAT = 3.6V

VSIMSEL = VMEM

5VSEL = 0.0V

RSIMOUT = 140

Figure 9. 2.8 V SIM RiseTime

CH1: 5VSEL

CH2: SIMOUT

VBAT = 3.6V

SIMEN = 0.0V

RSIMOUT = 250

Figure 10. 5 V SIM Response

CH1: 5VSEL

CH2: SIMOUT

VBAT = 3.6V

SIMEN = 0.0V

RSIMOUT = 250

Figure 11. 5 V SIM RiseTime

REV. 0





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