142472733AN678_0| Application Note
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AN-678 APPLICATION NOTE
Supporting Legacy 5 V SIM Cards with the ADP3408 and ADP3522
By Daryl Sugasawara
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 circuitry. Designed for newer handsets, they do not have the capability to generate a 5 V supply. The ADP3408 supports 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 outlines 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 batte
ry 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.
DIGITAL BASEBAND
BATTERY
VBAT SIMPROG
GPIOs
SIMEN
ADP3408 ADP3522
SIMDATA EN VBAT CHARGE PUMP 5V VSIM
LEVEL SHIFT
SIM CARD
Figure 1. 5 V SIM Block Diagram
BATTERY
DIGITAL BASEBAND
VBAT SIMPROG
GPIOs
SIMEN
ADP3408 ADP3522
SIMDATA EN VBAT CHARGE PUMP 5V VSIM
LEVEL SHIFT
D1
SIM CARD
Figure 2. Blocking Diode on SIM Output
REV. 0
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