User's Manual

LZT 123 1834 41
The wireless modem has insufficient internal capacitance to supply the
large current peaks during GSM burst transmission - use the following
general guidelines in designing the application power supply.
Fit a low ESR electrolytic capacitor close to the wireless modem (>1,000
µF, with an ESR < 100 m)
Ensure power supply to wireless modem line resistance is < 200 m
The module has approximately 40µF of internal capacitance across the
VCC pins. During initial power-up the host power supply will have to
charge this capacitance to the operating voltage. This initial in-rush
current may exceed the module’s normal peak current, sometimes
greater than an order of magnitude higher (depending upon the power
supply design) for a short duration (generally a few microseconds).
5.6 Voltage Reference (VREF)
Pin Name Direction Function
34 VREF Input (Output) Core voltage reference
GR64 provides a voltage reference interface for user applications.
Level shifters are integrated in the GR64 product. The integrated level shifters are
referenced to an internal IO regulator on the module side and to an application
voltage on the user side of the interface. There are two implementation of VREF,
dependent upon the users’ GR64 variant;
VREF as an output (DPY 102 1494/10 & DPY 102 1494/30 variants)
VREF as an input (DPY 102 1494/20 & DPY 102 1494/40 variants)
5.6.1 VREF as an Output
The version of GR64 without an integrated SIM holder provides a 2.8V reference to
the host side level shifter devices. This enables legacy users, and users of older
interface technology to connect directly to the GR64’s IO. The same reference voltage
is provided as an output on VREF. In this arrangement VREF can be used as a further
level shifter reference in the users application circuits, or to power external circuits,
since it has a 75mA current sourcing capability.
TIP
CAUTION