Inrush Currents - Sierra Wireless MC5725 Hardware Integration Manual

CDMA and GSM / UMTS Mini Card Hardware Integration Guide
Figure 2-4: Inrush model - MC57xx
Current Probe
Current
32m
+3.3VDC
250m
Power source
8.5m
Recommended host power rail
capacitance = 470 µF - 1000 µF
24
In the event of a discrepancy in values between this guide and the Product Specification Document (PSD), the PSD takes precedence.
2.
3.
Note: (MC57xx only) If the ambient temperature of the module
exceeds 60 °C, the RF level is automatically lowered for max power
transmission.

Inrush currents

Two power events can cause large inrush currents from the
host supply to the module on the power pins:
•
•
Figure 2‐4 and Figure 2‐5 show the inrush models for the
MC57xx and MC87xx. Application of the host's power supply
typically occurs when the switch is open;
floating (high impedance) when the switch is closed.
38m
ESR +
trace Z
ESR
60m
20m
RF Cap
4.7uF
2.8m
Proprietary and Confidential
The module sends a CnS notification (Return Radio
Temperature—0x0008) to the host indicating that it is now
in low power mode.
The host has the option, at this point, of driving
low (forcing the module to power off) to prevent damage
to the unit.
Application of the host's power supply
Host leaves # floating (high impedance) to power
W_Disable
up the module (as shown in Figure 2‐1 on page 17).
PWR (1,2,3,4,5)
15m
ESR +
ESR
trace Z
20m
40m
ON/OFF
(18)
Cin
Cin
2x 1uF
2x 1uF
1.7m
Regulator input capacitor
EM board decoupling cap
EM board decoupling cap
W_Disable#
# is left
W_Disable
MC57xx
LDO pass
element
1 ohm typ
10m
ESR
REG
80m
1uF
2.2m
Regulator
capacitor
2130114
output