Specifications

Power Interface
Rev 2.0 Apr.10 Proprietary and Confidential 25
Inrush current via application of host power supply
This event occurs when the host’s supply is enabled, charging the input
capacitors on the embedded module’s power rail. The switches shown in
Figure 2-4 on page 24 and Figure 2-5 on page 24 are open (typically) when this
event occurs.
Note: In some circum-
stances, depending on
temperature and the
components in use, two or
more regulators may
switch on at the same
time. The host power
system must be designed
to handle this possibility.
To limit the inrush current and stabilize the supply of power to the module,
sufficient capacitance must be added to the host power rail. The recommended
capacitance range is 470–1000 µF.
Peak current (IPEAK) is calculated using:
Inrush current via floating W_Disable#
The second event type occurs when the host leaves the W_Disable# signal
floating (high impedance) to power up the module. The switches shown in
Figure 2-4 on page 24 and Figure 2-5 on page 24 are closed when this event
occurs. This enables the power management system of the module, charging
several internal regulator output capacitors.
When W_Disable# is left floating (high impedance), the peak current is less than
500 mA (with a 30 µs rise time).
Timing requirements
Power ramp-up
During the first 0.5 seconds, as the internal voltage regulators are activated in
sequence, several current transients of up to 500 mA with a 30 µs rise time may
occur.
The supply voltage must remain within specified tolerances while this is occurring.
Power-up timing
The unit is ready to enumerate with a USB host within a maximum of 5.1 seconds
(depending on module type) after power-up. (Most modules enumerate within
4 seconds.) Figure 2-6 on page 26 illustrates the power-up timing sequence.
Note: The actual startup time may vary between the different module types (for example,
MC5727 versus MC8775).
IPEAK = VCC/RSERIES
RSERIES = impedance from power source (+ive) through to the
Regulator output capacitor)
+ (impedance from power source (-ive) through to GND
pins of the EM regulator)