Iridium 9523 Product Developer Manual page 20

Iridium Communications, Inc.
Iridium 9523 Product Developer's Guide
Revision 7.0
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The current peak in the VBOOST rail lasts for 8.3ms and repeats every 90ms (this is the period of a frame
in the Iridium air interface). When not transmitting, the VBOOST current returns to zero. The VBOOST
current was measured when the Iridium 9523 was connected to a 27V power source that could meet its
instantaneous power requirements (around 25W).
A block diagram of the VBOOST power supply in a typical Lithium-ion powered application is shown in
Figure 8.
Iridium's products use a boost-converter circuit based on the Linear Technology LT3580 to produce the
VBOOST rail. This circuit is not capable of supplying the instantaneous current needed by the power
amplifier. The Iridium 9523 therefore includes a total capacitance of 1000μF on the VBOOST rail to store
charge; these capacitors are depleted during the transmit time-slot (8.3ms) and replenished during the
remainder of the frame time (81.7ms). The voltage across the capacitors at the end of each transmit time-
slot must not fall below 10.5V; otherwise the output voltage of the buck converter will drop too low and
affect the transmitted waveform from the PA. The average current taken from the boost converter in this
configuration is around 300mA.
Iridium Communications, Inc.
Proprietary & Confidential Information
Only
Figure 7: Current Profile of Supply Rails
Current in VBOOST Rail
Current in VBAT Rail
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Figure 8: VBOOST block diagram
Page 20 of 27
Information Contained in this Guide
is Subject to Change Without Notice
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Distribution of Guide Restricted
to Product Developers
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