Tighten Supply Regulation for 2A USB Devices by Dynamically
Compensating for Voltage Drops in Wiring and Connectors
Design Note 1029
Tom Hack
Introduction
These days, the Universal Serial Bus (USB) is commonly
used to power tablet computers and high rate cell phone
battery chargers—applications never envisioned at the
inception of the USB standard in the mid-1990s. The
USB standard has changed significantly over this time.
For instance, USB 3.0 requires up to 900mA (six, 150mA
unit loads) during high bandwidth communication. A
dedicated charging port can supply as much as 1.8A. of 12V. To simplify the schematic, the protection circuitry
for load dump, reverse battery, 2-battery jump, spikes
and noise are not shown (consult Linear Technology’s
“Automotive Electronics Solutions” brochure for further
information.) The nominal switching frequency is set
above 455kHz to avoid interference in the IF of various
RF devices. It can be raised to 2MHz to avoid interfering
with AM and Travelers Information Station broadcasts,
but at the expense of some power supply efficiency. Such high, and highly variable, load currents can
produce significant and unpredictable voltage drops
in wiring and connectors, lowering the performance …