FOUR-PORT USB HUB POWER CONTROLLERS
SLVS287A – SEPTEMBER 2000 – REVISED FEBRUARY 2001
POST OFFICE BOX 655303
• DALLAS, TEXAS 75265
external capacitor requirements (continued)
A transient condition occurs because of a sudden increase in output current. The output capacitor reduces the
transient effect by providing the additional current needed by the load. Depending on the current demand at the
output, a voltage drop will occur across the internal resistance, ESR, of the capacitor. Using a low ESR capacitor
will help minimize this voltage drop. A larger capacitor will also reduce the voltage drop by supplying the current
demand for a longer time, versus that provided by a smaller capacitor.
An internal sense FET checks for overcurrent conditions. Unlike current-sense resistors, sense FETs do not
increase the series resistance of the current path. When an overcurrent condition is detected, the device
maintains a constant output current and reduces the output voltage accordingly. Complete shutdown occurs
only if the fault is present long enough to activate thermal limiting.
Three possible overload conditions can occur. In the first condition, the output has been shorted before the
device is enabled or before BP and SP have been applied. The TPS2070 and TPS2071 sense the short and
immediately switch into a constant-current output.
In the second condition, the short occurs while the device is enabled. At the instant the short occurs, very high
currents may flow for a very short time before the current-limit circuit can react. After the current-limit circuit has
tripped (reached the overcurrent trip threshold), the device switches into constant-current mode.
In the third condition, the load has been gradually increased beyond the recommended operating current. The
current is permitted to rise until the current-limit threshold is reached or until the thermal limit of the device is
exceeded. The TPS2070 and TPS2071 are capable of delivering current up to the current-limit threshold without
damaging the device. Once the threshold has been reached, the device switches into its constant-current mode.
The OCx output is asserted (active low) when an overcurrent or overtemperature condition is encountered and
will remain asserted until the overcurrent or overtemperature condition is removed. Connecting a heavy
capacitive load to an enabled device can cause momentary false overcurrent reporting from the inrush current
flowing through the device and charging the downstream capacitor. The TPS2070 and TPS2071 are designed
to reduce false overcurrent reporting by implementing an internal deglitch circuit. This circuit eliminates the need
for an external filter, which requires extra components. Also, using low-ESR electrolytic capacitors on the
outputs can reduce erroneous overcurrent reporting by providing a low-impedance energy source to lower the
inrush current flow through the device during hot-plug events. The OCx outputs are logic outputs thereby
requiring no pullup or pulldown resistors.