TPS2074, TPS2075

FOUR-PORT USB HUB POWER CONTROLLERS

SLVS288A – SEPTEMBER 2000 – REVISED FEBRUARY 2001

18

POST OFFICE BOX 655303

APPLICATION INFORMATION

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.

overcurrent

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 TPS2074 and TPS2075 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 TPS2074 and TPS2075 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.

OC response

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 TPS2074 and TPS2075 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.