TPS2148

TPS2158

SLVS373 – AUGUST 2001

13

www.ti.com

APPLICATION INFORMATION

USB

Function

Controller

5-V

Circuitry

3.3 V Circuitry

0.1

µF

10

µF

TPS2148

3.3 V

LDO

1.5 k

Ω

0.1

µF

4.7

µF

D+

D–

GND

5 V

Upstream Data Port

Figure 17. Example of a Peripheral Design With TPS2148

external capacitor requirements on power lines

A ceramic bypass capacitor (0.01-

µF to 0.1-µF) between VIN/SWIN1 and GND, close to the device, is

recommended to improve load transient response and noise rejection.

A bulk capacitor (4.7-

µF ) between VIN/SWIN1 and GND is also recommended, especially if load transients in

the hundreds of milliamps with fast rise times are anticipated.

A 66-

µF bulk capacitor is recommended from OUTx to ground, especially when the output load is heavy. This

precaution helps reduce transients seen on the power rails. Additionally, bypassing the outputs with a 0.1-

µF

ceramic capacitor improves the immunity of the device to short-circuit transients.

LDO output capacitor requirements

Stabilizing the internal control loop requires an output capacitor connected between LDO_OUT and GND. The

minimum recommended capacitance is a 4.7

µF with an ESR value between 200 mΩ and 10 Ω. Solid tantalum

electrolytic, aluminum electrolytic, and multilayer ceramic capacitors are all suitable, provided they meet the

ESR requirements.

overcurrent

A sense FET is used to measure current through the device. 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. Complete shut down 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 is shorted before the device is

enabled or before VIN has been applied. The TPS2148 and TPS2158 sense the short and immediately switches

to 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 TPS2148 and TPS2158 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.