TPS7101Q, TPS7133Q, TPS7148Q, TPS7150Q

TPS7101Y, TPS7133Y, TPS7148Y, TPS7150Y

LOW-DROPOUT VOLTAGE REGULATORS

SLVS092G – NOVEMBER 1994 – REVISED JANUARY 2003

26

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

The TPS71xx series of low-dropout (LDO) regulators is designed to overcome many of the shortcomings of

earlier-generation LDOs, while adding features such as a power-saving shutdown mode and a power-good

indicator. The TPS71xx family includes three fixed-output voltage regulators: the TPS7133 (3.3 V), the

TPS7148 (4.85 V), and the TPS7150 (5 V). The family also offers an adjustable device, the TPS7101 (adjustable

from 1.2 V to 9.75 V).

device operation

The TPS71xx, unlike many other LDOs, features very low quiescent currents that remain virtually constant even

with varying loads. Conventional LDO regulators use a pnp-pass element, the base current of which is directly

proportional to the load current through the regulator (IB = IC/β). Close examination of the data sheets reveals

that those devices are typically specified under near no-load conditions; actual operating currents are much

higher as evidenced by typical quiescent current versus load current curves. The TPS71xx uses a PMOS

transistor to pass current; because the gate of the PMOS element is voltage driven, operating currents are low

and invariable over the full load range. The TPS71xx specifications reflect actual performance under load.

Another pitfall associated with the pnp-pass element is its tendency to saturate when the device goes into

dropout. The resulting drop in

β forces an increase in IB to maintain the load. During power up, this translates

to large start-up currents. Systems with limited supply current may fail to start up. In battery-powered systems,

it means rapid battery discharge when the voltage decays below the minimum required for regulation. The

TPS71xx quiescent current remains low even when the regulator drops out, eliminating both problems.

Included in the TPS71xx family is a 4.85-V regulator, the TPS7148. Designed specifically for 5-V cellular

systems, its 4.85-V output, regulated to within

± 2%, allows for operation within the low-end limit of 5-V systems

specified to

± 5% tolerance; therefore, maximum regulated operating lifetime is obtained from a battery pack

before the device drops out, adding crucial talk minutes between charges.

The TPS71xx family also features a shutdown mode that places the output in the high-impedance state

(essentially equal to the feedback-divider resistance) and reduces quiescent current to under 2

µA. If the

shutdown feature is not used, EN should be tied to ground. Response to an enable transition is quick; regulated

output voltage is reestablished in typically 120

µs.

minimum load requirements

The TPS71xx family is stable even at zero load; no minimum load is required for operation.

SENSE-pin connection

The SENSE pin of fixed-output devices must be connected to the regulator output for proper functioning of the

regulator. Normally, this connection should be as short as possible; however, the connection can be made near

a critical circuit (remote sense) to improve performance at that point. Internally, SENSE connects to a

high-impedance wide-bandwidth amplifier through a resistor-divider network and noise pickup feeds through

to the regulator output. Routing the SENSE connection to minimize/avoid noise pickup is essential. Adding an

RC network between SENSE and OUT to filter noise is not recommended because it can cause the regulator

to oscillate.

external capacitor requirements

An input capacitor is not required; however, a ceramic bypass capacitor (0.047 pF to 0.1

µF) improves load

transient response and noise rejection if the TPS71xx is located more than a few inches from the power supply.

A higher-capacitance electrolytic capacitor may be necessary if large (hundreds of milliamps) load transients

with fast rise times are anticipated.