Datasheet
SLVS208I − MAY 1999 − REVISED JANUARY 2004
www.ti.com
15
APPLICATION INFORMATION
The TPS767xx family includes eight fixed-output voltage regulators (1.5 V, 1.8 V, 2.5 V, 2.7 V, 2.8 V, 3.0 V,
3.3 V, and 5.0 V), and an adjustable regulator, the TPS76701 (adjustable from 1.5 V to 5.5 V).
device operation
The TPS767xx features very low quiescent current, which remains 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 (I
B
= I
C
/β). The TPS767xx uses a PMOS transistor to pass current; because the gate of the PMOS is voltage
driven, operating current is low and invariable over the full load range.
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 I
B
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 TPS767xx quiescent current remains low even when
the regulator drops out, eliminating both problems.
The TPS767xx 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 2 µA. If the shutdown feature is not used, EN
should
be tied to ground.
minimum load requirements
The TPS767xx family is stable even at zero load; no minimum load is required for operation.
FB—pin connection (adjustable version only)
The FB pin is an input pin to sense the output voltage and close the loop for the adjustable option . The output voltage is
sensed through a resistor divider network to close the loop as shown in Figure 27. Normally, this connection should be as
short as possible; however, the connection can be made near a critical circuit to improve performance at that point.
Internally, FB connects to a high-impedance wide-bandwidth amplifier and noise pickup feeds through to the regulator
output. Routing the FB connection to minimize/avoid noise pickup is essential.
external capacitor requirements
An input capacitor is not usually required; however, a ceramic bypass capacitor (0.047 µF or larger) improves load transient
response and noise rejection if the TPS767xx 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.
Like all low dropout regulators, the TPS767xx requires an output capacitor connected between OUT and GND to stabilize
the internal control loop. The minimum recommended capacitance value is 10 µF and the ESR (equivalent series
resistance) must be between 50 mΩ and 1.5 Ω. Capacitor values 10 µF or larger are acceptable, provided the ESR is less
than 1.5 Ω. Solid tantalum electrolytic, aluminum electrolytic, and multilayer ceramic capacitors are all suitable, provided
they meet the requirements described above. Most of the commercially available 10 µF surface-mount ceramic capacitors,
including devices from Sprague and Kemet, meet the ESR requirements stated above.