Datasheet
SLVS230G − SEPTEMBER 1999 − REVISED JANUARY 2004
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15
APPLICATION INFORMATION
The TPS777xx and TPS778xx families include four fixed-output voltage regulators (1.5 V, 1.8 V, 2.5 V, and 3.3 V), and an
adjustable regulator, the TPS77x01 (adjustable from 1.5 V to 5.5 V for TPS77701 option and 1.2 V to 5.5 V for TPS77801
option).
device operation
The TPS777xx and TPS778xx feature 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 TPS777xx and TPS778xx use 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 TPS777xx and TPS778xx quiescent currents remain
low even when the regulator drops out, eliminating both problems.
The TPS777xx and TPS778xx families also feature 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 TPS777xx and TPS778xx families are 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 it is shown in Figure 26. 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 TPS777xx or TPS778xx are 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 TPS777xx and TPS778xx require 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 previously.