BASIC OPERATION
The REG104 series is a family of LDO (Low DropOut) linear
regulators. The family includes five fixed output versions
(2.5V to 5.0V) and an adjustable output version. An internal
DMOS power device provides low dropout regulation with
near constant ground pin current (largely independent of load
and dropout conditions) and very fast line and load transient
response. All versions include internal current limit and
thermal shutdown circuitry.
Figure 1 shows the basic circuit connections for the fixed
voltage models. Figure 2 gives the connections for the
adjustable output version (REG104A) and example resistor
values for some commonly used output voltages. Values for
other voltages can be calculated from the equation shown in
Figure 2.
None of the versions require an output capacitor for regulator
stability. The REG104 will accept any output capacitor type
less than 1µF. For capacitance values larger than 1µF the
effective ESR should be greater than 0.1Ω. This minimum
ESR value includes parasitics such as printed circuit board
traces, solder joints, and sockets. A minimum 0.1µF low ESR
capacitor connected to the input supply voltage is recom-
mended.
ENABLE
The Enable pin allows the regulator to be turned on and off.
This pin is active HIGH and compatible with standard TTL-
CMOS levels. Inputs below 0.5V (max) turn the regulator off
and all circuitry is disabled. Under this condition ground-pin
current drops to approximately 0.5µA.
When not used, the Enable pin may be connected to V
IN
.
Internal to the part, the Enable pin is connected to an input
resistor-zener diode circuit, as shown in Figure 3, creating a
nonlinear input impedance.
Enable
V
IN
0.1µF
In
REG104
Gnd
NR
C
NR
0.01µF
V
Z
= 10V
Out
C
OUT
V
OUT
Enable
175kΩ
Optional
FIGURE 1. Fixed Voltage Nominal Circuit for REG104.
FIGURE 3. Enable Pin Equivalent Input Circuit.
Enable
5
2
V
IN
0.1µF
1
REG104
4
3 Gnd
Pin numbers for SOT-223 package.
Optional
V
OUT
= (1 + R
1
/R
2
) • 1.295V
To reduce current through divider, increase resistor
values (see table at right).
As the impedance of the resistor divider increases,
I
ADJ
(~200nA) may introduce an error.
C
FB
improves noise and transient response.
5
I
ADJ
R
1
Adj
R
2
3
3.3
C
FB
0.01µF
C
OUT
Load
V
OUT
EXAMPLE RESISTOR VALUES
V
OUT
(V)
1.295
2.5
R
1
(Ω)
(1)
Short
12.1k
1.21k
16.9k
1.69k
20k
2.0k
37.4k
3.74k
R
2
(Ω)
(1)
Open
13k
1.3k
13k
1.3k
13k
1.3k
13k
1.3k
NOTE: (1) Resistors are standard 1% values.
FIGURE 2. Adjustable Voltage Circuit for REG104A.
8
REG104
SBVS025G
BURR-BROWN [ BURR-BROWN CORPORATION ]
