BASIC OPERATION
The REG103 series is a family of LDO (Low Drop-Out)
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 regula-
tion with near constant ground pin current (largely indepen-
dent of load and drop-out 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 (REG103A) and example resistor
values for some commonly used output voltages. Values for
other voltages can be calculated from the equation shown in
Figure 2. The SO-8 package provides two pins each for V
IN
and V
OUT
. Both sets of pins
MUST
be used and connected
adjacent to the device.
None of the versions require an output capacitor for regula-
tor stability. The REG103 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 mini-
mum 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
recommended.
INTERNAL CURRENT LIMIT
The REG103 internal current limit has a typical value of
700mA. A fold-back feature limits the short-circuit current
to a typical short-circuit value of 40mA. This circuit will
protect the regulator from damage under all load conditions.
A typical characteristic of V
OUT
versus I
OUT
is given in
Figure 3a.
Care should be taken in high current applications to avoid
ground currents flowing in the circuit board traces causing
voltage drops between points on the circuit. If voltage drops
occur on the circuit board ground that causes the load ground
voltage to be much lower than the ground voltage seen by
the ground pin on the REG103, the foldback current may
approach zero and the REG103 may not start up. In these
types of applications, a large value resistor can be placed
between V
IN
and V
OUT
to help “boost” up the output of the
REG103 during start-up, see Figure 3b. The value for the
“boost” resistor should be chosen so that the current through
the “boost” resistor is less than the minimum load current:
R
BOOST
> (V
IN
– V
OUT
)/I
LOAD
. Typically, a good value for a
“boost” resistor is 5kΩ.
ENABLE
V
IN
0.1µF
In
ERROR
Out
C
OUT
C
NR
0.01µF
V
OUT
REG103
Gnd
NR
Optional
FIGURE 1. Fixed Voltage Nominal Circuit for REG103.
ENABLE
5
8
V
IN
0.1µF
7
REG103
3
4 Gnd
Pin numbers for SO-8 package.
ERROR
6
1
2
I
ADJ
R
1
Adj
R
2
C
FB
0.01µF
V
OUT
C
OUT
Load
EXAMPLE RESISTOR VALUES
V
OUT
(V)
1.295
2.5
3
3.3
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
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
NOTE: (1) Resistors are standard 1% values.
FIGURE 2. Adjustable Voltage Circuit for REG103A.
8
REG103
SBVS010D
TI [ TEXAS INSTRUMENTS ]
