5
LT1123
1123fb
The LT1123 is designed to be used in conjunction with an
external PNP transistor. The overall specifications of a
regulator circuit using the LT1123 and an external PNP will
be heavily dependent on the specifications of the external
PNP. While there are a wide variety of PNP transistors
available that can be used with the LT1123, the specifica-
tions given in typical transistor data sheets are of little use
in determining overall circuit performance.
Linear Technology has solved this problem by cooperating
with Motorola to design and specify the MJE1123. This
transistor is specifically designed to work with the LT1123
as the pass element in a low dropout regulator. The
specifications of the MJE1123 reflect the capability of the
LT1123. For example, the dropout voltage of the MJE1123
is specified up to 4A collector current with base drive
currents that the LT1123 is capable of generating (20mA
to 120mA). Output currents up to 4A with dropout voltages
less than 0.75V can be guaranteed.
The following sections describe how specifications can be
determined for the basic regulator. The charts and graphs
are based on the combined characteristics of the LT1123
and the MJE1123. Formulas are included that will enable
the user to substitute other transistors that have been
characterized. A chart is supplied that lists suggested
resistor values for the most popular range of input volt-
ages and output current.
Basic Regulator Circuit
The basic regulator circuit is shown in Figure 1. The
LT1123 senses the voltage at its feedback pin and drives
the base of the PNP (MJE1123) in order to maintain the
output at 5V. The drive pin of the LT1123 can only sink
current; R
B
is required to provide pull up on the base of the
PNP. R
B
must be sized so that the voltage drop caused by
the minimum drive pin current is less than the emitter/
base voltage of the external PNP at light loads. The
recommended value for R
B
is 620Ω. For circuits that are
required to run at junction temperatures in excess of
100°C the recommended value of R
B
is 300Ω.
Figure 1. Basic Regulator Circuit
R
D
is used to limit the drive current available to the PNP
and to limit the power dissipation in the LT1123. Limiting
the drive current to the PNP will limit the output current of
the regulator which will minimize the stress on the regu-
lator circuit under overload conditions. R
D
is chosen
based on the operating requirements of the circuit, prima-
rily dropout voltage and output current.
Dropout Voltage
The dropout voltage of an LT1123-based regulator circuit
is determined by the V
CE
saturation voltage of the discrete
PNP when it is driven with a base current equal to the
available drive current of the LT1123. The LT1123 can sink
up to 150mA of base current (150mA typ, 125mA min)
when output voltage is up near the regulating point (5V).
The available drive current of the LT1123 can be reduced
by adding a resistor (R
D
) in series with the drive pin (see
the section below on current limit). The MJE1123 is
specified for dropout voltage (V
CE
sat.) at several values of
output current and up to 120mA of base drive current. The
chart below lists the operating points that can be guaran-
teed by the combined data sheets of the LT1123 and
MJE1123. Figure 2 illustrates the chart in graphic form.
Although these numbers are only guaranteed by the data
sheet at 25°C, Dropout Voltage vs Temperature (Figure 3)
clearly shows that the dropout voltage is nearly constant
over a wide temperature range.
APPLICATIO S I FOR ATIO
WUUU
R
D
R
B
620Ω
V
IN
V
OUT
= 5V
10µF ALUM
LT1123 F01
DRIVE
LT1123
GND
FB
+
MJE1123
