LT1123 LINER [Linear Technology], LT1123 Datasheet - Page 4

• Manufacturer Part Number: LT1123
• Description: Low Dropout Regulator Driver
• Manufacturer: LINER [Linear Technology]

PI FU CTIO S
Drive Pin: The drive pin serves two functions. It provides
current to the LT1123 for its internal circuitry including
startup, bias, current limit, thermal limit and a portion of
the base drive current for the output Darlington. The sum
total of these currents (450 A typical) is equal to the
minimum drive current. This current is listed in the speci-
fications as Drive Current with V
minimum current required by the drive pin of the LT1123.
The second function of the drive pin is to sink the base
drive current of the external PNP pass transistor. The
available drive current is specified for two conditions.
Drive current with V
available under nominal operating conditions, when the
device is regulating. Drive current with V
range of drive current available with the feedback pin
pulled low as it would be during startup or during a short
circuit fault. The drive current available when the feedback
pin is pulled low is less than the drive current available
when the device is regulating (V
in the curve of Drive Current vs V
Performance Characteristic curves. This can provide some
foldback in the current limit of the regulator circuit.
A
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 cooperat-
ing 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
LT1123
4
PPLICATI
U
U
O
U
FB
U
S
= 4.80V gives the range of current
I FOR ATIO
U
FB
FB
= 5V). This can be seen
FB
Voltage in the Typical
W
= 5.2V. This is the
FB
= 0.5V gives the
U
to 120mA). Output currents up to 4A with dropout volt-
ages 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
All internal circuitry connected to the drive pin is designed
to operate at the saturation voltage of the Darlington
output driver (1.4 – 2V). This allows a resistor to be
inserted between the base of the external PNP device and
the drive pin. This resistor is used to limit the base drive to
the external PNP below the value set internally by the
LT1123, and also to help limit power dissipation in the
LT1123. The operating voltage range of this pin is from
0V to 30V. Pulling this pin below ground by more than one
V
This condition can only occur if the power supply leads are
reversed and will not damage the device if the current is
limited to less than 200mA.
Feedback Pin (V
functions. It provides a path for the bias current of the
reference and error amplifier and contributes a portion of
the drive current for the Darlington output driver. The sum
total of these currents is the Feedback Pin Bias Current
(300 A typical). The second function of this pin is to
provide the voltage feedback to the error amplifier.
BE
will forward bias the substrate diode of the device.
FB
): The feedback pin also serves two