IC’s internal driver amplifier provides base current for the exter-
nal series pass transistor, at the “Driver Sink” for positive sup-
plies, or at the “Driver Source” for negative supplies. If the
supply output voltage increases, the regulator IC decreases base
current to the external series pass transistor, reducing the output
voltage; if the supply output voltage decreases, the regulator IC
increases base current to the external series pass transistor,
increasing the output voltage.
M.2.2.4
Regulator IC: Fault Logic
Refer to Figure M-1, Block Diagram, UC3834 Linear Regulator.
The fault monitoring circuit senses both under-voltage and over-
voltage conditions. Internal fault logic activates the Fault Alert
and pulls pin 10 to the regulator IC’s internal ground at pin 5,
which is the “V(in)-” terminal. When there is no fault, the “Fault
Alert” output is an open collector. If an out of tolerance condition
exists, pin 10 is an active “LOW”.
A “fault delay” capacitor from pin 11 to ground provides a delay
to prevent a “Fault Alert” when transient over-voltage or under-
voltage conditions occur.
The internal fault logic circuit is supplemented by external fault
circuitry on the DC Regulator. No Fault Alert outputs from the
IC are used on the DC Regulator.
A thermal shutdown circuit pulls the Error Amplifier output
LOW, turning off the IC’s internal drive transistor and external
pass transistor, when junction temperatures become excessive.
This protects the IC from over-dissipation. The under-voltage
will then cause a “Fault Alert” to be generated.
M.2.3
B+ (+5.75 VDC) Regulated Supplies (DC
Regulator)
There are two identical “A” and “B” supplies on the DX-25U DC
Regulator — one for each Modulation Encoder. Only the “A”
supply will be described here.
The B+ “A” supply uses regulator IC U3, and series pass transis-
tor Q1. The unregulated input at TP3 is +8 VDC, fused by F2,
from the Low Voltage power supply. Capacitors C17 and C20
bypass transients and high frequency noise on the unregulated
input. The output voltage can be measured at TP4.
M.2.3.1
Basic Regulator Circuit (U3, Q1)
The output voltage is determined by the reference voltage at pin
8 (Vref) and the voltage sample divider R16 and R19. The
regulator controls the output voltage so that the reference voltage
at pin 8 is equal to the voltage sample at pin 9. The output voltage,
then, is Vout = Vref/[R19/(R16 + R19)]. The reference voltage
for the positive voltage regulators is the in1.5 Volts from
pin 3.
The base of series pass transistor Q1 is driven by the “sink”
output of the IC, at pin 12. The “Source” at pin 13 goes to ground
through emitter resistor R14.
M.2.3.2
Error Amp Reference Circuit
The B+ supply includes a turn-on circuit, consisting of R20 and
CR6. Diodes CR4 and CR5 provide a reference voltage of
approxi1.2 Volts. Before the internal reference voltage
has come on during turn-on, Schottky diode CR6 is forward
biased, providing a voltage of slightly less than 1 Volt at pin 8,
the error amplifier non-inverting input. When the internal refer-
ence voltage increases, diode CR1 will be reverse biased and the
error amplifier reference voltage will be the +1.5 Volts from the
regulator IC’s internal reference.
M.2.3.3
Other Regulator Circuit Components
Additional components include:
a. Supply voltage for the IC’s internal circuits: Resistor R17
to pin 1 supplies voltage for the regulator’s internal cir-
cuits, and current sense inputs at pins 6 and 7 are tied to
pin 1. This supply voltage is not fused.
b. Feedback Loop Compensation: Capacitor C10 and resistor
R12, from pins 14 and 15 to ground, forms feedback loop
compensation.
c. Output protection: Protection at the output includes zener
diode CR7 and bypass capacitors C14 and C16.
d. Reverse voltage protection: Diode CR3 across series pass
transistor Q1 provides protection if a reverse voltage is
accidentally applied at the unregulated input.
M.2.3.4
Other Positive Regulated Supplies in the Transmitter
Operation of all regulated supplies using the UC3834 linear
regulator IC is similar to operation of the B+ supply on the DC
Regulator. Voltage sample divider resistances, compensation
components, and IC driver emitter resistances will be different
Figure M-2. Basic Positive and Negative Voltage
Regulator Circuits
DX-25U
M-2
888-2297-002
WARNING: Disconnect primary power prior to servicing.
