Switching Regulator - Motorola ASTRO XTS 3000 Service Manual

by a single bicolor LED on the top and (on models II and III) a four-
line, twelve-character display.
The controller schedules the activities of the DSP through the host
port interface. This includes setting the operational modes and
parameters of the DSP. The controlling of the DSP is analogous to
programming analog signaling ICs on standard analog radios.

Switching Regulator

All of the digital circuitry on the vocoder and controller boards is
supplied with 5-volt regulated dc by a switched-mode regulator on the
controller board (see Figure 3-4). The fundamental parts of the
regulator are U709, L119, C180, D104, C174, C175, and U726. Module
U709 is a pulse-width modulating (PWM), switched, regulator
controller. Coil L119 is an energy storage element, C180 is an output
ripple filter, and D104 is a Schottky diode switch. Capacitors C174 and
C175 are added for UNSW_B+ ripple filtering, and are necessary for the
stability of the regulator. Module U726 is a supply supervisory IC,
which provides a system reset function when the output of the
regulator falls out of regulation, typically around 4.7 Vdc.
This switched-mode regulator works by supplying just sufficient
energy to the storage element to maintain the output power of the
regulator at 5Vdc. It can be related to a flywheel in the sense that just
enough energy can be added to a spinning flywheel to keep it spinning
at a constant speed. This is in contrast to a typical linear type regulator,
which basically shunts unused current to ground through an active
resistive divider. The switched-mode regulator is much more energy
efficient. It can be noted that input current to the regulator is less than
the load current. In fact, as input voltage to the regulator goes up,
current supplied to the regulator actually goes down for a constant
load.
Module U709 works off of a clock with a nominal operating frequency
of 160kHz (kit number NCN6128), or 260kHz (kit number NCN6167).
This may vary a little, based on the load and input voltage. Regulation
is maintained by varying the duty cycle of a clock output driving L119.
This signal is referred to as Lx on U709 (refer to Waveform W1). As long
as the clock output is high, current flows from the supply into L119,
allowing energy to be stored. When the clock output goes low, diode
D104 conducts, allowing current to continue to flow from ground
through L119. A pulse width on the Lx signal can be obtained, which
provides the correct amount of energy to keep the output in
regulation. Capacitor C180 is an output filter that reduces ripple on
the output from the clock transitions.
Module U709 is supplied directly from the unswitched battery supply.
It is turned on and off through the control line connected to SHDN*/
ON/OFF*. This is the same control line from the MCU, which controls
the series pass element Q106, that switches SW_B+. A voltage level of
approximately 2 Vdc is required to turn the regulator on.
6-5