1730–Series Theory of Operation
4–26
U5 is a current-mode Pulse Width Modulator (PWM). A current-mode PWM
uses two feedback loops. The inner current-feedback loop directly controls the
switcher mosfet peak current. The outer voltage-feedback loop programs the
inner loop peak current trip point.
U5 pin 2 is the inverting input of an internal op-amp. The non-inverting input is
set to 2.5 V by an internal voltage reference. Current from the peak detector
flows through R83 and R79. R84 provides a 100
m
A offset. The voltage at U5
pin 1 will vary in order to maintain U5 pin 2 at 2.5 V.
The voltage at U5 pin 1 is modified by an internal circuit and sets the trip point
of the internal comparator. U5 pin 3 is the external input to the comparator. R88
and C52, connected to U5 pin 4, set the internal oscillator to 80 kHz.
The circuit works as follows: The oscillator resets the latch and U5 pin 6 goes
high, turning the switcher mosfet on. The current through the switcher mosfet
increases, causing the voltage across R96 to increase. This voltage is divided by
R87 and R101, and is applied to the comparator (pin 3). When the voltage at U5
pin 3 reaches the comparator trip point, the latch toggles and the switcher mosfet
is turned off. This process is repeated at an 80 kHz rate.
C58 increases the PWM noise immunity by rolling off the internal op-amp
frequency response. R82 holds the switcher mosfet off as the circuit is powering
up. R81 slows the turn-on of the switcher mosfet while CR27 speeds up the turn
off.
Pulse Width Modulator
Summary of Contents for 1730 Series
Page 4: ......
Page 12: ...Contents viii...
Page 17: ...Introduction and Specifications...
Page 18: ......
Page 32: ...1730 Series Introduction 1 14...
Page 33: ...Operating Instructions...
Page 34: ......
Page 62: ...1730 Series Operating Instructions 2 28...
Page 64: ...Service Safety Summary S 2 1730 Series B070000 Above...
Page 66: ......
Page 67: ...Installation...
Page 68: ......
Page 82: ...1730 Series Installation 3 14...
Page 83: ...Theory of Operation...
Page 84: ......
Page 115: ...Checks and Adjustments...
Page 116: ......
Page 159: ...Maintenance...
Page 160: ......
Page 180: ...1730 Series Maintenance 6 20 3 Remove the board by slipping it through the front panel opening...
Page 184: ...1730 Series Maintenance 6 24 Figure 6 8 Repackaging a 1730 Series instrument...
Page 185: ...Options...
Page 186: ......
Page 189: ...Replaceable Electrical Parts...
Page 190: ......
Page 222: ...1730 Series Replaceable Electrical Parts 8 32...
Page 223: ...Diagrams Circuit Board Illustrations...
Page 224: ......
Page 246: ......
Page 247: ...Replaceable Mechanical Parts...
Page 248: ......
Page 255: ...1730 Series Waveform Monitor FIG 1 EXPLODED VIEW A1 A3 A2 A3A1 A10 A11...
Page 256: ...1730 Series B070000 Up...
Page 257: ......
Page 258: ......