SLUA110 Datasheet, PDF (13/19 Pages) Texas Instruments – PRACTICAL CONSIDERATIONS IN CURRENT MODE POWER SUPPLIES

APPLICATION NOTE
VOLTAGE CONTROLLED OSCILLATOR
GENERAL CONFIGURATION
VARIABLE FREQUENCY OPERATION
FIXED 50% DUTY CYCLE
OSCILLATORS WITH SINGLE PIN PROGRAMMING
U-111
At the beginning of an oscillator cycle, Ct begins charging
and the PWM output is turned on. Transistor Q1 is driven
from the output and also turns on with the PWM output, thus
discharging Ct and pulling this node to ground. As this
occurs, the oscillator is “frozen” with the PWM output fully
ON. On-time can be controlled in the conventional manner
by comparing the error amplifier output voltage with the
current sense input voltage. This results in a current con-
trolled “on-time” and fixed “off-time” mode of operation.
Other variations are possible with different inputs to the
current sense input.
When the PWM output goes low (off ), transistor Q1 also turns
off and Ct begins charging to its upper threshold.The off-time
generated by this approach will be longer for a given Rt/Ct
combination than first anticipated using the oscillator"charg-
ing” equations or curves. Timing capacitor Ct now begins
charging from Vsat of Q1 (approx. 0V) instead of the internal
oscillator lower threshold of approximately 1 volt.
FIXED “OFF-TIME”, CURRENT
CONTROLLED “ON-TIME”
UC3851 / UC3844A / UC3845A
*GROUND RAMP OR CURRENT SENSE INPUT
OSCILLATORS WITH SEPARATE RT & CT PINS
SCHEMATIC
UC3823 / UC3825 / UC3847
*GROUND RAMP OR CURRENT SENSE INPUT
USE NONINV E/a INPUT FOR REVERSE V/F OPERATION
Fixed “Off -Time” Applications
Obtaining a fixed “off-time” and a variable “on-time” can
easily be accomplished with most current-mode PWM IC’s.
In these applications, the Rt/Ct timing components are used
to generate the “off-time” rather than the traditional “on-
time.” Implementation is shown schematically in Figure 3
along with the pertinent waveforms.
3-118
WAVEFORMS
Figure 35.