11-06-2013, 04:24 PM
Pulse Width Modulator Control Circuits
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INTRODUCTION
The SG3525A, SG3527A pulse width modulator control circuits offer improved performance and lower external parts count when implemented for controlling all types of switching power supplies. The on–chip +5.1 V reference is trimmed to 1% and the error amplifier has an input common–mode voltage range that includes the reference voltage, thus eliminating the need for external divider resistors. A sync input to the oscillator enables multiple units to be slaved or a single unit to be synchronized to an external system clock. A wide range of deadtime can be programmed by a single resistor connected between the CT and Discharge
pins. These devices also feature built–in soft–start circuitry, requiring only an
external timing capacitor. A shutdown pin controls both the soft–start circuitry and the output stages, providing instantaneous turn off through the PWM latch with pulsed shutdown, as well as soft–start recycle with longer shutdown commands. The under voltage lockout inhibits the outputs and the changing of the soft–start capacitor when VCC is below nominal. The output
stages are totem–pole design capable of sinking and sourcing in excess of
200 mA. The output stage of the SG3525A features NOR logic resulting in a low output for an off–state while the SG3527A utilized OR logic which gives a high output when off.
APPLICATION INFORMATION
Since both the compensation and soft–start terminals (Pins 9 and 8) have current source pull–ups, either can readily accept a pull–down signal which only has to sink a maximum of 100 A to turn off the outputs. This is subject to the added requirement of discharging whatever external capacitance may be attached to these pins.
An alternate approach is the use of the shutdown circuitry of Pin 10 which has been improved to enhance the available shutdown options. Activating this circuit by applying a positive signal on Pin 10 performs two functions: the PWM
latch is immediately set providing the fastest turn–off signal to the outputs; and a 150 A current sink begins to discharge the external soft–start capacitor. If the shutdown command is short, the PWM signal is terminated without significant discharge of the soft–start capacitor, thus, allowing, for example, a convenient implementation of pulse–by–pulse current limiting. Holding Pin 10 high for a longer duration, however, will ultimately discharge this external capacitor, recycling slow turn–on upon release.