The TL497A is a fixed-on-time, variable-frequency voltage regulator controller. The block diagram of the TL497 A is shown in Figure 6-26.
The on-time is controlled by an external capacitor connected between the frequency control pin (pin 3) and ground. This capacitor, CT, is charged by an internal constant-current generator to a predetermined threshold. The charging current and threshold vary proportionately with VCC; thus, the on-time remains constant over the allowable input voltage range.
The output voltage is controlled by two series resistors, in parallel with the supply output. The resistance ratios are calculated to supply 1.2 V to the comparator input (pin I) at the desired output voltage. This feedback voltage is compared to the 1.2 V bandgap reference by the high-gain error amplifier. When the output voltage falls below the desired voltage, the error amplifier enables the oscillator circuit, which charges and discharges CT.
The npn output transistor is driven “on” during the charging cycle of CT. The internal transistor can switch currents up to 500 mA. It is current driven to allow operation from either the positive supply voltage or ground. An internal diode matched to the current
characteristics of the output transistor is included on the chip and may be used for blocking or commutating purposes.
The TL497A also contains current-limiting circuitry which senses the peak currents in the switching regulator and protects the inductor against saturation and the output transistor against overstress. The current limit is adjustable and is set by a single sense resistor between pins 13 and 14. The current-limit circuitry is activated when 0.5 V is developed across current-limit resistor RCL.
The TL497A contains all the active elements required for constructing a single-ended dc-to-dc converter. The output transistor and the rectifier are uncommitted allowing maximum flexibility in the choice of circuit configuration.
The TL497A’s primary feature is design simplicity. Using six external components; three resistors, two capacitors, and one inductor, the step-up, step-down, and inverting power supplies shown in Figure 6-27 may be constructed.