A5974D datasheet(9/46 Pages) STMICROELECTRONICS

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A5974D

Functional description

The main internal blocks are shown in the device block diagram in Figure 3. They are:



A voltage regulator supplying the internal circuitry. From this regulator, a 3.3 V

reference voltage is externally available.



A voltage monitor circuit which checks the input and the internal voltages.



A fully integrated sawtooth oscillator with a frequency of 250 kHz

15%, including also

the voltage feed-forward function and an input/output synchronization pin.



Two embedded current limitation circuits which control the current that flows through

the power switch. The pulse-by-pulse current limit forces the power switch OFF cycle-

by-cycle if the current reaches an internal threshold, while the frequency shifter

reduces the switching frequency in order to significantly reduce the duty cycle.



A transconductance error amplifier.



A pulse width modulator (PWM) comparator and the relative logic circuitry necessary to

drive the internal power.



A high-side driver for the internal P-MOS switch.



An inhibit block for standby operation.



A circuit to implement the thermal protection function.

Figure 3. Block diagram

5.1

Power supply and voltage reference

The internal regulator circuit (shown in Figure 4) consists of a start-up circuit, an internal

voltage pre-regulator, the bandgap voltage reference and the bias block that provides

current to all the blocks. The starter supplies the start-up currents to the entire device when

the input voltage goes high and the device is enabled (inhibit pin connected to ground). The

pre-regulator block supplies the Bandgap cell with a pre-regulated voltage VREG that has

a very low supply voltage noise sensitivity.

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