Functional description

AN1518

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3

Functional description

The main internal blocks are shown in the device block diagram in Figure 4. 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 stand-by operation.

●

A circuit to implement the thermal protection function.

3.1

Power supply & voltage reference

The internal regulator circuit (shown in Figure 5) consists of a start-up circuit, an internal

voltage Preregulator, 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

very low supply voltage noise sensitivity.

3.2

Voltages monitor

(UVLO).

Figure 5.

Internal circuit