L5973AD データシートの表示（PDF） - STMicroelectronics

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L5973AD

2 A switch step down switching regulator

STMicroelectronics 

Functional description

L5973AD

4.1

Power supply and voltage reference

The internal regulator circuit (shown in Figure 4) 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 gives the start-up currents to the whole device when the input voltage goes high

and the device is enabled (inhibit pin connected to ground).

The Preregulator block supplies the Bandgap cell with a preregulated voltage VREG that has

a very low supply voltage noise sensitivity.

4.2

Voltages monitor

An internal block senses continuously the VCC, Vref and Vbg. If the voltages go higher than

their thresholds, the regulator starts to work. There is also an hysteresis on the VCC

(UVLO).

Figure 4. Internal regulator circuit

VCC

STARTER

D00IN1126

IC BIAS

PREREGULATOR

VREG

BANDGAP

VREF

4.3

Oscillator and synchronizator

Figure 5 shows the block diagram of the oscillator circuit.

The clock generator provides the switching frequency of the device that is internally fixed at

500 kHz. The frequency shifter block acts reducing the switching frequency in case of strong

overcurrent or short circuit. The clock signal is then used in the internal logic circuitry and is

the input of the Ramp Generator and Synchronizator blocks.

The Ramp Generator circuit provides the sawtooth signal, used to realize the PWM control

and the internal voltage feed forward, while the Synchronizator circuit generates the

synchronization signal. Infact the device has a synchronization pin that can works both as

Master and Slave.

As Master to synchronize external devices to the internal switching frequency.

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