HTSSOP16 Ver la hoja de datos (PDF) - STMicroelectronics

Número de pieza

componentes Descripción

Fabricante

HTSSOP16

4 A continuous (more than 5 A pulsed) step-down switching regulator with synchronous rectification

STMicroelectronics 

L5988D

Functional description

5.2

Oscillator and synchronization

The generation of the internal saw-tooth waveform is based on the constant current charge /

discharge of an internal capacitor. The current generator is designed to get a switching

frequency of 400 kHz ± 10% in case the FSW pin is left floating.

The current mirror connected to FSW (see Figure 5) pin acts increasing / decreasing the

value of the internal charging current to adjust the oscillator frequency. Since the internal

circuitry forces the FSW voltage bias at 1.235 V, the user can easily source / sink current in

this pin connecting a pull up resistor to VREF or a pull down to GND respectively.

Figure 5. Oscillator circuit block diagram

VREF

Clock

Clock

Generator

Synchronization

SYNCH

Ramp

Generator

Sawtooth

The value of the pull up resistor versus VREF to decrease the oscillator frequency follows

the formula:

Equation 1

R1K = 4----0---0-----8–---.-F-5---S----W-1---0---K-3---H-----z---- + 0.95

In the same way to increase the switching frequency the pull down resistor is selected using

the formula:

Equation 2

R2K = -F---S----W-----1--K-8---H-----1z---0----3–-----4---0---0-- – 2.1

Table 11 on page 26 shows some resistor values to adjust the oscillator frequency.

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