L5970AD 데이터 시트보기 (PDF) - STMicroelectronics

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L5970AD

1.5A SWITCH STEP DOWN SWITCHING REGULATOR

STMicroelectronics 

Figure 5. Internal Regulator Circuit

VCC

L5970AD

STARTER

PREREGULATOR

VREG

BANDGAP

IC BIAS

D00IN1126

VREF

3.3 OSCILLATOR & SYNCHRONIZATOR

Figure 6 shows the block diagram of the oscillator circuit.

The Clock Generator provides the switching frequency of the device that is internally fixed at 500KHz. 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 volt-

age 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.

As Slave to synchronize itself by external signal.

In particular, connecting together two devices, the one with the lower switching frequency works as Slave and

the other one works as Master.

To synchronize the device, the SYNC pin has to pass from a low level to a level higher than the synchronization

threshold with a duty cycle that can vary approximately from 10% to 90%, depending also on the signal frequen-

cy and amplitude.

The frequency of the synchronization signal must be at least higher than the internal switching frequency of the

device (500KHz).

Figure 6. Oscillator Circuit

FREQUENCY

SHIFTER

Ibias_osc

CLOCK

GENERATOR

CLOCK

t

RAMP

GENERATOR

RAMP

D00IN1131

SYNCHRONIZATOR

SYNC

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