AN557 datasheet(9/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

Calculating dt from (22) and (23) and equalizing, it follows that :

Calculating dvC and equalizing it to ∆Vo, it follows that :

(24) for +

∆Io

(25) for -

∆Io

From these two expressions the dependence of overshoots and undershoots on the L and C values may

be observed. To minimize

∆Vo it is therefore necessary to reduce the inductance value L and to increase

the capacitance value C. Should other auxiliary functions be required in the circuit like reset or crowbar

protections and very variable loads may be present, it is worthwhile to take special care for minimizing

these overshoots, which could cause spurious operation of the crowbar, and the under-shoot, which could

trigger the reset function.

DEVICE DESCRIPTION

For a better understanding of how the device functions, a description will be given of the principle blocks

that compose the device. The block diagram of the device is shown in fig.6

POWER SUPPLY

The device contains a stabilized regulator (Vstart = 12V) that provides power to the analogic and digital

control blocks as well as the section of the bootstrap. The Vstart voltage also powers the blocks that op-

erates the internal reference voltage of 5.1V, with a precision of ±2%, necessary for the feedback.

OSCILLATOR, SYNC. AND VOLTAGE FEED-FORWARD FUNCTIONS

The oscillator block generates a sawtooh wave signal that sets the switching frequency of the system. This

signal, compared with the output voltage of the error amplifier, generates the PWM signal that will then

sent to the power output stage. The oscillator also contains the voltage feedforward function that, being

completely integrated, does not require additional external components to function. The VFF function op-

erates with supply voltages from 15V to 45V. The

∆V/∆t of the sawtooh is directly proportional to the supply

voltage Vi.

As Vi increases, the conduction time (ton) of the power transistor decreases in such way as to provide to

the coil, and therefore to the load, the product Volt x Sec constant.

Figure 6. Block Diagram of the 10A Monolithic Regulator L4970A.

--------

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∆

–

()

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∆

C V

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