AN557 datasheet(23/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

THERMAL SHUTDOWN

The thermal protection intervenes when the junction temperature reaches 150°C; it intervenes directly on

the output stage turning it off quikly and in the meantime discharging the soft start capacitor.

The reference voltage and the oscillator will continue to work regularly.

The thermal shutdown has a hysteresis, after its intervention, it is necessary to wait for the junction tem-

perature to lower around 30°C before the device will begin to work properly again.

The device restart to work by using the soft start function.

Table 2. High Current Switching Regulator ICs.

APPLICATIONS

Even though the regulators of the L4970A family has been designed to work only in step down configura-

tion we will see next how these regulators can be use in large range of applications. In same cases the

L4970A device will be used as an example for the entire family assembled in Multiwatt package and the

L4974A will be used for the types in powerdip package.

Anyway the suggested applications can be extended to any other device of the family by adjusting if nec-

essary the external components using the given equation for the calculation.

Typical Application

The Fig. 29 shows the electrical diagram of the typical application, complete with all the auxiliary functions.

The same application suggested in the data sheet as test circuit and is the same used for the final dynamic

test. All our devices are 100% tested both in static and dynamic conditions.

Included in the dynamic test are obviously the external components: the coil, catch diode and output ca-

pacitor which have been defined for all regulators.

Shown below are the electrical diagrams of 5 devices that compose the family of this regulator complete

Parameter

L497X FAMILY

L4970A

L4977A

L4975A

L4974A

L4972A

L4972AD

Surf. Mount.

Max. Input Operating

Voltage

50V

Output Voltage Range

5.1V (

±2%) to 40V

Max. Output Current

10A

3.5A

Power Switch

RDSON at 25°C

0.13

Ω typ.

Switching Mode Control

System

Continuous Mode, Direct Duty Cycle Control with Voltage Feed-Forward

Max. Switching Freq.

500KHz

200KHz

Efficiency

VINPUT = 35V

VOUT = 5.1V

10A

80%

at 200KHz

80%

at 200KHz

85%

at 200KHz

3.5A

85%

at 100KHz

85%

at 100KHz

85%

at 100KHz

Current Limiting

Constant Current

Soft Start

Yes

Reset and Power Fail

Yes

Synch

Yes

Crowbar

Package

Max. Rth j-case (pin)

Rth j-amb

Multiwatt15

°C/W

Multiwatt15

°C/W

Multiwatt15

°C/W

Powerdip

16+2+2

°C/W

Powerdip

16+2+2

°C/W

SO20L

°C/W

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