L4972 Просмотр технического описания (PDF) - STMicroelectronics

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L4972

2A SWITCHING REGULATOR

STMicroelectronics 

L4972A-L4972AD

CIRCUIT OPERATION

The L4972A is a 2A monolithic stepdown switching

regulatorworking in continuousmode realized inthe

new BCD Technology. This technology allows the

integration of isolatedvertical DMOS power transis-

tors plus mixed CMOS/Bipolar transistors.

The device can deliver 2A at an output voltage ad-

justable from 5.1V to 40V and contains diagnostic

and control functions that make it particularly suit-

able for microprocessor based systems.

BLOCK DIAGRAM

The block diagram shows the DMOS power tran-

sistors and the PWM control loop. Integrated func-

tions include a reference voltage trimmed to 5.1V

± 2%,soft start, undervoltagelockout, oscillator with

feedforward control, pulse by pulse current limit,

thermal shutdown and finally the reset and power

fail circuit. The reset and power fail circuit provides

an output signal for a microprocessor indicating the

status of the system.

Device turn on is around 11V with a typical 1V hys-

terysis, this threshold porvides a correct voltage for

the driving stage of the DMOS gate and the hyste-

rysis prevents instabilities.

An externalbootstrapcapacitorchargeto 12V by an

internal voltage reference is needed to provide cor-

rect gate drive to the power DMOS. The driving cir-

cuit is able to source and sink peak currents of

around 0.5A to the gate of the DMOS transistor. A

typical switching time of the current in the DMOS

transistor is 50ns. Due to the fast commutation

switching frequencies up to 200kHz are possible.

The PWM control loop consists of a sawtooth oscil-

lator, error amplifier, comparator, latch and the out-

put stage. An error signal is producedby comparing

theoutputvoltagewiththeprecise5.1V ± 2% onchip

reference. This error signal is then compared with

the sawtooth oscillator in order to generate frixed

frequency pulse width modulated drive for the out-

put stage. A PWM latch is included to eliminate

multiple pulsing within a period even in noisy envi-

ronments.

The gain and stabilityof the loop can be adjustedby

an external RC network connected to the output of

the error amplifier. A voltage feedforward control

has been added to the oscillator, this maintains su-

perior line regulation over a wide input voltage

range. Closing the loop directly gives an outputvol-

tage of 5.1V, higher voltages areobtained by insert-

ing a voltage divider.

At turn on, outputovercurrents are preventedby the

soft start function (fig. 2). The error amplifier is in-

itially clamped by an externalcapacitor,Css, and al-

lowed to rise linearly under the charge of an internal

constant current source.

Output overload protection is provided by a current

limit circuit. The load current is sensedby a internal

metalresistor connectedto a comparator.When the

load current exceeds a preset threshold, the output

of the comparator sets a flip flop which turns off the

power DMOS. The next clock pulse, from an internal

40kHz oscillator, will reset the flip flopand the power

DMOS will again conduct. This current protection

method,ensuresa constantcurrent outputwhenthe

systemis overloadedor shortcircuited and limitsthe

switching frequency, in this condition,to 40kHz. The

Reset and Power fail circuit (fig. 4), generates an

output signal when the supply voltage exceeds a

threshold programmed by an external voltage di-

vider. The reset signal, is generated with a delay

time programmedby a externalcapacitor on the de-

lay pin. When the supply voltage falls below the

threshold or the output voltage goes below 5V, the

resetoutput goes low immediately. The reset output

is an open drain.

Fig. 4A shows the case when the supply voltage is

higher than the threshold, but the output voltage is

not yet 5V.

Fig. 4B shows the case when the output is 5.1V, but

the supply voltage is not yet higher than the fixed

threshold.

The thermal protection disables circuit operation

when the junction temperature reaches about

150°C and has a hysterysis to prevent unstable

conditions.

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