l4972.pdf

L4972A

L4972AD

. 2A OUTPUT CURRENT

2A SWITCHING REGULATOR

. 5.1V TO 40V OUTPUT VOLTAGE RANGE

MULTIPOWER BCD TECHNOLOGY

. INTERNAL CURRENT LIMITING

. PRECISE 5.1V ± 2% ON CHIP REFERENCE

. RESET AND POWER FAIL FUNCTIONS

. INPUT/OUTPUT SYNC PIN

. UNDER VOLTAGE LOCK OUT WITH HYS-

. PWM LATCH FOR SINGLE PULSE PER PE-

TERETIC TURN-ON

. VERY HIGH EFFICIENCY

. SWITCHING FREQUENCY UP TO 200KHz

POWERDIP

(16 + 2 + 2)

SO20

. THERMAL SHUTDOWN

. CONTINUOUS MODE OPERATION

ORDERING NUMBERS : L4972A (Powerdip)

L4972AD (SO20)

DESCRIPTION

The L4972A is a stepdown monolithic power switch-

ing regulator delivering 2A at a voltage variable from

5.1 to 40V.

Realized with BCD mixed technology, the device

uses a DMOS output transistor to obtain very high

efficiency and very fast switching times. Features of

the L4972A include reset and power fail for micro-

processors, feed forward line regulation, soft start,

limiting current and thermal protection. The device

is mounted in a Powerdip 16 + 2 + 2 and SO20 large

plastic packages and requires few external compo-

nents. Efficient operation at switching frequencies

up to 200KHz allows reduction in the size and cost

of external filter component.

BLOCK DIAGRAM

January 1995

1/23

This is advanced information on a new product now in development or undergoing evaluation. Details are subject to change without notice.

. 0 TO 90% DUTY CYCLE RANGE

. INTERNAL FEED-FORWARD LINE REG.

RIOD

L4972A-L4972AD

ABSOLUTE MAXIMUM RATINGS

Symbol

Parameter

Value

Unit

V 11

Input Voltage

V 11

Input Operating Voltage

V 20

Output DC Voltage

Output Peak Voltage at t = 0.1

-1

-5

s f = 200khz

I 20

Maximum Output Current

Internally Limited

V I

Boostrap Voltage

Boostrap Operating Voltage

V 11 +15

V 4 ,V 8

Input Voltage at Pins 4, 12

V 3

Reset Output Voltage

I 3

Reset Output Sink Current

V 2 ,V 7 ,V 9 ,V 10 Input Voltage at Pin 2, 7, 9, 10

I 2

Reset Delay Sink Current

I 7

Error Amplifier Output Sink Current

I 8

Soft Start Sink Current

P tot

Total Power Dissipation at T PINS

5 / 3.75(*)

1.3/1 (*)

at T amb =70

C (No copper area on PCB)

T J ,T stg

Junction and Storage Temperature

-40 to 150

(*) SO-20

PIN CONNECTION (top view)

THERMAL DATA

Symbol

Parameter

Powerdip

SO-20

R th j-pins

R th j-amb

Thermal Resistance Junction-Pins

max

C/W

Thermal Resistance Junction-ambient

max

C/W

2/23

L4972A-L4972AD

PIN FUNCTIONS

N o

Name

Function

BOOTSTRAP

A C boot capacitor connected between this terminal and the output allows to drive

properly the internal D-MOS transistor.

RESET DELAY

A C d capacitor connected between this terminal and ground determines the reset

signal delay time.

RESET OUT

Open Collector Reset/power Fail Signal Output. This output is high when the supply

and the output voltages are safe.

RESET INPUT

Input of Power Fail Circuit. The threshold is 5.1V. It may be connected via a divider

to the input for power fail function. It must be connected to the pin 14 an external 30K

resistor when power fail signal not required.

5, 6

15, 16

GROUND

Common Ground Terminal

FREQUENCY

COMPENSATION

A series RC network connected between this terminal and ground determines the

regulation loop gain characteristics.

SOFT START

Soft Start Time Constant. A capacitor is connected between thi sterminal and ground

to define the soft start time constant.

FEEDBACK INPUT

The Feedback Terminal of the Regulation Loop. The output is connected directly to

this terminal for 5.1V operation; It is connected via a divider for higher voltages.

SYNC INPUT

Multiple L4972A’s are synchronized by connecting pin 10 inputs together or via an

external syncr. pulse.

SUPPLY VOLTAGE

Unregulated Input Voltage.

12, 19 N.C.

Not Connected.

V ref

5.1V V ref Device Reference Voltage.

V start

Internal Start-up Circuit to Drive the Power Stage.

OSCILLATOR

R osc . External resistor connected to ground determines the constant charging current

of C osc .

OSCILLATOR

C osc . External capacitor connected to ground determines (with R osc ) the switching

frequency.

OUTPUT

Regulator Output.

3/23

L4972A-L4972AD

CIRCUIT OPERATION

The L4972A is a 2A monolithic stepdown switching

regulator working in continuousmode realized in the

new BCD Technology. This technology allows the

integration of isolated vertical DMOS power transi-

stors 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 suita-

ble for microprocessor based systems.

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 ran-

ge. Closing the loop directly gives an output vol-tage

of 5.1V, higher voltages are obtained by inserting a

voltage divider.

At turn on, output overcurrents 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 sensed by a internal

metal resistor 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 flop and the power

DMOS will again conduct. This current protection

method, ensuresa constant current output when the

system is overloaded or short circuited and limits the

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 programmed by a external capacitor on the de-

lay pin. When the supply voltage falls below the

threshold or the output voltage goes below 5V, the

reset output 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.

2%, soft start, undervoltage lockout, 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 hy-

sterysis, this thresholdporvides a correct voltage for

the driving stage of the DMOS gate and the hyste-

rysis prevents instabilities.

An external bootstrap capacitor charge to 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 swit-

ching 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 produced by comparing

the output voltage with the precise5.1V ± 2% on chip

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 mul-

tiple pulsing within a period even in noisy

environments.

The gain and stability of the loop can be adjusted by

4/23

BLOCK DIAGRAM

The block diagram shows the DMOS power tran-

sistors and the PWM control loop. Integrated fun-

ctions include a reference voltage trimmed to 5.1V

L4972A-L4972AD

Figure 1 : Feedforward Waveform.

Figure 2 : Soft Start Function.

Figure 3 : Limiting Current Function.

5/23

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