L4972A
L4972AD
June 2000
2A SWITCHING REGULATOR
.
2A OUTPUT CURRENT
.
5.1V TO 40V OUTPUT VOLTAGE RANGE
.
0 TO 90% DUTY CYCLE RANGE
.
INTERNAL FEED-FORWARD LINE REG.
.
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-
TERETIC TURN-ON
.
PWM LATCH FOR SINGLE PULSE PER PE-
RIOD
.
VERY HIGH EFFICIENCY
.
SWITCHING FREQUENCY UP TO 200KHz
.
THERMAL SHUTDOWN
.
CONTINUOUS MODE OPERATION
DESCRIPTION
The L4972Ais a stepdownmonolithicpower switch-
ing regulator delivering 2A at a voltagevariable 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
BLOCK DIAGRAM
POW ERDIP
(16 + 2 + 2)
the L4972A include reset and power fail for micro-
processors, feed forward line regulation, soft start,
limiting current and thermal protection. The device
is mountedin a Powerdip16 + 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.
ORDERING NUMBERS : L4972A (Powerdip)
L4972AD (SO20)
1/23
This is advanced information on a new product now in development or undergoing evaluation. Details are subject to change without notice.
SO20
MULTIPOWER BCD TECHNOLOGY
PIN CONNECTION (top view)
THERMAL DATA
Symb ol
Parameter
Po werdi p
SO- 20
R
th j-pins
R
th j-amb
Thermal Resistance Junction-Pins
max
Thermal Resistance Junction-ambient
max
12
C/W
60
C/W
16
C/W
80
C/W
ABSOLUTE MAXIMUM RATINGS
Symbo l
Parameter
Valu e
Unit
V
11
Input Voltage
55
V
V
11
Input Operating Voltage
50
V
V
20
Output DC Voltage
Output Peak Voltage at t = 0.1
s f = 200khz
-1
-5
V
V
I
20
Maximum Output Current
Internally Limited
V
I
Boostrap Voltage
Boostrap Operating Voltage
65
V
11
+ 15
V
V
V
4
, V
8
Input Voltage at Pins 4, 12
12
V
V
3
Reset Output Voltage
50
V
I
3
Reset Output Sink Current
50
mA
V
2
, V
7
, V
9
, V
10
Input Voltage at Pin 2, 7, 9, 10
7
V
I
2
Reset Delay Sink Current
30
mA
I
7
Error Amplifier Output Sink Current
1
A
I
8
Soft Start Sink Current
30
mA
P
tot
Total Power Dissipation at T
PINS
90
C
at T
amb
= 70
C (No copper area on PCB)
5 / 3.75(*)
1.3/1 (*)
W
W
T
J
, T
stg
Junction and Storage Temperature
-40 to 150
C
(*) SO-20
L4972A-L4972AD
2/23
PIN FUNCTIONS
N
o
Name
F unctio n
1
BOOTSTRAP
A C
boot
capacitor connected between this terminal and the output allows to drive
properly the internal D-MOS transistor.
2
RESET DELAY
A C
d
capacitor connected between this terminal and ground determines the reset
signal delay time.
3
RESET OUT
Open Collector Reset/power Fail Signal Output. This output is high when the supply
and the output voltages are safe.
4
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
7
FREQUENCY
COMPENSATION
A series RC network connected between this terminal and ground determines the
regulation loop gain characteristics.
8
SOFT START
Soft Start Time Constant. A capacitor is connected between thi sterminal and ground
to define the soft start time constant.
9
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.
10
SYNC INPUT
Multiple L4972A's are synchronized by connecting pin 10 inputs together or via an
external syncr. pulse.
11
SUPPLY VOLTAGE
Unregulated Input Voltage.
12, 19
N.C.
Not Connected.
13
V
ref
5.1V V
ref
Device Reference Voltage.
14
V
start
Internal Start-up Circuit to Drive the Power Stage.
17
OSCILLATOR
R
osc
. External resistor connected to ground determines the constant charging current
of C
osc
.
18
OSCILLATOR
C
osc
. External capacitor connected to ground determines (with R
osc
) the switching
frequency.
20
OUTPUT
Regulator Output.
L4972A-L4972AD
3/23
The L4972A is a 2A monolithic stepdown switching
regulatorworking in continuousmode realized inthe
new BCD Technology. This technology allows the
integration of isolated vertical 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 externalbootstrapcapacitor chargeto 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
theoutputvoltagewiththe 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
multiple pulsing within a period even in noisy envi-
ronments.
The gain and stability of 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 output vol-
tage of 5.1V, higher voltages areobtained by insert-
ing a voltage divider.
At turn on, outputovercurrents are prevented by 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
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 flop and the power
DMOS will again conduct. This current protection
method,ensuresa constantcurrent outputwhen the
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 programmed by a externalcapacitor 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.
CIRCUIT OPERATION
L4972A-L4972AD
4/23
Figure 1 : Feedforward Waveform.
Figure 2 : Soft Start Function.
Figure 3 : Limiting Current Function.
L4972A-L4972AD
5/23
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