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INTRODUCTION TO A 10A MONOLITHIC SWITCHING REGULATOR IN MULTIPOWER-BCD TECHNOLOGY 1738 - Datasheet Archive [ST]

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AN487

INTRODUCTION TO A 10A MONOLITHIC SWITCHING REGULATOR IN MULTIPOWER-BCD TECHNOLOGY

Document Number:          1738
Date Update:              8/6/95
Pages:                    12
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APPLICATION NOTE
INTRODUCTION TO A 10A MONOLITHIC SWITCHING
REGULATOR IN MULTIPOWER-BCD TECHNOLOGY
by C.Diazzi
Switched mode techniques led to the develop-
ment of high efficiency circuits offering space sav-
ing and a reduction in costs, mainly of the
heatsink and output LC filter. For these applica-
tions a new technology, called MULTIPOWER-
BCD, has been developed which allows the inte-
gration on the same chip of isolated power DMOS
elements, Bipolar transistors and CMOS logic.
The technology is particularly suitable for the
problems rising in the switch mode field, due to
the characteristics of high efficiency, fast switch-
ing speed, no secondary breakdown of the power
DMOS element.
The great flexibility that we have at our disposal
for the choice of the signal and driving sections
components allows optimization and compact-
ness of the system. With MULTIPOWER-BCD it
has been possible to implement the family L497X,
a new series of fully integrated switching regula-
tors suitable for DC-DC converters working in
Buck configuration. The complete family consists
of five devices which differ each other only by the
output current value (2A, 3.5A, 5A, 7A, 10A) they
can deliver to the load. The devices rated at 2A
and 3.5A are assembled in Power Dip (16+2+2),
while the others are assembled in the Multiwatt15
package. Each device integrates a DMOS output
power stage, a control section, limiting current
and supervisor functions like Reset and Power
Fail signal for microprocessors applications.
Output voltage can be adjusted starting from the
internal reference voltage (5.1V) up to 40V, allow-
ing a maximum output power of 80W for the 2A
version and of 400W for the 10A version. Maxi-
mum operating supply voltage is 55V.
THE TECHNOLOGY
The technology architecture is based on the verti-
cal DMOS silicon gate process that allows a
channel length of 1.5 micron ; using a junction
isolation technique it has been possible to mix on
the same chip Bipolar and CMOS transistors
along with the DMOS power components (Fig. 2).
Figure 1 shows how this process brings a rapid
increase in power IC complexity compared to
conventional bipolar technology.
AN487/0592
The L497X series of high current switching regulator ICs exploit Multipower-BCD technology to
achieve very high output currents with low power dissipation - up to 10A in the Multiwatt power
package and 3.5A in a DIP package .
Figure 1:
BCD process and increase in power ICs complexity.
1/12
In the 70's class B circuits and DC circuits al-
lowed output power in the range of 70W. By 1980
,with the introduction of switching techniques in
power ICs, output powers up to 200W were
reached ; with BCD technology the output power
increased up to 400W.
FUNCTIONS AND BLOCK DIAGRAM
The complete block diagram of the high power
L4970A is shown in fig.3. Each block is analysed
in the following.
POWER SUPPLY
The device is provided with an internal stabilized
power supply ( Vstart =12V ), that provides the
supply voltage to the analog and digital control
blocks and also the supply voltage to the boot-
strap section. The Vstart voltage supplies also the
internal Reference Voltage section that provides
accurate 5.1V voltage to the control loop.
Through trimming techniques the 5.1V reference
is within +- 2% limits.
OSCILLATOR and FEDFORWARD
The oscillator block (fig.4) generates the sawtooth
Figure 2:
Cross section of the BCD mixed technology.
Figure 3:
Block diagram of the 10A monolithic regulator L4970A.
APPLICATION NOTE
2/12
waveform that sets the switching frequency of the
system. The signal, compared with the output
voltage of the error amplifier, generates the PWM
signal to be sent to the power output stage. The
oscillator features a voltage feed-forward tech-
nique which is completely integrated and doesn't
require any external component. Feed-forward
function works in the supply voltage range 15-
45V. The rate of increase of the sawtooth wavw-
form is directly proportional to the input voltage
Vcc. As Vcc increases, the output pulse-width
(transistor on-time) decreases in such a manner
as to provide a constant "volt-second' product to
the inductance(fig.5).
From fig.5 it is shown that the duty cycle changes
due to the ramp increase when Vcc increases.
The error amplifier output doesn't have to change
to keep the loop in regulation. This feature in-
creases significantly the line regulation perform-
ance.
A resistor, between Rosc and GND , defines a
current that is mirrored internally to charge the os-
cillator capacitor on the Cosc pin. The voltage at
pin.Rosc is a function of Vcc value for the imple-
mentation of the feed-forward function (oscillator
slope proportional to Vcc). A comparator is sens-
ing the voltage across Cosc capacitor and dis-
carge it when the ramp exceedes an upper
threshold proportional to Vcc for the implementa-
tion of the feed-forward function. The Cosc dis-
charge current is internally controlled at a value of
about 20 mA. The lower threshold of the compa-
rator is about 1.3V (2VBE). Here are reported ba-
sic equations for the oscillator:
I
CHARGE
=
V
CC
-
9V
BE
R
osc.
for 15V  60V at I
D
= 1mA V
GS
= 0V
R
DS(ON)
= 100m
W
at I
D
= 10A T
j
= 25
5
C V
GS
= 10V
R
DS(ON)
= 150m
W
at I
D
= 10A T
j
= 150
5
C V
GS
= 10V
V
TH
= 3V at I
D
= 1mA
Figure 26:
Gate-charge curve for the power
DMOS.
APPLICATION NOTE
11/12
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for the
consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No
license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifications mentioned
in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. SGS-
THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without express
written approval of SGS-THOMSON Microelectronics.
{ 1995 SGS-THOMSON Microelectronics - All Rights Reserved
SGS-THOMSON Microelectronics GROUP OF COMPANIES
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APPLICATION NOTE
12/12

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