Williams 10 - LT Magazine 1991_2011.pdf
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The LT1074 Family of Step Down
Switching Regulators
by Jim Williams
A substantial percentage of DC
regulator requirements involve reduc-
tion or step down of a primary voltage.
Linear regulators do this, but they
don’t achieve the efficiency of switch-
ers. The theory supporting step down
or “buck” switching regulation is well
established, and has been exploited
for some time. However, conveniently
applied ICs allowing practical imple-
mentations haven’t been available. A
new power IC device, the LT1074,
permits broad application of step down
regulators with minimal complexity
and low cost. Further, more complex
step down regulator functions are
possible with it also.
The LT1074 is a 5A bipolar switch-
ing regulator requiring minimal exter-
nal parts for operation. While the block
diagram of Figure 1. reveals a complex
device, basic operation is still fairly
straightforward. A description of the
main circuit elements and their pin
functions is as follows.
The LT1074 uses a special con-
trolled saturation Darlington NPN
output switch, with the emitter out-
continued on page 8
INPUT SUPPLY
320 A
m
10 A
m
0.3V
+
6V
REGULATOR
AND BIAS
-POWER
SHUTDOWN
m
6V TO ALL
CIRCUITRY
–
CURRENT
LIMIT
COMP
0.035
+
CURRENT
LIMIT
SHUTDOWN
2.35V
C2
+
250
W
–
–
100
W
I *
LIM
SHUTDOWN*
4.5V
10k
EXTLM*
2.4V
–
SYNC
FREQ*
+
SYNC
FREQ BOOST
FREQ SHIFT
R
100kHz
OSCILLATOR
R/S
LATCH
G1
S
Q
SYNC
R
OUTPUT
VOLTAGE
MONITOR
V
IN
COMOUT*
+
400
15
W
W
Z
+
C1
ANALOG
MULTIPLIER
XY
Z
A1
ERROR
AMP
STATUS*
–
X
PULSE WIDTH
COMPARATOR
2.21V
–
SWITCH
OUTPUT
(V )
Y
SW
FB
V
20V (EQUIVALENT)
*AVAILABLE ONLY ON 11-PIN SIP PACKAGE
C
Figure 1. LT1074/LT1076 Block Diagram
Linear Technology Magazine • June 1991
7
continued from page 7
When the LT1074 (internal) switch
closes, input voltage appears at the
inductor, and current flowing through
the inductor-capacitor combination
builds over time. When the switch
opens, current flow ceases and the
magnetic field around the inductor
collapses. Faraday teaches that the
voltage induced by the collapsing mag-
netic field is opposite to the originally
applied voltage. As such, the voltage
at the inductor’s left end heads nega-
tive, and is clamped by the diode. The
charge accumulated on the capacitor
has no discharge path, leaving an
output DC potential. This potential is
lower than the input, because the
inductor limits current during the
switch on-time.
Ideally, there are no dissipative
elements in this voltage step down
conversion. Although the output volt-
age is lower than the input, there is
no energy lost in this conversion. In
practice, the circuit elements do have
losses, but step down efficiency is
still higher than with inherently dis-
sipative (e.g. voltage divider) ap-
proaches. In this circuit, feedback
additional new elements appear. The
RC components at the LT1074 VC
pin provide frequency compensation,
stabilizing the feedback loop. Output
sensing resistors R1/R2 are selected
to scale the output to the desired
voltage V
OUT
, generally as noted in
the figure, with values shown in this
case for 5V.
Performance wise, the circuit op-
erates over an input range of 10-40V,
and has a maximum output of 5A.
Efficiency is about 80% at a current
of 1A, while output ripple is about
25mV with the filtering as shown.
With these and other switching regu-
lators, power components are critical
to performance, and should be rated
for switching use at the currents
anticipated.
Regulated negative outputs with
the LT1074 are easily obtained also,
using a simple two terminal induc-
tor. The basic positive to negative
converter of Figure 3 demonstrates
this, essentially grounding the in-
ductor, steering diode current to what
is now a negative output. This design
accomplishes the plus-to-minus DC
level shift by connecting the
LT1074 GND pin direct to
the negative output, requir-
ing an isolated heat sink.
put at pin VSW. This switch uses an
isolated design, allowing voltage
swings up to 40V below the ground
pin. In addition, the switch also has
a continuous current monitor. The
oscillator of the LT1074 operates at
100kHz, driving the switch through a
control latch. Duty cycle control
comes from a pulse width compara-
tor, which in turn is driven by the
main error amplifier through the ana-
log multiplier. This multiplier allows
a loop gain independent of input volt-
age, optimizing transient response.
The error amp of the LT1074 com-
pares a sample of the output pre-
sented to the FB pin to an internal
2.21V (
2.5%) reference. Loop com-
pensation is accomplished by a simple
RC network at the amplifier output
(VC pin), to ground.
While the above describes the ba-
sic operational loop of the LT1074
design, accessory internal functions
also exist. These are I
LIM
, FREQ, STA-
TUS, COMOUT, SHUTDOWN and
EXTLIM pins (available only in the 11
pin package). As alluded, there are
multiple power packages
used with the LT1074., a 4
lead TO-3 (K), a 5 lead TO-
220 (T), and the 11 lead SIP
package (V), which permits
the optional clock synchro-
nization, micropower shut-
down, current limit pro-
gramming and other fea-
tures. The LT1074 is avail-
able in two basic voltage
grades, the LT1074 for
45V(max) inputs, and the
LT1074HV, usable to 64V.
There is also a 2.5A rated
device, the LT1076.
±
=
L1
50
m
V = 10V
TO 40V
V = 5V
AT 5A
IN
OUT
V
IN
V
SW
R1 **
2.8k
1%
LT1074
MBR745
Feedback is sensed from
the grounded positive out-
put terminal, and the regu-
lator again forces its feed-
back pin 2.21V above its GND
pin. Output voltage scaling
is numerically as in Figure 2,
with a negative sign. Circuit
ground is common to input
and output, making system
use easy.
Overall performance is as noted,
and is similar to the positive buck
converter of Figure 2, but with some
unique distinctions. On the plus side,
note that the input/output voltages
of this configuration are seen in se-
continued on page 9
FB
GND
V
C
R2**
2.21k
1%
R3
2k
+
+
+
C3*
100
m
C2
0.1 F
C1
500
m
m
* OPTIONAL - USE IF CONVERTER IS MORE THAN 2"
FROM RAW SUPPLY FILTER CAPACITOR
PULSE ENGINEERING, INC. #PE-92114
=
OUT
( )
R1
R2
** V = + 1 * 2.21
A3 • F2
Figure 2. LT1074 Step Down Regulator (5V output)
controls the switch, to regulate out-
put voltage. The switch on-time (e.g.
inductor charge time) is varied to
maintain the output against changes
in either input or loading.
With respect to a practical circuit
using the LT1074 regulator, some
Applications
Figure 2 is a practical LT1074 volt-
age step down or “buck” circuit, us-
ing minimum componentry. It closely
follows a voltage step down concep-
tual model, described as follows.
8
Linear Technology Magazine • June1991
continued from page 8
common. This option uses an op amp
as a precision feedback level shifter.
A1 facilitates loop closure, providing
a scaled inversion of the negative
output to the LT1074 FB pin. Preci-
sion resistors R1/R2 set negative
output voltage as noted (values shown
for a -5V output). The VC pin of the
LT1074 is left open, and the RC net-
work around A1 gives frequency com-
pensation.
Advantages of this circuit com-
pared to Fig. 3 is that the LT1074
package can directly contact a
grounded heat sink. Additionally, this
circuit permits ground referred ad-
dressing of the regulator’s control
pins. Disadvantages are that it re-
quires a higher minimum input volt-
age, plus an additional active device..
ries by the LT1074, as V
IN
. This has
the effect of allowing a very low abso-
lute level for the positive input, down
to as low as 5V, making the circuit an
efficient 5V to -5V power converter.
On the down side, note in this in-
stance the LT1074 control pins are
floating off ground, presenting some
potential problems with control in-
terfacing (when necessary).
Figure 4, another variant, is used
when it is desirable to operate the
case (GND) pin of the regulator at
Higher Output Currents
with Tapped Inductors
Buck (step-down) convertors have
a switch current at least as high as
regulator output current. This limits
LT1074 output current to 5A in the
simple buck convertor topology. A
slightly modified version (shown on
the data sheet) can double available
output current to 10A when input
voltage is a minimum of 20V. The
modified version uses a 3 to 1 tapped
inductor which generates current gain
in the inductor.
During switch “on” time current
flows through the entire inductor to
the output and can have a maximum
value of 5A. When the switch turns
off, the voltage at the tap flies nega-
tive and current flows to the output
through just 1/4 of the inductor.
Energy conservation requires that
this current be four times the current
which was flowing in the entire in-
ductor. Average current delivered to
the output is between 5A and 20A, as
determined by operating switch duty
cycle. For low input voltages, switch
duty cycle is very high, and maxi-
mum output current is only slightly
above 5A. For input voltage above
20V, duty cycle is low enough to
deliver 10A output current.
C3*
200
m
F
+
†
L1
22
m
H
R1
2.80k
12V TO 40V
V
IN
V
SW
LT1074
FB
GND
V
D1
MBR745
C
+
C1**
1000
C2
m
F
R2
2.21k
R3
††
–5V
1.5A
A3 • F3
*
OPTIONAL – USE IF CONVERTER IS MORE THAN 2"
FROM RAW SUPPLY FILTER CAPACITOR
LOWER OUTPUT RIPPLE CAN BE OBTAINED BY
PARALLELING SEVERAL LOWER VALUE CAPACITORS.
AN OUTPUT FILTER OF 5
m
H, 100
m
F WILL GIVE
20:1 RIPPLE ATTENUATION WITH AN ESR OF 0.1
W
ON THE 100
m
F CAPACITOR
PULSE ENGINEERING, INC. #PE-51590
MAXIMUM OUTPUT CURRENT IS 1.5A AT V
IN
= 5V
3A AT V
IN
= 15V AND 3.5A AT V
IN
= 30V
**
†
††
Figure 3. Positive to Negative Converter
MBR745
V = –5V
OUT
=
L1
55
m
12V
INPUT
V
IN
V
SW
1000
m
F
+
The voltage on the LT1074 switch
pin flies negative to about 17V during
switch “off” time due to the trans-
former action of the inductor. Leak-
age inductance, however, would
cause, at switch turn-off, the switch
voltage to briefly fly negative without
limit. Clamps are needed to protect
the LT1074.
0.33
m
F
LT1074
4.7k
R1*
22.6k
1%
R2
1
0k 1%
V
C
FB
GND
12V INPUT
–
IN4148
LT1006
NC
+
=
A1
L1 = PULSE ENGINEERING, INC. #PE-92116
( )
R1
R2
* V = – 2.21
OUT
*
A3 • F4
Figure 4. Positive to Negative Converter with Op Amp Level Shift
Linear Technology Magazine • June 1991
9
An LT1123 Ultra Low Dropout 5V
Regulator
Jim Williams and Dennis O'Neill
Switching regulator post regula-
tion, battery powered apparatus, and
other applications often require low
V
IN
-V
OUT
, or dropout, linear regula-
tors. For post regulators this is needed
for high efficiency. In battery circuits
lifetime is significantly effected by regu-
lator dropout. The LT1123, a new low
cost reference/control IC, is designed
specifically for cost-effective duty in
such applications. Used in conjunc-
tion with a discrete PNP power tran-
sistor, the 3 lead TO-92 unit allows
very high performance positive leg
regulator designs. The IC contains a
5V bandgap reference, error ampli-
fier, NPN darlington driver, and cir-
cuitry for current and thermal limit-
ing.
A low dropout example is the simple
5V circuit of Fig. 1, using the LT1123
and an MJE1123 silicon PNP. In op-
eration, the LT1123 sinks Q1 base
current through the DRIVE pin, to
servo control the FB (feedback) pin to
5V. R1 provides pull-up current to
turn Q1 off, and R2 is a drive limiter.
The 10
more than 6 inches from the input
source, the optional 10
regulation are typically within 5 milli-
volts, and initial accuracy is typically
inside 1%. Additionally, the regulator
is fully short circuit protected, with a
no load quiescent current of 1.3mA.
Figure 2 shows typical circuit drop-
out characteristics, in comparison with
other IC regulators. Even at 5A the
LT1123/MJE1123 circuit dropout is
less than 0.5V, decreasing to only
50mV at 1A. Totally monolithic regu-
lators cannot approach these figures,
primarily because their power tran-
sistors do not offer the MJE1123 com-
bination of high beta and excellent
saturation. For example, dropout is
ten times lower than in 138 types, and
significantly better than all the other
IC types. Because of Q1’s high beta,
base drive loss is only 1-2% of output
current even at high output currents.
This maintains high efficiency under
the low V
IN
-V
OUT
conditions the circuit
will typically see. As an exercise, the
MJE1123 was replaced with a 2N4276
germanium device. This provided even
lower dropout performance, but limit-
ing couldn't be production guaran-
teed without screening.
F input ca-
pacitor (C
IN
) should be added.
Normally, such configurations re-
quire external protection circuitry.
Here, the MJE1123 has been coopera-
tively designed by Motorola and LTC
for use with the LT1123. The device is
specified for saturation voltage for
currents up to 4 amperes, with base
drive equal to the minimum LT1123
drive current specification. In addi-
tion, the MJE1123 is specified for
min/max beta at high current. Be-
cause of this factor and the defined
LT1123 drive, simple current limiting
is practical. In limit, excessive output
current causes the LT1123 to drive Q1
hard until the LT1123 current limits.
Maximum circuit output current is
then a product the LT1123 current
and the beta of Q1. The foldback char-
acteristic of the LT1123’s drive cur-
rent combined with the MJE1123 beta
and safe area characteristics provide
reliable short circuit limiting. Thermal
limiting can also be accomplished, by
mounting the active devices with good
thermal coupling.
Performance of the circuit is no-
table, as it has lower dropout than any
monolithic regulator. Line and load
m
F output capacitor (Cout) pro-
vides frequency compensation. The
LT1123 is designed to tolerate a wide
range of capacitor ESR so that low
cost aluminum electrolytics can be be
used for C
OUT
. If the circuit is located
m
3.0
2.5
Q1
M
JE112
3
LT138
+5V
OUT
INPUT
2.0
+
+
C
1
IN
C
10 F
R1
600
OUT
m
m
F
1.5
W
LT1084
R2
20
1.0
W
LT1123/2N4276
LT1185
DRIVE
0.5
LT1123/MJE1123
U1
LT1123
FB
0
0
1
2
3
4
5
GND
OUTPUT CURRENT (A)
* = OPTIONAL (SEE TEXT)
MJE 1123 = MOTOROLA
A8 • F2
A8 • F1
Figure 2. LT1123 regulator dropout voltage
vs. output current
Figure 1. The LT1123 5V regulator features ultra-low dropout.
14
Linear Technology Magazine • June1991
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