DN53 - High Performance Frequency Compensation Gives DC-to-DC Converter 75µs Response With High Stability

advertisement
High Performance Frequency Compensation Gives DC-to-DC
Converter 75µs Response with High Stability – Design Note 53
Ron Vinsant
This Design Note describes four high performance,
low cost, 1.75A step-down converter circuits based on
the LT®1076 five terminal switching regulator. All four
circuits have exceptional transient response; indeed,
it is superior to most three terminal linear regulators.
Transient response is important to loads that are
switched on and off or that require high peak currents.
Examples are digital circuits that are turned on and off,
disk drive motors, stepper motors and linear amplifiers.
The frequency compensation schemes shown in this
Design Note, when compared to the usual R and C technique, allow greater variation in output capacitor ESR
without causing stability problems. This is important
in applications where wide temperature variations occur (which changes capacitor ESR) such as industrial
control, automotive and military, and when the use
of multiple capacitor vendors with different capacitor
specifications is required.
Phase margin is always more than 50° and gain margin
is a minimum of 18dB. Bode plots are available from
the factory upon request.
The efficiency of these circuits is typically 80% with
output ripple less than 50mV. Input voltages can be
as high as 45V. Input ripple rejection is an exceptional
60dB due to the feedforward architecture of the LT1076.
These circuits use a small number of external parts that
are available off-the-shelf.
Many of the problems associated with five terminal
switching regulators have been addressed by these
circuits. Start-up overshoot is less than 5% with the
optional soft start circuit. On recovery from a short
circuit, a 10% overshoot is realized.
For a 15V output, line regulation is typically 0.06%
(10mV) for a 20V to 40V input voltage change. Load
regulation is difficult to measure; in fact, it is only 1mV
to 2mV at the point of regulation. This applies to all
output voltages.
10/91/153_conv
Each circuit has been built in our lab and evaluated for
stability, temperature, component life and tolerance. Two
circuit options are shown: a simple soft start circuit and
an output voltage adjustment (see Figure 1).
Inductors
The inductors shown in Table 3 are designed around
two different core materials. The first is powered iron
based for low cost. The second is tape wound steel for
smaller size and higher efficiency but greater cost. For
rapid evaluation of these circuits, powered iron cores
are available in sample quantities from Micrometals
at 1-800-356-5977. Completed inductors are available
from Coiltronics at 305-781-8900.
Capacitors
Ripple current in the output capacitor is 150mA maximum with the input voltage at 40V and maximum load.
At 35°C ambient estimated life-time with the specified
capacitor and full load is 28 years.
The input capacitor, which undergoes higher stress,
has a ripple current of 830mA maximum at 14V input
and maximum load. The life-time of this capacitor is
14 years at 35°C. If the ambient temperature is higher,
the life of the capacitor will be cut in half for every
10°C increase. The ESR specification affects the output
ripple as well as frequency compensation. Its value of
capacitance is not critical.
The capacitors in the frequency compensation network
should be at least X7R ceramic, never Z5U, and, if
broad temperature operation is expected, polyester or
polycarbonate film caps should be used.
Manufacturing technologies must also be taken into
account. If an IR furnace is used for soldering, use
only ceramic capacitors. A wave or hand soldering
operation is suggested for both film and electrolytic
capacitors. This is an area of continuing development
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
so be sure to contact the capacitor manufacturers for
temperature profiles.
Layout
In order to achieve proper performance it is important
to lay out the circuit as the schematic indicates. Use
a single point ground at the output of the converter
as shown. The term “short” indicates that the trace
should be as short as possible between the two points
shown. These traces should have a minimum width of
0.2 inches in 2oz. copper for a length of less than 1.5
inches. Traces longer than this should be avoided on
the heavily shaded portions of the schematic.
015*0/"-40'545"35'03
07&34)0050/45"3561
+VIN
5
VIN
+
D3
/
C3
Q'
R5
8
R2
8
C6
Q'
C7
˜'
.,4
&43Ā
4&&/05&
+V065
%
FB
GND VC
2
3
Heat Sinking
Any heatsink of 30°C/W (~2 square inches) or lower
will keep the LT1076 at an acceptable temperature up to
a 70°C ambient. See LT1076 data sheet for further
information.
#0-%-*/&*/%*$"5&4)*()$633&/51"5)4
4)035-&/(5)4
-
4
V3
-5
D2
/
$
˜'
7
VSW
Output Adjustment
A potentiometer can be added to the output divider
string, provided the string does not change its overall
resistance value. A table showing resistance values is
shown with the schematic.
C4
SEE
53*.
015*0/
3
36k
8
5%
+
R4
L
8
1
015*0/"065165
70-5"(&
53*.
C2
˜'
7
&43Ā
4&&/05&
4*/(-&10*/5
GND
35/
35/
NOTE 1:%0/0546#4*5565&$0.10/&/548*5)065$0.1-&5&&7"-6"5*0/
NOTE 2:$"/%$.645#&Ā.*/&43"5300.5&.1&3"563&¡$
6/*5&%$)&.*$0/49&7#.9--413"(6&%'%.%
R2
8
5%
"
R3
8
R3
8
R4
L
8
R3
1
+5V 2.74k Ā
7 L Ā
7 L Ā
NOTE 3:"--$"14&9$&15$"/%$"3&8*."',$0393$&3".*$
œ50-&3"/$&8*."
%/t5"
Figure 1. High Performance DC-to-DC Converter
Table 1. Components
#
1
2
3
4
VIN
8V-20V
8V-40V
15V-40V
18V-40V
VOUT
+5V
+5V
+12V
+15V
e (%) at VIN
83% at 10
76% at 24
86% at 24
86% at 24
L (μH)
75
91
180
240
Table 2. Performance
#
1
2
3
4
VOUT
+5V
+5V
+12V
+15V
MIN
LOAD
0.200
0.175
0.175
0.175
REGULATION
(MIN TO MAX)
LOAD
LINE
0.1%
15mV
0.1%
15mV
0.1%
15mV
0.1%
15mV
D1
MBR330P
MBR350
MUR415
MUR415
R2 (5%)
1.5k
1.5k
1.2k
1.2k
R3 (1%)
2.80k
2.80k
9.79k
12.7k
C4 (10%)
0.0068μF
0.0068μF
0.01μF
0.01μF
Table 3. Inductor
RIPPLE
REJECTION
50Hz-400Hz
60dB
60dB
60dB
60dB
OUTPUT
RIPPLE
50mV
50mV
50mV
50mV
L
NUMBER
COILTRONICS
SMALLER
(μH) TURNS
CORE
P/N
TOROID
75
37 #18 T68-52A CTX75-2-52 CTX75-2-KM
91
38 #18 T80-52B CTX91-2-52 CTX91-2-KM
180
53 #18 T80-52B CTX180-2-52 CTX180-2-KM
240
61 #18 T80-52B CTX240-2-52 CTX240-2-KM
Note 1: ∆L with DC current is 20% max.
Note 1: VIN = 24V except #1 at 14V.
Note 2: Temperature = 25°C.
Note 3: Periodic and random deviation (P.A.R.D.).
With optional adjustment = ±2.5%.
Without optional adjustment = ±4.5%.
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn53f_conv BA/GP 1091 180K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1991
Similar pages