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| Número de pieza | CM2596-ADJ | |
| Descripción | 3A STEP-DOWN VOLTAGE REGULATOR | |
| Fabricantes | COSMOS WEALTH | |
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DATA
SHEET
20 AUGUST 2004
NO. _____
REV 1-04
PRELIMINARY
CM2596-XX SERIES • 3A STEP-DOWN VOLTAGE REGULATORS
CM2596-xx series
3A STEP-DOWN VOLTAGE REGULATOR
REPLACEMENT OF:
LM 2596
CONTENTS
GENERAL DESCRIPTION, FEATURES AND
APPLICATIONS
PIN DESCRIPTION
INTERNAL BLOCK DIAGRAM
ABSOLUTE MAXIMUM RATINGS
OPERATING CONDITIONS
ELECTRICAL CHARACTERISTICS
CM 2596-3.3
CM 2596-5.0
CM 2596-12.0
CM 2596-ADJ
ALL OUTPUT VOLTAGE VERSIONS
ELECTRICAL CHARACTERISTICS (NOTES)
STANDARD TEST CIRCUITS AND LAYOUT GUIDES
APPLICATION INFORMATION
INPUT CAPACITOR
Page
1
2
2
2
2
3
3
3
3
3
3
4
4
5
5
CONTENTS
FEEDFORWARD CAPACITOR
CATCH DIODE
INDUCTOR SELECTION
DISCONTINUOUS MODE OPERATION
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
OPEN CORE INDUCTORS
THERMAL CONSIDERATIONS
DELAYED STARTUP
UNDERVOLTAGE LOCKOUT
INVERTING REGULATOR SHUTDOWN METHODS
PHYSICAL DIMENSIONS AND MARKING DIAGRAM
TO-220-5
TO-220-5h
TO-263-5
ORDERING INFORMATION
Page
5
6
6
7
7
8
9
9
10
11
12
12
12
12
13
GENERAL DESCRIPTION
The CM2596 series of regulators are monolithic
integrated circuits that provide all the active
functions for a step-down (buck) switching
regulator, capable of driving a 3A load with
excellent line and load regulation. These devices
are available in fixed output voltages of 3.3V, 5V,
12V, and an adjustable output version.
Requiring a minimum number of external
components, these regulators are simple to use
and include internal frequency compensation,
and a fixed-frequency oscillator.
The CM2596 series operates at a switching
frequency of 150 kHz thus allowing smaller sized
filter components than what would be needed
with lower frequency switching regulators.
Available in a standard 5-lead TO-220 package
with several different lead bend options, and a 5-
lead TO-263 surface mount package.
A standard series of inductors are available from
several different manufacturers optimized for
use with the CM2596 series. This feature greatly
simplifies the design of switch-mode power
supplies.
Other features include a guaranteed ±4%
tolerance on output voltage under specified input
voltage and output load conditions, and ±15% on
the oscillator frequency. External shutdown is
included, featuring typically 80 µA standby
current. Self protection features include a two
stage frequency reducing current limit for the
output switch and an over temperature shutdown
for complete protection under fault conditions.
1
FEATURES
• 3.3V, 5V, 12V, and adjustable output versions
• Adjustable version output voltage range, 1.2V to
37V ±4% max over line and load conditions
• Available in TO-220 and TO-263 packages
• Guaranteed 3A output load current
• Input voltage range up to 40V
• Requires only 4 external components
• Excellent line and load regulation
specifications
• 150 kHz fixed frequency internal oscillator
• TTL shutdown capability
• Low power standby mode, IQ typically 80 µA
• High efficiency
• Uses readily available standard inductors
• Thermal shutdown and current limit
protection
APPLICATIONS
• Simple high-efficiency step-down (buck) regulator
• On-card switching regulators
• Positive to negative converter
1 page  
CM2596-XX SERIES • 3A STEP-DOWN VOLTAGE REGULATORS
APPLICATION INFORMATION
EXTERNAL COMPONENTS
INPUT CAPACITOR
CIN — a low ESR aluminum or tantalum bypass capaci-
tor is needed between the input pin and ground pin.
It must be located near the regulator using short
leads. This capacitor prevents large voltage tran-
sients from appearing at the input, and provides the
instantaneous current needed each time the switch
turns on.
The important parameters for the Input capacitor
are the voltage rating and the RMS current rating.
Because of the relatively high RMS currents flowing
in a buck regulator’s input capacitor, this capacitor
should be chosen for its RMS current rating rather
than its capacitance or voltage ratings, although the
capacitance value and voltage rating are directly
related to the RMS current rating.
The RMS current rating of a capacitor could be
viewed as a capacitor’s power rating. The RMS cur-
rent flowing through the capacitors internal ESR pro-
duces power which causes the internal temperature
of the capacitor to rise. The RMS current rating of a
capacitor is determined by the amount of current
required to raise the internal temperature approxi-
mately 10°C above an ambient temperature of 105°
C. The ability of the capacitor to dissipate this heat
to the surrounding air will determine the amount of
current the capacitor can safely sustain. Capacitors
that are physically large and have a large surface
area will typically have higher RMS current ratings.
For a given capacitor value, a higher voltage electro-
lytic capacitor will be physically larger than a lower
voltage capacitor, and thus be able to dissipate more
heat to the surrounding air, and therefore will have
a higher RMS current rating.
The consequences of operating an electrolytic ca-
pacitor above the RMS current rating is a shortened
operating life. The higher temperature speeds up the
evaporation of the capacitor’s electrolyte, resulting
in eventual failure.
Selecting an input capacitor requires consulting the
manufacturers data sheet for maximum allowable
RMS ripple current. For a maximum ambient tem-
perature of 40°C, a general guideline would be to
select a capacitor with a ripple current rating of
approximately 50% of the DC load current. For ambi-
ent temperatures up to 70°C, a current rating of 75%
of the DC load current would be a good choice for a
conservative design. The capacitor voltage rating
must be at least 1.25 times greater than the maxi-
mum input voltage, and often a much higher voltage
capacitor is needed to satisfy the RMS current re-
quirements.
A graph shown in Figure 3 shows the relationship
between an electrolytic capacitor value, its voltage
rating, and the RMS current it is rated for. These
curves were obtained from the Nichicon «PL» series
of low ESR, high reliability electrolytic capacitors
designed for switching regulator applications. Other
capacitor manufacturers offer similar types of ca-
pacitors, but always check the capacitor data sheet.
«Standard» electrolytic capacitors typically have
much higher ESR numbers, lower RMS current ratings
and typically have a shorter operating lifetime. Be-
cause of their small size and excellent performance,
2200
2000
1800
Typical RMS current
ratings of low ESR
Electrolytic Capacitors.
100 kHz, 105°C
1600
1400
680µF
470µF
330µF
270µF
1200
1000
800
180µF
120µF
600
400
200
0
10 20 30 40 50 60 70
CAPACITOR VOLTAGE RATING (V)
FIGURE 3
RMS CURRENT RATINGS FOR LOW ESR
ELECTROLYTIC CAPACITORS (TYPICAL)
surface mount solid tantalum capacitors are often
used for input bypassing, but several precautions
must be observed. A small percentage of solid tanta-
lum capacitors can short if the inrush current rating
is exceeded. This can happen at turn on when the
input voltage is suddenly applied, and of course,
higher input voltages produce higher inrush currents.
Several capacitor manufacturers do a 100% surge
current testing on their products to minimize this
potential problem. If high turn on currents are ex-
pected, it may be necessary to limit this current by
adding either some resistance or inductance before
the tantalum capacitor, or select a higher voltage
capacitor. As with aluminum electrolytic capacitors,
the RMS ripple current rating must be sized to the
load current.
FEEDFORWARD CAPACITOR
(ADJUSTABLE OUTPUT VOLTAGE VERSION)
CFF — a Feedforward Capacitor CFF, shown across R2
in Figure 1 is used when the output voltage is greater
than 10V or when COUT has a very low ESR. This ca-
pacitor adds lead compensation to the feedback loop
and increases the phase margin for better loop sta-
bility. For CFF selection, see the design procedure
section.
OUTPUT CAPACITOR
COUT — an output capacitor is required to filter the
output and provide regulator loop stability. Low im-
pedance or low ESR Electrolytic or solid tantalum
capacitors designed for switching regulator applica-
tions must be used. When selecting an output ca-
pacitor, the important capacitor parameters are: the
100 kHz Equivalent Series Resistance (ESR), the RMS
ripple current rating, voltage rating, and capaci-
tance value. For the output capacitor, the ESR value
is the most important parameter.
The output capacitor requires an ESR value that has
an upper and lower limit. For low output ripple volt-
age, a low ESR value is needed. This value is deter-
mined by the maximum allowable output ripple volt-
age, typically 1% to 2% of the output voltage. But if
the selected capacitor’s ESR is extremely low, there
is a possibility of an unstable feedback loop, result-
5
5 Page 
CM2596-XX SERIES • 3A STEP-DOWN VOLTAGE REGULATORS
APPLICATION INFORMATION (CONTINUED)
This example uses the CM2596-5.0 to generate a -5V
output, but other output voltages are possible by
selecting other output voltage versions, including
the adjustable version. Since this regulator topology
can produce an output voltage that is either greater
than or less than the input voltage, the maximum
output current greatly depends on both the input
and output voltage. The curve shown in Figure 16
provides a guide as to the amount of output load
current possible for the different input and output
1.8
1.6 L = 32µH
1.4
1.2
1.0
1.0
1.0
1.0
1.0
0 5 10 15 20
INPUT VOLTAGE (V)
-5V OUT
-12V OUT
-20V OUT
25
FIGURE 16 INVERTING REGULATOR TYPICAL LOAD CURRENT
voltage conditions. The maximum voltage appearing
across the regulator is the absolute sum of the input
and output voltage, and this must be limited to a
maximum of 40V. For example, when converting
+20V to -12V, the regulator would see 32V between
the input pin and ground pin. The CM2596 has a
maximum input voltage spec of 40V.
Additional diodes are required in this regulator con-
figuration.
Diode D1 is used to isolate input voltage ripple or
noise from coupling through the CIN capacitor to the
output, under light or no load conditions. Also, this
diode isolation changes the topology to closley re-
semble a buck configuration thus providing good
closed loop stability. A Schottky diode is recom-
mended for low input voltages, (because of its
lower voltage drop) but for higher input voltages, a
fast recovery diode could be used.
Without diode D3, when the input voltage is first
applied, the charging current of CIN can pull the
output positive by several volts for a short period of
time. Adding D3 prevents the output from going
positive by more than a diode voltage.
Because of differences in the operation of the in-
verting regulator, the standard design procedure is
not used to select the inductor value. In the major-
ity of designs, a 33 µH, 3.5A inductor is the best
choice. Capacitor selection can also be narrowed
down to just a few values.
Using the values shown in Figure 15 will provide
good results in the majority of inverting designs.
This type of inverting regulator can require rela-
tively large amounts of input current when starting
up, even with light loads. Input currents as high as
the CM2596 current limit (approx 4.5A) are needed
for at least 2 ms or more, until the output reaches
its nominal output voltage. The actual time de-
pends on the output voltage and the size of the
output capacitor. Input power sources that are cur-
rent limited or sources that can not deliver these
currents without getting loaded down, may not
work correctly. Because of the relatively high
startup currents required by the inverting topology,
the delayed startup feature (C1, R1 and R2) shown
in Figure 15 is recommended. By delaying the regu-
lator startup, the input capacitor is allowed to
charge up to a higher voltage before the switcher
begins operating. A portion of the high input cur-
rent needed for startup is now supplied by the input
capacitor (CIN). For severe start up conditions, the
input capacitor can be made much larger than nor-
mal.
INVERTING REGULATOR SHUTDOWN
METHODS
To use the ON/OFF pin in a standard buck configu-
ration is simple, pull it below 1.3V ( at 25°C, refer-
enced to ground) to turn regulator ON, pull it above
1.3V to shut the regulator OFF. With the inverting
configuration, some level shifting is required, be-
cause the ground pin of the regulator is no longer at
ground, but is now setting at the negative output
voltage level. Two different shutdown methods for
inverting regulators are shown in Figure 17 and 18.
+V
OFF
0 ON
+VIN
"
#
D1
1N5823
SHUTDOWN
INPUT
"
R4
6.2k
CIN +
470µF
"VIN
1
CM2596
" "Q1 5 3
2N3906
ON/OFF
GND
#
R1
10k
#
-5V
# " VOUT
FIGURE 17
INVERTING REGULATOR GROUND REFERENCED
SHUTDOWN
INPUT VOLTAGE
+4.5V TO +25V "
D1
1N5823
SHUTDOWN
+5V
0 OFF
INPUT
"
ON
R3
470
"# # VIN 1 CM2596-5.0
+ CIN
470µF
" "R1 5
3
# #47k ON/OFF GND
R2
47k
# # # " -5V
VOUT
INVERTING REGULATOR GROUND REFERENCED
FIGURE 18
SHUTDOWN USING OPTO DEVICE
11
11 Page | ||
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| PDF Descargar | [ Datasheet CM2596-ADJ.PDF ] | |
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