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PDF SC2440 Data sheet ( Hoja de datos )
| Número de pieza | SC2440 | |
| Descripción | 2.5 MHz Dual Switching Regulator | |
| Fabricantes | Semtech Corporation | |
| Logotipo |  |
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POWER MANAGEMENT
Description
The SC2440 is an adjustable frequency dual current-
mode switching regulator with 2A integrated switches.
Its high frequency operation allows the use of small
inductors and capacitors, resulting in very compact power
supplies. The SC2440 is suitable for next generation XDSL
modems requiring operating frequencies in excess of 1.5
MHz. The two channels operate at 180° out of phase
for reduced input voltage ripples. Separate soft start/
shutdown pins allow independent control and output
sequencing for latch-up prevention. The SC2440 can also
be externally synchronized up to 2.5 MHz per channel.
Current-mode PWM control allows fast transient
response with simple loop compensation. Cycle-by-cycle
current limiting and hiccup overload protection reduce
power dissipation during overload.
SC2440
2.5 MHz Dual Switwcwhw.iDnatgaShReeet4Ug.cuomlator
with Integrated 2A Switches
Features
u Up to 2.5 MHz/Channel Programmable Switching
Frequency
u Fixed Frequency Current-mode Control
u Wide Input Voltage Range 2.8V to 20V
u Out of Phase Switching Reduces Ripple
u Cycle-by-cycle Current-limiting
u Independent Shutdown/soft-start Pins
u Independent Hiccup Overload Protection
u Independent Power-Good Indicators
u Two 2A Integrated Switches
u External Synchronization
u Thermal Shutdown
u Thermally Enhanced 16-pin TSSOP Package
Applications
u XDSL and Cable Modems
u Set-up Boxes
u Point of Load Applications
u CPE Equipment
u DSP Power Supplies
u Disk Drives
Typical Application Circuit
R6
100K
R5 C5
FB1
PGOOD1 BOOST1
15.4K C6 470pF
C7 10pF
22nF
C10
R9
15K
22nF C9
R7 10pF C8
24.3K 220pF
COMP1
SW1
SS1 SC2440
SYNC
ROSC
IN
VIN
12V
C15
10µF
SS2
COMP2
SW2
FB2 BOOST2
GND PGOOD2
C4
0.1µF
R8
100K
D3
C2 1N4148
0.1µF
L1
3.3µH
D1
UPS120
C1
10µF
R1
23. 3K
R2
10K
D2
UPS120
L2
4.4µH
D4
1N4148
C3
10µF
R3
40. 2K
R4
10K
OUT1
3.3V /2A
OUT2
5V/2 A
L1: Sumida CR43
L2: F alco D04012
Figure 1. 1.3MHz 12V to 3.3V and 5V Step-down Converter
Revision: March 5, 2007
1
Efficiency vs Load Current
95
VIN = 12V
90
VOUT2 = 5V
85
80
VOUT1 = 3.3V
75
70
65
60
55
50
0
0.5 1 1.5
Load Current (A)
2
www.semtech.com
1 page  
POWER MANAGEMENT
Block Diagrams
PGOOD1
14
COMP1
15
FB1
16
SS1
13
-
+
POWER
GOOD
100mV
-
EA
+
SC2440
www.DataSheet4U.com
+
PWM
-
+
Σ
+
SLOPE
COMP 1
+
ISEN
-
+
ILIM
-
20mV
BOOST1
1
3 IN
7.7mΩ
S
Q
R
POWER
TRANSISTOR
FB1
2 SW1
Soft-Start
1V 0.74V
And
O ve rloa d
OVLD
SS2
REFERENCE
& THERMAL
FAULT
Hiccup
Control 1
12 SHUTDOWN
SLOPE SLOPE
COMP 1 COMP 2
ROSC
5
SYNC
4
OSCILLATOR
SLOPE COMP
CLK1
FREQUENCY
DIVIDER CLK2
Figure 2. SC2440 Functional Diagram (One of Two Converters Shown)
FB
0.74V
SS
+
-
1.8µA
1V/2 V
S
Q
R
OVLD
FAULT
2.6µA
Figure 3. Details of the Soft-Start and Overload Hiccup Control Circuit
© 2005 Semtech Corp.
5
www.semtech.com
5 Page 
SC2440
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POWER MANAGEMENT
Applications Information
output voltage will not be able to reach its set value in
continuous-conduction mode.
the set frequency of master oscillator because the
amplitudes of the internal sawtooth ramp and slope
compensation ramp will both be significantly reduced.
Example: Determine the maximum operating frequency
of a dual 3.3V to 1.8V and 3.3V to 2.5V switching
regulator using the SC2440.
Assuming that VD = 0.45V, VCESAT = 0.25V and VIN = 2.97V
(10% low line), the duty ratios D1 and D2 of the 1.8V
and 2.5V converters can be calculated using (2).
D1
=
1.8 + 0.45
2.97 + 0.45 − 0.25
= 0.71
D2
=
2.5 + 0.45
2.97 + 0.45 − 0.25
=
0.93 .
The maximum operating frequencies of the 1.8V and the
2.5V
converters are therefore
1 − D1
120ns
=
2.4MHz
and
1 − D2
120ns
=
580KHz
respectively.
Example: Choose the
synchronize the SC2440
value of
to 2MHz
pReOrSCchtaonneexlt.ernally
The required synchronizing clock frequency = 2 times
the channel frequency = 4MHz.
For
has
a
a
gtoivleernanRcOeSCo, fth±e15fr%e.e-running
channel
frequency
Set the nominal free-running channel frequency to
2MHz
1.15
= 1.73MHz
to ensure
locking.
LtfrhoeeoqkuTienynpgcicuyapol ftCh1he.a7gr3araMcptHehzri.“sCtihcasn, nReOlSCFr=eq9u.e3n1cKyΩvsf.oRr OaSC”seint
With ±15% tolerance, the set channel frequency can vary
from 0.85 ⋅(1.73) = 1.47MHz to 1.15 ⋅(1.73) = 2MHz .
Therefore
Transient headroom requires that channel frequency be
lower than 580KHz.
Synchronizing Frequency
Lowest Free − running Frequency
=
2
1.47
= 1.36 .
External Synchronization
Inductor Selection
The SYNC input buffer is positive-edge triggered and TTL-
compatible ( VIL < 0.8V and VIH > 2V ). The free-running
master oscillator generates a periodic sawtooth ramp
between two threshold voltages. A faster external clock
applied to the SYNC pin discharges the internal ramp
before it reaches its upper threshold, thus locking the
internal oscillator. As shown in Figure 2, the master
oscillator is being synchronized not the individual phases
(see Figure 2). The synchronizing frequency should be
twice the desired channel frequency. Bench test shows
that an external clock with frequency ranging from slightly
below twice to at least 3.5 times the channel free-
running frequency is capable of locking the master
oscillator. To ensure frequency locking, the external clock
frequency should be at least twice the highest free-
running channel frequency. The frequency of the
synchronizing clock should not be higher than 1.6 times
The inductor ripple
step-down converter
ciunrrceonnttin∆uILofuosr-caonndounc-tsiyonncmhroodneouiss
∆IL
=
(VOUT
+
VD )(1
fL
− D)
=
(VOUT
+ VD )(VIN − VOUT − VCESAT )
(VIN + VD − VCESAT )fL
(3)
where f is the switching frequency and L is the
inductance.
In current-mode control, the slope of the modulating
(sensed switch current) ramp should be steep enough
to lessen jittery tendency but not so steep that large
flux swing decreases efficiency. Inductor ripple current
∆aIgL oboedtwceoemnp2r5om-4i0s%e.
of the peak inductor
Inductors so chosen
current limit is
are optimized
© 2005 Semtech Corp.
11
www.semtech.com
11 Page | ||
| Páginas | Total 26 Páginas | |
| PDF Descargar | [ Datasheet SC2440.PDF ] | |
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