SC2440TETRT Semtech, SC2440TETRT Datasheet - Page 11

• Manufacturer Part Number: SC2440TETRT
• Description: IC SW REG DUAL 2.5MHZ 16-TSSOP
• Manufacturer: Semtech
• Type: Step-Down (Buck)r

Specifications of SC2440TETRT

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 2
• Current - Output: 2A
• Frequency - Switching: 1.4MHz
• Voltage - Input: 2.8 ~ 20 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Power - Output: -
• Other names: SC2440TETR

output voltage will not be able to reach its set value in
continuous-conduction mode.
Example
of a dual 3.3V to 1.8V and 3.3V to 2.5V switching
regulator using the SC2440.
Assuming that V
(10% low line), the duty ratios
and 2.5V converters can be calculated using (2).
The maximum operating frequencies of the 1.8V and the
2.5V converters are therefore
Transient headroom requires that channel frequency be
lower than 580KHz.
External Synchronization
The SYNC input buffer is positive-edge triggered and TTL-
compatible (
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
that an external clock with frequency ranging from slightly
below twice to at least 3.5 times the channel
running frequency is capable of locking the master
oscillator. To ensure frequency locking, the external clock
frequency should be at least twice
running channel
synchronizing clock should not be higher than 1.6 times
twice
POWER MANAGEMENT
Applications Information
120
1
2005 Semtech Corp.
D
D
D
ns
1
2
2
:
. 2
. 2
580
channel frequency. The frequency of the
Determine the maximum operating frequency
97
97
1
2
V
KHz
8 .
5 .
IL
. 0
. 0
D
= 0.45V, V
45
45
respectively. .
0
channel frequency. Bench test shows
. 0
. 0
8 .
45
45
V
. 0
. 0
and
25
25
CESAT
V
IH
. 0
. 0
twice the highest
71
= 0.25V and V
93
D
2
1
120
1
V
.
and
). The free-running
D
ns
1
D
channel free-
highest free-
2
2
4 .
of the 1.8V
MHz
IN
= 2.97V
and
11
the set frequency of master oscillator because the
amplitudes of the internal sawtooth ramp and slope
compensation ramp will both be significantly reduced.
Example: Choose the value of R
synchronize the SC2440 to 2MHz per channel
The required synchronizing clock frequency = 2 times
the channel frequency = 4MHz.
For a given R
has a tolerance of ±15%.
Set the nominal free-running channel
Looking up the graph “Channel Frequency vs. R
the Typical Characteristics, R
frequency of 1.73MHz.
With ±15% tolerance, the set channel frequency can vary
from
Therefore
Inductor Selection
The inductor ripple current I
step-down converter in continuous-conduction mode is
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
a good compromise. Inductors so chosen are optimized
2
. 1
I
I
MHz
L
L
15
between 25-40% of the peak inductor current limit is
Lowest
V (
. 0
Synchroniz
OUT
85
. 1
73
Free
. 1
V
fL
MHz
OSC
D
73
)(
, the free-running channel
1
running
ing
to ensure locking.
) D
. 1
Frequency
47
V (
MHz
Frequency
OUT
V (
L
to
OSC
IN
for a non-synchronous
V
channel frequency to
D
= 9.31K
. 1
)(
V
channel frequency
V
15
D
IN
OSC
. 1
channel.
2
47
www.semtech.com
SC2440
V
. 1
V
to externally
CESAT
OUT
73
. 1
for a set
)
36
fL
V
2
CESAT
OSC
MHz
.
” in
(3)
)
.