SC2440 SEMTECH [Semtech Corporation], SC2440 Datasheet - Page 9

• Manufacturer Part Number: SC2440
• Description: 2.5 MHz Dual Switching Regulator with Integrated 2A Switches
• Manufacturer: SEMTECH [Semtech Corporation]

Setting the Output Voltage
The regulator output voltage is set with an external
resistive divider (Figure 4) with its center tap tied to the
FB pin.
The percentage error due the input bias current of the
error amplifier is
Example: Determine the output voltage error of a
From (1),
This error is at least an order of magnitude lower than
the ratio tolerance resulting from the use of 1% resistors
in the divider string.
Table 1. The 12V to 5V Converter in Figure 1 is modified
© 2005 Semtech Corp.
POWER MANAGEMENT
Applications Information
V
OUT
R
f
∆
R
(
∆
V
0
1
2
M
1
V
V
1
5 .
3 .
5 .
OUT
V
=
H
OUT
OUT
=
) z
OUT
=
5
Figure 4. V
51
R
V
to run at Different Frequencies.
=
2
R
=
converter with
1 .
V (
9
5
1
4
−
3
5
0 .
K ( Ω)
−
K
6 .
0 .
OUT
2
15
15
Ω
nA
−
⋅
nA
5 (
1
) 1
2
0
OUT
4
7 .
⋅
C (
VOUT
−
4 .
⋅
100
100
S (
o
4
is set with a Resistive Divider
) 1
1
t l i
u
R1
F (
R2
m
o r
V
L
l a
d i
15nA
=
i n
2
⋅
1
o c
a
(µ ) H
s c
⋅
(
205
R (
51
V
C
R
D
D
R
0
2
R
4
1
4
7
⎪⎢
3
1 .
0
=
3
2 -
FB
K
1
R
1 -
K
) 2
R
51
Ω
0
SC2440
) 7
2
⎪⎢
) 0
)
205
1 .
.
K
R
7
1
2
3
Ω
4
2
K ( Ω)
2
) K
4 .
3 .
4 .
.
=
−
C
. 0
4
2
2
8
7
2
2
p (
0
061
0
0
) F
%
C
9
2
1
1
.
p (
2
0
0
) F
(1)
9
Choosing the Operating Frequency
The free-running frequency of the master
set with an external resistor from the ROSC pin to ground.
Channel frequency is one-half of that of the master
oscillator. A graph of channel
shown in the “Typical Performance Characteristics”.
Before choosing the operating frequency, tradeoffs
among efficiency, operating duty cycle, component size
and EMI interferences must be considered. High
frequency operation reduces the size of passive
components but switching losses are higher. Lowering
the switching frequency improves efficiency. However the
required inductor and capacitor are larger. Channel
frequencies between 1 and 2MHz are good compromises.
In order to quantify the tradeoff between switching
frequency and efficiency, the 12V to 5V DC-DC converter
in Figure 1 is modified to run at 500KHz and 2.5MHz
while keeping the inductor ripple current constant. The
modified component values are tabulated in Table 1 and
efficiencies at these frequencies are shown in Figure 5.
The efficiency of the 1.3MHz 5V regulator in Figure 1 is
also plotted for the ease of comparison. The efficiency
at 500KHz is only marginally higher than that at 1.3MHz.
The peak efficiency at 2.5MHz is only 2% lower compared
to those at lower frequencies.
Figure 5. Efficiencies of 500KHz, 1.3MHz and 2.5MHz
12V to 5V Step-down Converters.
90
85
80
75
0.0
Efficiency vs Load Current
0.5
channel
channel
channel
channel frequency against R
Load Current (A)
1.3MHz
1.0
1.5
master
master
master
master oscillator is
500KHz
2.5MHz
V
V
IN
OUT
=12V
www.semtech.com
=5V
SC2440
2.0
OSC
is