SC2440 Просмотр технического описания (PDF) - Semtech Corporation
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SC2440 Datasheet PDF : 26 Pages
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SC2440
POWER MANAGEMENT
Applications Information
Setting the Output Voltage
Choosing the Operating Frequency
The regulator output voltage is set with an external
resistive divider (Figure 4) with its center tap tied to the
FB pin.
VOUT
R1
15nA
SC2440
FB
R2
Figure 4. VOUT is set with a Resistive Divider
The free-running frequency of the master oscillator is
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 frequency against ROSC is
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.
R1 = R2(VOUT − 1)
(1)
The percentage error due the input bias current of the
error amplifier is
∆VOUT
VOUT
=
− 15nA ⋅100 ⋅ (R1⎪⎢R2)
1V
.
Example: Determine the output voltage error of a
VOUT = 5V converter with R2 = 51.1KΩ .
From (1),
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.
R1 = 51.1KΩ ⋅ (5 − 1) = 205KΩ
∆VOUT
VOUT
=
− 15nA ⋅100 ⋅ (51.1K⎪⎢205K)
1V
= −0.061% .
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.
Efficiency vs Load Current
90
500KHz
85
1.3MHz
2.5MHz
f (MHz)
0.5
1.3
2.5
R9 (KΩ)
53.6
15.0
4.02
L2 (µH)
10 (Coiltronics DR73-100)
4.44 (Falco D04012)
2.7 (Sumida CR43-2R7)
R7 (KΩ)
12.4
24.3
32.4
C8 (pF)
470
220
220
C9 (pF)
22
10
10
80
75
0.0
VIN=12V
VOUT =5V
0.5
1.0
1.5
2.0
Load Current (A)
Table 1. The 12V to 5V Converter in Figure 1 is modified Figure 5. Efficiencies of 500KHz, 1.3MHz and 2.5MHz
to run at Different Frequencies.
12V to 5V Step-down Converters.
© 2005 Semtech Corp.
9
www.semtech.com
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