EL7554(2005) 查看數據表（PDF） - Intersil
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EL7554 Datasheet PDF : 14 Pages
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EL7554
Figure 18 shows the waveforms.
goes HI, where VREF is the regulator reference voltage.
VREF=1.26.
- STN
C 0.1µF
VO
+ STP
R
200K
VO
EL7554
VIN
TST
FIGURE 21. ADJUSTABLE START-UP
In this application, CIN and COUT may be increased to
reduce input/output ripple because the pulse skipping nature
of the method.
2. CASCADE START-UP
In this configuration, EN pin of Regulator 2 is connected to
the PG pin of Regulator 1 (Figure 22). VO2 will only start
after VO1 is good.
EN
PG
VO2
VO1
VIN
EL7554
EL7554
VO1
VO2
FIGURE 22. CASCADE START-UP
3. LINEAR START-UP
In the linear start-up tracking configuration, the regulator with
lower output voltage, VO2, tracks the one with higher output
voltage, VO1. The waveform is shown in Figure 19.
C
-
- STN
+
VO2
+ STP
R
VO1
EL7554
VIN
EL7554
VIN
VO1
VO2
FIGURE 23. LINEAR START-UP TRACKING
4. OFFSET START-UP
Compared with the cascade start-up, this configuration
allows Regulator 2 to begin the start-up process when VO1
reaches a particular value of VREF*(1+RB/RA) before PG
VREF
RB
-
VO2
+
RA
VO1
EL7554
VIN
EL7554
VIN
VREF(1+RB/RA)
VO1
VO2
FIGURE 24. OFFSET START-UP TRACKING
Component Selection
INPUT CAPACITOR
The main functions of the input capacitor(s) are to maintain
the input voltage steady and to filter out the pulse current
passing through the upper switch. The root-mean-square
value of this current is:
I I N ,RMS
=
-----V----O------×----(---V----I--N-----–-----V----O-----)
VIN
×
IO
≈
1/2
(I
O
)
for a wide range of VIN and VO.
For long-term reliability, the input capacitor or combination of
capacitors must have the current rating higher than IIN,RMS.
Use X5R or X7R type ceramic capacitors, or SPCAP or
POSCAP types of Polymer capacitors for their high current
handling capability.
INDUCTOR
The NFET positive current limit is set at about 0.5A. For
optimal operation, the peak-to-peak inductor current ripple
∆IL should be less than 1A. The following equation gives the
inductance value:
L = -(--VV----I-I-NN------×–----V-∆---O-I--L--)--×--×---F--V--S--O--
The peak current the inductor sees is:
ILPK
=
IO
+
-∆----I--L-
2
When inductor is chosen, make sure the inductor can handle
this peak current and the average current of IO.
OUTPUT CAPACITOR
If there is no holding time requirement for output; output
voltage ripple and transient response are the main deciding
factors in choosing the output capacitor. Initially, choose the
11
FN7360.2
January 31, 2005
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