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EL7554
goes HI, where V REF is the regulator reference voltage.
V REF =1.26.
-
STN
C
0.1μF
V O
+
EL7554
STP
R
200K
V IN
T ST
V O
V O2
V REF
-
+
R A
R B
V O1
FIGURE 21. ADJUSTABLE START-UP
EL7554
V IN
EL7554
V IN
In this application, C IN and C OUT 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). V O2 will only start
after V O1 is good.
V REF (1+R B /R A )
V O1
V O2
FIGURE 24. OFFSET START-UP TRACKING
Component Selection
INPUT CAPACITOR
EN
PG
The main functions of the input capacitor(s) are to maintain
V O2
EL7554
V O1
EL7554
V IN
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 IN,RMS = ----------------------------------------------- × I O ≈ 1/2 ( I O )
V O1
V O2
FIGURE 22. CASCADE START-UP
3. LINEAR START-UP
In the linear start-up tracking configuration, the regulator with
lower output voltage, V O2 , tracks the one with higher output
voltage, V O1 . The waveform is shown in Figure 19.
V O × ( V IN – V O )
V IN
for a wide range of V IN and V O .
For long-term reliability, the input capacitor or combination of
capacitors must have the current rating higher than I IN,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 5A. For
-
-
STN
C
optimal operation, the peak-to-peak inductor current ripple
Δ I L should be less than 1A. The following equation gives the
( V IN – V O ) × V O
V IN × Δ I L × F S
V O2
+
EL7554
V O1
V IN
+
EL7554
STP
R
V IN
inductance value:
L = --------------------------------------------
The peak current the inductor sees is:
V O1
V O2
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 V O1
reaches a particular value of V REF *(1+R B /R A ) before PG
11
2
Δ I L
I LPK = I O + --------
When inductor is chosen, make sure the inductor can handle
this peak current and the average current of I O .
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
FN7360.5
November 5, 2007
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