LT3570
15
3570fb
APPLICATIONS INFORMATION
compensation resistor, R
C
, is usually in the range of 5k to
50k. A good technique to compensate a new application
is to use a 50k potentiometer in place of R
C
, and use a
1nF capacitor for C
C
. By adjusting the potentiometer while
observing the transient response, the optimum value for
R
C
can be found. Figures 3a to 3c illustrate this process
for the circuit of Figure 1 with load current stepped from
100mA to 500mA for the buck converter. Figure 3a shows
the transient response with R
C
equal to 1.6k. The phase
margin is poor as evidenced by the excessive ringing in
the output voltage and inductor current. In Figure 3b,
the value of R
C
is increased to 5.75k, which results in a
more damped response. Figure 3c shows the result when
R
C
is increased further to 25k. The transient response
is nicely damped and the compensation procedure is
complete. The same procedure is used to compensate
the boost converter.
Soft-Start
The soft-start time is programmed with an external capaci-
tor to ground on SS. An internal current source charges it
with a nominal 4.5糀. The voltage on the soft-start pin is
used to control the feedback voltage. The soft-start time
is determined by the equation:
t
SS
= 0.2 " C
SS
where C
SS
is in nF and t
SS
is in ms. In the event of a
commanded shutdown, ULVO on the input or a thermal
shutdown, the capacitor is discharged automatically. The
soft-start will remain low and only charge back up after
the fault goes away and the voltage on SS is less than
approximately 100mV.
Figure 3a. Transient Response Shows Excessive Ringing
Figure 3b. Transient Response is Better
Figure 3c. Transient Response Well Damped
I
OUT
500mA/DIV
V
OUT
200約/DIV
3570 F03a
I
OUT
500mA/DIV
V
OUT
200約/DIV
3570 F03b
I
OUT
500mA/DIV
V
OUT
200約/DIV
3570 F03c
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