lt3570 Linear Technology Corporation, lt3570 Datasheet - Page 15

• Manufacturer Part Number: lt3570
• Description: 1.5a Buck Converter, 1.5a Boost Converter And Ldo Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
compensation resistor, R
50k. A good technique to compensate a new application
is to use a 50k potentiometer in place of R
1nF capacitor for C
observing the transient response, the optimum value for
R
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
margin is poor as evidenced by the excessive ringing in
the output voltage and inductor current. In Figure 3b,
the value of R
more damped response. Figure 3c shows the result when
R
is nicely damped and the compensation procedure is
complete. The same procedure is used to compensate
the boost converter.
C
C
can be found. Figures 3a to 3c illustrate this process
is increased further to 25k. The transient response
Figure 3a. Transient Response Shows Excessive Ringing
500mA/DIV
V
I
OUT
OUT
C
is increased to 5.75k, which results in a
C
. By adjusting the potentiometer while
C
, is usually in the range of 5k to
200μs/DIV
C
equal to 1.6k. The phase
500mA/DIV
Figure 3c. Transient Response Well Damped
V
I
OUT
OUT
C
, and use a
3570 F03a
200μs/DIV
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 4.5μA. The voltage on the soft-start pin is
used to control the feedback voltage. The soft-start time
is determined by the equation:
where C
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.
t
SS
= 0.2 • C
500mA/DIV
SS
V
I
Figure 3b. Transient Response is Better
OUT
OUT
is in nF and t
SS
3570 F03c
SS
200μs/DIV
is in ms. In the event of a
LT3570
3570 F03b
15
3570fa