Datasheet

LT3579/LT3579-1
29
35791fa
For more information www.linear.com/LT3579
APPENDIX
Table 7 shows a list of several discrete PMOS manufa-
cturers. Consult the manufacturers for detailed information
on their entire selection of PMOS devices.
Table 7. Discrete PMOS Manufacturers
Vishay www.vishay.com
Fairchild Semiconductor www.fairchildsemi.com
Central Semiconductor www.centralsemi.com
COMPENSATION – ADJUSTMENT
To compensate the feedback loop of the LT3579, a series
resistor-capacitor network in parallel with an optional single
capacitor must be connected from the V
C
pin to GND. For
most applications, choose a series capacitor in the range
of 1nF to 10nF with 2.2nF being a good starting value.
The optional parallel capacitor should range in value from
22pF to 180pF with 47pF being a good starting value. The
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 100kΩ potentiometer in place of the series resistor
R
C
. With the series and parallel capacitors at 2.2nF and
47pF respectively, adjust the potentiometer while observing
the transient response and the optimum value for R
C
can
be found. Figures 17a to 17c illustrate this process for the
circuit of Figure 20 with a load current stepped between
0.7A and 1.5A. Figure 17a shows the transient response
with R
C
equal to 1k. The phase margin is poor as evidenced
by the excessive ringing in the output voltage and inductor
current. In Figure 17b, the value of R
C
is increased to 3.5k,
which results in a more damped response. Figure 17c
shows the results when R
C
is increased further to 8k. The
transient response is nicely damped and the compensation
procedure is complete.
COMPENSATION – THEORY
Like all other current mode switching regulators, the
LT3579 needs to be compensated for stable and efficient
operation. Two feedback loops are used in the LT3579: a
fast current loop which does not require compensation,
and a slower voltage loop which does. Standard Bode plot
analysis can be used to understand and adjust the voltage
feedback loop.
Figure 17a. Transient Response Shows Excessive Ringing
Figure 17b. Transient Response Is Better
Figure 17c. Transient Response Is Well Damped
100µs/DIV
V
OUT
500mV/DIV
AC COUPLED
I
L
2A/DIV
I
LOAD
1A/DIV
35791
F17a
R
C
= 1k
100µs/DIV
V
OUT
500mV/DIV
AC COUPLED
I
L
2A/DIV
I
LOAD
1A/DIV
35791
F17b
R
C
= 3.5k
100µs/DIV
V
OUT
500mV/DIV
AC COUPLED
I
L
2A/DIV
I
LOAD
1A/DIV
35791
F17c
R
C
= 8k