Datasheet

LT8582
27
8582f
APPENDIX
Table 7 shows a list of several discrete PMOS manufactur-
ers. Consult the manufacturers for detailed information
on their entire selection of PMOSs.
Table 7. Discrete PMOS Manufacturers
Vishay www.vishay.com
ON Semiconductor www.onsemi.com
Fairchild Semiconductor www.fairchildsemi.com
Diodes Incorporated www.diodes.com
COMPENSATION – ADJUSTMENT
To compensate the feedback loop of the LT8582, a series
resistor capacitor network in parallel with an optional
single capacitor should 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 start-
ing value. The optional parallel capacitor should range in
value from 22pF to 220pF with 47pF being a good starting
value. The compensation resistor, R
C
, is usually in the
range of 5k to 50k with 10k being a good starting value.
A good technique to compensate a new application is to
use a 100k potentiometer in place of the series resistor
R
C
. With the series and parallel capacitors at 4.7nF and
47pF respectively, adjust the potentiometer while observ-
ing the transient response and the optimum value for R
C
can be found. Figures 14a to Figure 14c illustrate this
process for the circuit of Figure 17 with a load current
stepped between 300mA and 800mA. Figure 14a 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 14b, the
value of R
C
is increased to 3.15k, which results in a more
damped response. Figure 14c shows the results when R
C
is
increased further to 6.49k. The transient response is nicely
damped and the compensation procedure is complete.
Compensation – Theory
Like all other current mode switching regulators, the
LT8582 needs to be compensated for stable and efficient
operation. Two feedback loops are used in the LT8582: 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.
As with any feedback loop, identifying the gain and phase
contribution of the various elements in the loop is critical.
Figure 15 shows the key equivalent elements of a boost
converter. Because of the fast current control loop, the
power stage of the chip, inductor and diode have been
replaced by a combination of the equivalent transcon-
ductance amplifier g
mp
and the current controlled current
source (which converts I
VIN
to (ηV
IN
/V
OUT
) • I
VIN
). g
mp
acts as a current source where the peak input current,
I
VIN
, is proportional to the V
C
voltage. η is the efficiency
of the switching regulator and is typically about 88% at
higher currents.
Figure 14a. Transient Response Shows Excessive Ringing
Figure 14b. Transient Response Is Better
Figure 14c. Transient Response Is Well Damped
V
OUT1
AC-COUPLED
500mV/DIV
I
LOAD
400mA/DIV
I
L1
1A/DIV
100µs/DIV
8582 F14a
V
OUT
AC-COUPLED
500mV/DIV
I
LOAD
400mA/DIV
I
L
1A/DIV
100µs/DIV
8582 F14b
V
OUT
AC-COUPLED
500mV/DIV
I
LOAD
400mA/DIV
I
L
1A/DIV
100µs/DIV
8582 F14c