Datasheet

LT3581
26
3581fa
For more information www.linear.com/LT3581
V
OUT
AC-COUPLED
500mV/DIV
I
L
1A/DIV
3581 F15a
50µs/DIV
V
OUT
AC-COUPLED
500mV/DIV
I
L
1A/DIV
3581 F15b
50µs/DIV
appenDix
found. Figures 15a to 15c illustrate this process for the
circuit of Figure 18 with a load current stepped between
540mA and 800mA. Figure 15a shows the transient re
-
sponse 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 15b, the value of R
C
is
increased to 3k, which results in a more damped response.
Figure 15c shows the results when R
C
is increased further
to 10.5k. The transient response is nicely damped and the
compensation procedure is complete.
Figure 15a. Transient Response Shows Excessive Ringing
Figure 15b. Transient Response is Better
Figure 15c. Transient Response is Well Damped
V
OUT
AC-COUPLED
500mV/DIV
I
L
1A/DIV
3581 F15c
50µs/DIV
COMPENSATION – THEORY
Like all other current mode switching regulators, the LT3581
needs to be compensated for stable and efficient operation.
Two feedback loops are used in the LT3581: 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 16 shows the key equivalent elements
of a boost converter. Because of the fast current control
loop, the power stage of the IC, inductor and diode
have been replaced by a combination of the equivalent
transconductance 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 80%.
Note that the maximum output currents of the g
mp
and
g
ma
stages are finite. The output of the g
mp
stage is
limited by the minimum switch current limit (see Electrical
Specifications) and the output of the g
ma
stage is nominally
limited to about ±12μA.
1.215V
REFERENCE
I
VIN
η
•
V
IN
V
OUT
•
I
VIN
V
OUT
C
OUT
C
PL
R
ESR
R
L
R
O
V
C
R
C
C
C
C
F
R1
FB
R2
R2
–
+
–
+
3581 F16
g
mp
g
ma
C
C
: COMPENSATION CAPACITOR
C
OUT
: OUTPUT CAPACITOR
C
PL
: PHASE LEAD CAPACITOR
C
F
: HIGH FREQUENCY FILTER CAPACITOR
g
ma
: TRANSCONDUCTOR AMPLIFIER INSIDE IC
g
mp
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
R
C
: COMPENSATION RESISTOR
R
L
: OUTPUT RESISTANCE DEFINED AS V
OUT
/I
LOAD(MAX)
R
O
: OUTPUT RESISTANCE OF g
ma
R1, R2; FEEDBACK RESISTOR DIVIDER NETWORK
R
ESR
: OUTPUT CAPACITOR ESR
Figure 16. Boost Converter Equivalent Model