LT3581 LINER [Linear Technology], LT3581 Datasheet - Page 26

• Manufacturer Part Number: LT3581
• Description: 3.3A Boost/Inverting DC/DC Converter with Fault Protection
• Manufacturer: LINER [Linear Technology]

LT3581
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
evidenced by the excessive ringing in the output voltage
and inductor current. In Figure 15b, the value of R
increased to 3k, which results in a more damped response.
Figure 15c shows the results when R
to 10.5k. The transient response is nicely damped and the
compensation procedure is complete.

AC-COUPLED
AC-COUPLED
AC-COUPLED
500mV/DIV
500mV/DIV
500mV/DIV
Figure 15a. Transient Response Shows Excessive Ringing
1A/DIV
1A/DIV
1A/DIV
V
V
V
OUT
OUT
OUT
Figure 15c. Transient Response is Well Damped
I
I
I
L
L
L
Figure 15b. Transient Response is Better
C
equal to 1k. The phase margin is poor as
50µs/DIV
50µs/DIV
50µs/DIV
C
is increased further
3581 F15a
3581 F15b
3581 F15c
C
is
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have been replaced by a combination of the equivalent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
current source (which converts I
g
I
the switching regulator and is typically about 80%.
Note that the maximum output currents of the g
g
limited by the minimum switch current limit (see Electrical
Specifications) and the output of the g
limited to about ±12μA.
VIN
mp
ma
C
F
, is proportional to the V
acts as a current source where the peak input current,
stages are finite. The output of the g
R
C
C
Figure 16. Boost Converter Equivalent Model
C
V
R
C
–
+
O
C
C
C
C
g
g
R
R
R
R1, R2; FEEDBACK RESISTOR DIVIDER NETWORK
R
ma
mp
C
OUT
PL
F
C
L
O
ESR
g
: HIGH FREQUENCY FILTER CAPACITOR
: COMPENSATION CAPACITOR
: OUTPUT RESISTANCE DEFINED AS V
: COMPENSATION RESISTOR
: OUTPUT RESISTANCE OF g
mp
: PHASE LEAD CAPACITOR
: TRANSCONDUCTOR AMPLIFIER INSIDE IC
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
: OUTPUT CAPACITOR
: OUTPUT CAPACITOR ESR
g
ma
+
–
I
VIN
C
REFERENCE
mp
voltage. η is the efficiency of
1.215V
V
and the current controlled
•
R2
R2
OUT
VIN
V
ma
IN
• I
FB
to ηV
ma
VIN
OUT
stage is nominally
/I
C
IN
LOAD(MAX)
PL
/V
mp
OUT
R1
stage is
V
R
mp
• I
C
OUT
ESR
OUT
3581 F16
VIN
and
3581f
R
).
L