Datasheet

9
LT1310
sn1310 1310fs
100mA to 200mA. Figure 3a shows the transient response
with R
C
equal to 3k. The phase margin is poor as evi-
denced by the excessive ringing in the output voltage and
inductor current. In Figure 3b, the value of R
C
is increased
to 6k, which results in a more damped response. Figure
3c shows the results when R
C
is increased further to 15k.
The transient response is nicely damped and the compen-
sation procedure is complete.
Compensation—Theory
Like all other current mode switching regulators, the
LT1310 needs to be compensated for stable and efficient
operation. Two feedback loops are used in the LT1310: a
V
OUT
100mV/DIV
AC COUPLED
I
L
0.5A/DIV
R
C
= 3k 200µs/DIV 1310 F03a
Figure 3a. Transient Response Shows Excessive Ringing
V
OUT
100mV/DIV
AC COUPLED
I
L
0.5A/DIV
R
C
= 6k 200µs/DIV 1310 F03b
Figure 3b. Transient Response is Better
V
OUT
100mV/DIV
AC COUPLED
I
L
0.5A/DIV
R
C
= 15k 200µs/DIV 1310 F03b
Figure 3c. Transient Response is Well Damped
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 4 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power stage of the IC, inductor and diode have been
replaced by the equivalent transconductance amplifier
g
mp
. g
mp
acts as a current source where the output current
is proportional to the V
C
voltage. Note that the maximum
output current of g
mp
is finite due to the current limit in the
IC.
From Figure 4, the DC gain, poles and zeroes can be
calculated as follows:
Output Pole: P1=
2
2• •R
Error Amp Pole: P2 =
1
2• •R
Error Amp Zero: Z1=
1
2• •R
DC Gain: A =
1.25
V
L
O
C
OUT
π
π
π
•
•
•
••••
C
C
C
gRgR
OUT
C
C
ma O mp L
In addition to the elements from Figure 4, current mode
control aslo results in some other poles and zeroes. These
are as follows:
RHP Zero: Z2 =
Output Zero: Z3 =
Current Mode Pole: P3 >
VR
VL
ESR C
f
IN L
OUT
OUT
S
2
2
2
1
2
3
•
•• •
•• •
π
π
The Current Mode zero is a right half plane zero which can
be an issue in feedback control design, but is manageable
with proper external component selection.
APPLICATIO S I FOR ATIO
WUUU