lt1310emse-trpbf Linear Technology Corporation, lt1310emse-trpbf Datasheet - Page 9

• Manufacturer Part Number: lt1310emse-trpbf
• Description: 1.5a Boost Dc/dc Converter With Phase-locked Loop
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
100mA to 200mA. Figure 3a shows the transient response
with R
denced by the excessive ringing in the output voltage and
inductor current. In Figure 3b, the value of R
to 6k, which results in a more damped response. Figure
3c shows the results when R
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
AC COUPLED
AC COUPLED
AC COUPLED
100mV/DIV
100mV/DIV
100mV/DIV
Figure 3a. Transient Response Shows Excessive Ringing
0.5A/DIV
0.5A/DIV
0.5A/DIV
C
V
V
V
OUT
OUT
Figure 3c. Transient Response is Well Damped
OUT
equal to 3k. The phase margin is poor as evi-
I
I
I
L
L
L
R
R
R
Figure 3b. Transient Response is Better
C
C
C
= 6k
= 15k
= 3k
U
200 s/DIV
200 s/DIV
200 s/DIV
U
C
is increased further to 15k.
W
1310 F03b
1310 F03b
1310 F03a
C
is increased
U
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power stage of the IC, inductor and diode have been
replaced by the equivalent transconductance amplifier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
is proportional to the V
output current of g
IC.
From Figure 4, the DC gain, poles and zeroes can be
calculated as follows:
In addition to the elements from Figure 4, current mode
control aslo results in some other poles and zeroes. These
are as follows:
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.
mp
Output Pole: P1=
Error Amp Pole: P2 =
Error Amp Zero: Z1=
DC Gain: A =
RHP Zero: Z2 =
Output Zero: Z3 =
Current Mode Pole: P3 >
. g
mp
acts as a current source where the output current
V
1.25
mp
OUT
2
is finite due to the current limit in the
• •
2 • • R
2
C
•
V
• •
voltage. Note that the maximum
g
IN
2 • • R
ma
2 • • R
2
V
OUT
•
ESR C
•
R
2
f
L
3
R
S
1
L
2
•
O
1
1
C
•
C
•
•
O
L
OUT
g
•
•
mp
OUT
C
C
C
C
•
R
L
LT1310
sn1310 1310fs
9