LT1946 LINER [Linear Technology], LT1946 Datasheet - Page 7

• Manufacturer Part Number: LT1946
• Description: 1.2MHz Boost DC/DC Converter with 1.5A Switch and Soft-Start
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Compensation—Theory
Like all other current mode switching regulators, the
LT1946 needs to be compensated for stable and efficient
operation. Two feedback loops are used in the LT1946: 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 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power stage of the IC, inductor and diode have been
replaced by the equivalent transconductance amplifier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:
mp
DC GAIN: A =
RHP Zero: Z3 =
High Frequency Pole: P3 >
Output Pole: P1=
Error Amp Pole: P2 =
Error Amp Zero: Z1=
ESR Zero:
. g
mp
acts as a current source where the output current
Z
2
V
mp
1.25
U
OUT
2
2
is finite due to the current limit in the
• •
• •
2 • •R
C
V
•
IN
voltage. Note that the maximum
g
U
2 • •R
2 • •R
V
ESR C
2
ma
OUT
•
1
R
2
L
•
L
R
•
2
•
f
3
1
O
S
C
1
•
C
O
OUT
OUT
L
•
W
•
g
•
C
C
mp
C
C
•
R
L
U
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.
Using the circuit of Figure 1 as an example, the following
table shows the parameters used to generate the Bode plot
shown in Figure 5.
Table 3. Bode Plot Parameters
From Figure 5, the phase is 120 when the gain reaches
0dB giving a phase margin of 60 . This is more than
adequate. The crossover frequency is 25kHz, which is
about three times lower than the frequency of the right half
plane zero Z2. It is important that the crossover frequency
be at least three times lower than the frequency of the RHP
zero to achieve adequate phase margin.
Parameter
R
V
C
V
C
C
C
g
g
V
R
R
C
R
C
C
C
g
g
R
R
R
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
OUT
OUT
f
ma
mp
L
IN
S
ma
mp
C
OUT
O
C
C
L
O
L
C
: COMPENSATION CAPACITOR
: OUTPUT RESISTANCE DEFINED AS V
: COMPENSATION RESISTOR
: OUTPUT RESISTANCE OF g
: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
R
: OUTPUT CAPACITOR
Figure 4. Boost Converter Equivalent Model
O
–
+
g
mp
Value
18.6
49.9
470
3.3
5.4
1.2
20
10
40
g
8
5
ma
+
–
ma
Units
mho
MHz
M
mho
REFERENCE
k
pF
V
V
H
F
1.250V
OUT
DIVIDED BY I
Comment
Application Specific
Application Specific
Not Adjustable
Adjustable
Adjustable
Application Specific
Application Specific
Not Adjustable
Not Adjustable
Application Specific
Not Adjustable
1946 F04
R1
R2
LOAD(MAX)
LT1946
C
OUT
sn1946 1946fs
R
L
V
OUT
7