Datasheet
LT1946A
8
sn1946a 1946afs
ESR Zero:
Z
ESR C
OUT
2
1
2
=
•• •π
RHP Zero:
Z
VR
VL
IN L
OUT
3
2
2
2
=
•
•• •π
High Frequency Pole:
P
F
S
3
3
>
Using the circuit of Figure 1 as an example, Table 3 shows
the parameters used to generate the bode plot shown in
Figure 5.
Table 3. Bode Plot Parameters
Parameter Value Units Comment
R
L
28 Ω Application Specific
C
OUT
2.2 µF Application Specific
R
O
10 MΩ Not Adjustable
C
C
270 pF Adjustable
R
C
27.4 kΩ Adjustable
V
OUT
12 V Application Specific
V
IN
5 V Application Specific
G
MA
40 µmho Not Adjustable
G
MP
5 mho Not Adjustable
L 2.2 µH Application Specific
F
S
2.7 MHz Not Adjustable
ESR 10 mΩ Not Adjustable
From Figure 5, the phase when the gain reaches 0dB is
122° giving a phase margin of 58°. This is more than
adequate. The cross-over frequency is 90kHz, which is
about 30 times lower than the frequency of the right half
plane zero Z2. It is important that the cross-over frequency
be at least 3 times lower than the frequency of the RHP zero
to achieve adequate phase margin.
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power stage of the IC, inductor, and diode have been
replaced by the equivalent transconductance amplifier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:
P
RC
L OUT
1
2
2
=
•• •π
Error Amp Pole:
P
RC
OC
2
1
2
=
•• •π
Error Amp Zero:
Z
RC
CC
1
1
2
=
•• •π
DC Gain:
A
V
GRGR
OUT
MA O MP L
=
125.
••••
–
+
G
MP
–
+
1.250V
REFERENCE
V
C
R
C
C
C
C
OUT
V
OUT
R
O
R
1
R
2
R
L
G
MA
G
MA
: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
G
MP
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
C
OUT
: OUTPUT CAPACITOR
R
L
: OUTPUT RESISTANCE DEFINED AS V
OUT
DIVIDED BY I
LOAD
(MAX)
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
R
O
: OUTPUT RESISTANCE OF G
MA
R
C
: COMPENSATION RESISTOR
C
C
: COMPENSATION CAPACITOR
ESR
Figure 4. Boost Converter Equivalent Model
APPLICATIO S I FOR ATIO
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