Datasheet
LTC3813
25
3813fb
Finally, choose a convenient resistor value for R1 (10k
is usually a good value). Now calculate the remaining
values:
(K is a constant, used in the calculations)
f = chosen crossover frequency
G = 10
(GAIN/20)
(this converts GAIN in dB to G in
absolute gain)
TYPE 2 Loop:
K = tan
BOOST
2
+ 45°
C2 =
1
2 •f•G•K•R
1
C1= C2 K
2
1
()
R2 =
K
2 •f•C1
R
B
=
V
REF
(R1)
V
OUT
V
REF
TYPE 3 Loop:
K = tan
2
BOOST
4
+ 45°
C2 =
1
2 •f•G•R
1
C1= C2 K 1
()
R2 =
K
2 •f•C
1
R3 =
R1
K
1
C3 =
1
2fK
•R3
R
B
=
V
REF
(R1)
V
OUT
V
REF
SPICE or mathematical software can be used to generate
the gain/phase plots for the compensated power supply to
do a sanity check on the component values before trying
them out on the actual hardware. For software, use the
following transfer function:
T(s) = A(s)H(s)
where H(s) was given in equation 1 and A(s) depends on
compensation circuit used:
Type 2:
A (s)=
1+ s•R3•C2
s•R1• C2+ C3
()
•1+ s•R3•
C2 • C3
C2 + C3
Type 3:
A (s)=
1
s•R1• C2+ C3
( )
•
1+ s• R1+ R3
()
•C3
()
•1+ s•R2•C1
()
1+ s•R3•C3
()
•1+ s•R2•
C1• C2
C1+ C2
For SPICE, simulate the previous PSPICE code with
calculated compensation values entered and generate a
gain/phase plot of V
OUT
/V
OUTIN
.
Fault Conditions: Current Limit
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage.
In the LTC3813, the maximum sense voltage is controlled
by the voltage on the V
RNG
pin. With peak current control,
the maximum sense voltage and the sense resistance
determine the maximum allowed inductor peak current.
The corresponding output current limit is:
I
LIMIT
=
V
SNS(MAX)
R
DS(ON)
T
1
2
I
L
The current limit value should be checked to ensure that
I
LIMIT(MIN)
> I
OUT(MAX)
. The minimum value of current limit
generally occurs at the lowest V
IN
at the highest ambient
temperature, conditions that cause the largest power loss
in the converter. Note that it is important to check for
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