LTC3813 Datasheet, PDF (25/32 Pages) Linear Technology – 100V Current Mode Synchronous Step-Up Controller

LTC3813
APPLICATIONS INFORMATION
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
=
2
•
f
•
1
G
•
K
•
R1
( ) ^{C1= C2 K2
1}
R2 = ^K
2 • f • C1
R_B
=
V_REF (R1)
V_OUT
V_REF
TYPE 3 Loop:
K
=
tan²


BOOST
4
+
45°^
C2 =
1
2 • f • G • R1
C1= C2(K
1)
R2
=
2
•
K
f•
C1
R3 = ^R1
K
1
C3 =
1
2f K • 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)
=
s
•
R1•
(C2
+
1+ s • R3 • C2
C3)
•



1+
s
•
R3
•
C2 • C3
C2 + C3


Type 3:
A
(s)
=
s
•
R1•
1
(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
3813fb
25