Datasheet
LTC3814-5
22
38145fc
TYPE 2 Loop:
K = tan
BOOST
2
+ 45°
C2 =
1
2 •f•G•K•R1
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•R1
C1= C2 K 1
()
R2 =
K
2 •f•C1
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 2 and A(s) depends on
compensation circuit used:
Type 2:
A (s)=
1+ s•R2•C1
s•R1• C1+C2
()
•1+ s•R2•
C1• C2
C1+C2
⎛
⎝
⎜
⎞
⎠
⎟
Type 3:
A (s)=
1
s•R1• C1+C2
()
•
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 LTC3814-5, 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 valley current.
The corresponding output current limit is:
I
LIMIT
=
V
SNS(MAX)
R
DS(ON)
ρ
T
−
1
2
ΔI
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
self-consistency between the assumed MOSFET junction
temperature and the resulting value of I
LIMIT
which heats
the MOSFET switches.
Caution should be used when setting the current limit
based upon the R
DS(ON)
of the MOSFETs. The maximum
current limit is determined by the minimum MOSFET
on-resistance. Data sheets typically specify nominal
and maximum values for R
DS(ON)
, but not a minimum.
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