Datasheet

24
LTC1703
1703fa
CURRENT LIMIT PROGRAMMING
Programming the current limit on the LTC1703 is straight-
forward. The I
MAX
pin sets the current limit by setting the
maximum allowable voltage drop across QB (the bottom
MOSFET) before the current limit circuit engages. The
voltage across QB is set by its on-resistance and the
current flowing in the inductor, which is the same as the
output current. The LTC1703 current limit circuit inverts
the voltage at I
MAX
before comparing it with the negative
voltage across QB, allowing the current limit to be set with
a positive voltage.
To set the current limit, calculate the expected voltage
drop across QB at the maximum desired current:
V
PROG
= (I
ILIM
)(R
DS(ON)
) + CF
I
LIM
should be chosen to be quite a bit higher than the
expected operating current, to allow for MOSFET R
DS(ON)
changes with temperature. Setting I
LIM
to 150% of the
maximum normal operating current is usually safe and will
adequately protect the power components if they are
chosen properly. The CF term is an approximate factor that
corrects for errors caused by ringing on the switch node
(illustrated in Figure 6). This correction factor will change
depending on the layout and the components used, but
100mV is usually a good starting point. However, to
provide adequate margin and to accommodate for offsets
and external variations, it is recommended that V
PROG
be
calculated with CF = 100 ± 50mV. V
PROG
is then pro-
grammed at the I
MAX
pin using the internal 10µA pull-up
and an external resistor:
R
ILIM
= V
PROG
/10µA
The resulting value of R
ILIM
should be checked in an actual
circuit to ensure that the I
LIM
circuit kicks in as expected.
MOSFET R
DS(ON)
specs are like horsepower ratings in
automobiles, and should be taken with a grain of salt.
Circuits that use very low values for R
IMAX
(<20k) should
be checked carefully, since small changes in R
IMAX
can
cause large I
LIM
changes when the 100mV correction
factor makes up a large percentage of the total V
PROG
value. If V
PROG
is set too low, the LTC1703 may fail to
start up.
Now calculate the remaining values:
(K is a constant used in the calculations)
ƒ = chosen crossover frequency
G = 10
(GAIN/20)
(this converts GAIN in dB to G in absolute
gain)
Type 2 Loop:
K Tan
BOOST
C
GKR
CCK
R
K
C
R
VR
VV
B
REF
OUT REF
=+°
⎛
⎝
⎜
⎞
⎠
⎟
=
πƒ
=
()
=
πƒ
=
()
2
45
2
1
21
12 1
2
21
1
2
–
–
Type 3 Loop:
K Tan
BOOST
C
GR
CCK
R
K
C
R
R
K
C
KR
R
VR
VV
B
REF
OUT REF
=+°
⎛
⎝
⎜
⎞
⎠
⎟
=
πƒ
=
()
=
πƒ
=
()
=
πƒ
=
()
2
4
45
2
1
21
12 1
2
21
3
1
1
3
1
23
1
–
–
–
APPLICATIO S I FOR ATIO
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