Datasheet

24
LTC1702A
1702afa
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
–10mV 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 = –10 ± 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
(<10k) should
be checked carefully, since small changes in R
IMAX
can
cause large I
LIM
changes. If V
PROG
is set too low, the
LTC1702A may fail to start up.
Accuracy Trade-Offs
The V
DS
sensing scheme used in the LTC1702A is not
particularly accurate, primarily due to uncertainty in the
R
DS(ON)
from MOSFET to MOSFET. A second error term
arises from the ringing present at the SW pin, which
causes the V
DS
to look larger than (I
LOAD
)(R
DS(ON)
) at the
beginning of QB’s on-time. These inaccuracies do not
prevent the LTC1702A current limit circuit from protecting
itself and the load from damaging overcurrent conditions,
but they do prevent the user from setting the current limit
to a tight tolerance if more than one copy of the circuit is
being built. The 50% factor in the current setting equation
above reflects the margin necessary to ensure that the
circuit will stay out of current limit at the maximum normal
load, even with a hot MOSFET that is running quite a bit
higher than its R
DS(ON)
spec.
FCB OPERATION/SECONDARY WINDINGS
The FCB pin can be used in conjunction with a secondary
winding on one side of the LTC1702A to generate a third
regulated voltage output. This output can be directly
regulated at the FCB pin. In theory, a fourth output could
APPLICATIONS INFORMATION
WUU
U
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
–
–
–
CURRENT LIMIT PROGRAMMING
Programming the current limit on the LTC1702A 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 LTC1702A 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:
VIR CF
PROG LIM DS ON
=
()
()
+
()
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