ltc1702a Linear Technology Corporation, ltc1702a Datasheet - Page 24

• Manufacturer Part Number: ltc1702a
• Description: Dual 550khz Synchronous 2-phase Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LTC1702A
Type 3 Loop:
CURRENT LIMIT PROGRAMMING
Programming the current limit on the LTC1702A is straight-
forward. The I
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
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:
I
expected operating current, to allow for MOSFET R
changes with temperature. Setting I
maximum normal operating current is usually safe and will
adequately protect the power components if they are
24
LIM
K Tan
R
R
R
C
C
C
V
2
1
3
PROG
2
3
B
should be chosen to be quite a bit higher than the
=
=
=
=
=
=
=
C K
2
2
2
( )
V
2
K
=
πƒ
πƒ
πƒ
OUT
( )
R
V
2
–
( )
REF
K
1
⎛
⎜
⎝
1
I
GR
C
LIM
1
–
1
BOOST
MAX
1
K R
–
MAX
1
( )
1
R
V
4
(
1
REF
3
pin sets the current limit by setting the
R
U
before comparing it with the negative
DS ON
+
(
45
INFORMATION
)
U
)
°
⎞
⎟
⎠
+
CF
W
LIM
to 150% of the
U
DS(ON)
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
calculated with CF = –10 ± 50mV. V
grammed at the I
and an external resistor:
The resulting value of R
circuit to ensure that the I
MOSFET R
automobiles, and should be taken with a grain of salt.
Circuits that use very low values for R
be checked carefully, since small changes in R
cause large I
LTC1702A may fail to start up.
Accuracy Trade-Offs
The V
particularly accurate, primarily due to uncertainty in the
R
arises from the ringing present at the SW pin, which
causes the V
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
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
DS(ON)
R
ILIM
DS
from MOSFET to MOSFET. A second error term
= V
sensing scheme used in the LTC1702A is not
DS(ON)
PROG
DS
LIM
to look larger than (I
DS(ON)
/10µA
MAX
changes. If V
specs are like horsepower ratings in
pin using the internal 10µA pull-up
ILIM
spec.
LIM
should be checked in an actual
circuit kicks in as expected.
PROG
LOAD
IMAX
is set too low, the
PROG
)(R
(< 10k) should
DS(ON)
is then pro-
IMAX
PROG
) at the
1702afa
can
be