LTC1704 Linear Technology, LTC1704 Datasheet - Page 22

• Manufacturer Part Number: LTC1704
• Description: 550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1704/LTC1704B
is the same as the output current. The LTC1704 current
limit circuit inverts the voltage at I
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:
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
chosen properly. Note that the ringing on the switch node
can cause error for the current limit threshold (illustrated
in Figure 6). This factor will change depending on the
layout and the components used. V
grammed at the I
and an external resistor:
The resulting value of R
tual circuit to ensure that the current circuit kicks in as
expected. MOSFET R
ratings in automobiles, and should be taken with a grain of
salt. Circuits that use very low values for R
should be checked carefully, since small changes in R
can cause large I
ing makes up a large percentage of the total V
If V
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. Another important error is due
to poor PCB layout. Care should be taken to ensure that
proper kelvin sensing of the SW pin is provided. These
inaccuracies do not prevent the LTC1704 current limit
circuit from protecting itself and the load from damaging
overcurrent conditions, but they do prevent the user from
22
LIMIT
DS(ON)
V
R
PROG
PROG
IMAX
should be chosen to be quite a bit higher than the
DS
from MOSFET to MOSFET. A second error term
is set too low, the LTC1704 may fail to start up.
= V
= (I
sensing scheme used in the LTC1704 is not
PROG
DS
LIMIT
to look larger than (I
LIMIT
MAX
/10 A
)(R
U
DS(ON)
changes when the switch node ring-
pin using the internal 10 A pull-up
DS(ON)
IMAX
U
)
should be checked in an ac-
specs are like horsepower
MAX
W
LOAD
LIMIT
PROG
before comparing
)(R
to 150% of the
DS(ON)
is then pro-
IMAX
PROG
U
(< 10k)
) at the
DS(ON)
value.
IMAX
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 neces-
sary 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
REGULATION OVER COMPONENT
TOLERANCE/TEMPERATURE
DC Regulation Accuracy
The LTC1704’s switcher controller initial DC output accu-
racy depends mainly on internal reference accuracy and
internal op amp offset. Two LTC1704 specs come into
play: feedback voltage and feedback voltage line regula-
tion. The feedback voltage spec is 800mV 12mV over the
full temperature range and is specified at the FB pin, which
encompasses both reference accuracy and any op amp
offset. This accounts for 1.5% error at the output with a 5V
input supply. The feedback voltage line regulation spec
adds an additional 0.1%/V term that accounts for change
in reference output with change in input supply voltage.
With a 5V supply, the errors contributed by the LTC1704
itself add up to no more than 1.5% DC error at the output.
The output voltage setting resistors (see R1 and R2 in the
Typical Applications) are the other major contributor to DC
error. At a typical 1.xV output voltage, the resistors are of
roughly the same value, which tends to halve their error
terms, improving accuracy. Still, using 1% resistors for
R1 and R2 will add 1% to the total output error budget.
Using 0.1% resistors in just those two positions can nearly
halve the DC output error for very little additional cost.
Load Regulation
Load regulation is affected by feedback voltage, feedback
amplifier gain and external ground drops in the feedback
path. Feedback voltage is covered above and is within
1.5% over temperature. A full range load step might
require a 10% duty cycle change to keep the output
constant, requiring the COMP pin to move about 100mV.
With amplifier gain at 85dB, this adds up to only a 10 V
shift at FB, negligible compared to the reference accuracy
terms.
DS(ON)
spec.
1704bfa