LTC3774
15
3774fc
For more information www.linear.com/LTC3774
APPLICATIONS INFORMATION
The Typical Application on the first page of this data sheet
is a basic LTC3774 application circuit. The LTC3774 is
designed and optimized for use with a very low DCR
value by utilizing a novel approach to reduce the noise
sensitivity of the sensing signal by a factor of 14dB. DCR
sensing is becoming popular because it saves expensive
current sensing resistors and is more power efficient,
especially in high current applications. However, as the
DCR value drops below 1mΩ, the signal-to-noise ratio
is low and current sensing is difficult. LTC3774 uses an
LTC proprietary technique to solve this issue. In general,
external component selection is driven by the load require
-
ment, and begins with the DCR and inductor value. Next,
power MOSFETs are selected. Finally, input and output
capacitors are selected.
Current Limit Programming
The ILIM pin is a 5-level logic input which sets the maxi
-
mum current limit of the controller. When ILIM is either
grounded
,
floated or tied to INTV
CC
, the typical value for
the maximum current sense threshold will be 10mV,
20mV or 30mV, respectively. Setting ILIM to one-fourth
INTV
CC
and three-fourths INTV
CC
for maximum current
sense thresholds of 15mV and 25mV. Setting I
LIM
using
a resistor divider off of INTV
CC
will allow the maximum
current sense threshold setting to not change when the
5.5V LDO is in dropout at start-up. Please note that the
I
LIM
pin has an internal 500k pull-down to GND and a 500k
pull-up to INTV
CC
.
Which setting should be used? For the best current limit
accuracy, use the highest setting that is applicable to the
output requirements.
SNSD
+
, SNSA
+
and SNS
–
Pins
The SNSA
+
and SNS
–
pins are the inputs to the current
comparators, while the SNSD
+
pin is the input of an
internal amplifier. The operating input voltage range is
0V to 3.5V for all three sense pins. All the positive sense
pins that are connected to the current comparator or the
amplifier are high impedance with input bias currents
of less than 1µA, but there is also a resistance of about
300k from the SNS
–
pin to ground. The SNS
–
should
be connected directly to V
OUT
. The SNSD
+
pin connects
to the filter that has a R1•C1 time constant matched to
L/DCR of the inductor. The SNSA
+
pin is connected to
the second filter with the time constant one-fifth that
of R1•C1. Care must be taken not to float these pins
during normal operation. Filter components, especially
capacitors, must be placed close to the LTC3774, and
the sense lines should run close together to a Kelvin con
-
nection underneath the current sense element (Figure3).
Because the LTC3774 is designed to be used with a very
low DCR value to sense inductor current, without proper
care, the parasitic resistance, capacitance and inductance
will degrade the current sense signal integrity, making
the programmed current limit unpredictable. As shown
in Figure 4, resistors R1 and R2 are placed close to the
output inductor and capacitors C1 and C2 are close to
the IC pins to prevent noise coupling to the sense signal.
Figure 3. Sense Lines Placement with Inductor DCR
C
OUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR
3774 F03
