LTC1735CGN#TR Linear Technology, LTC1735CGN#TR Datasheet - Page 17

• Manufacturer Part Number: LTC1735CGN#TR
• Description: IC REG SW SYNC STEPDWN HE 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1735CGN#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 6 V
• Current - Output: 3A
• Frequency - Switching: 300kHz
• Voltage - Input: 4 ~ 30 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
Topside MOSFET Driver Supply (C
An external bootstrap capacitor C
BOOST pin supplies the gate drive voltage for the topside
MOSFET. Capacitor C
though external diode D
is low. Note that the voltage across C
drop below INTV
turned on, the driver places the C
gate-source of the MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node voltage
SW rises to V
The value of the boost capacitor C
greater than the total input capacitance of the topside
MOSFET. In most applications 0.1 F to 0.33 F is ad-
equate. The reverse breakdown on D
than V
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If you make a change
and the input current decreases, then you improved the
efficiency. If there is no change in input current, then there
is no change in efficiency.
SENSE
The common mode input range of the current comparator
is from 0V to 1.1(INTV
step-down applications is guaranteed throughout this
range allowing output voltages anywhere from 0.8V to 7V.
A differential NPN input stage is used and is biased with
internal resistors from an internal 2.4V source as shown
in the Functional Diagram. This causes current to either be
sourced or sunk by the sense pins depending on the
output voltage. If the output voltage is below 2.4V current
will flow out of both sense pins to the main output. This
forces a minimum load current that can be fulfilled by the
IN(MAX)
+
/SENSE
Figure 4. Setting the LTC1735 Output Voltage
IN
.
and the BOOST pin rises to V
–
CC
Pins
LTC1735
U
. When the topside MOSFET is to be
B
in the Functional Diagram is charged
CC
V
OSENSE
SGND
B
). Continuous linear operation in
from INTV
U
1735 F04
B
47pF
needs to be 100 times
B
W
B
B
, D
CC
voltage across the
V
B
B
connected to the
OUT
when the SW pin
B
must be greater
is about a diode
R2
R1
)
IN
+ INTV
U
CC
.
V
of the sense pins is:
Since V
can choose R1 in Figure 4 to have a maximum value to
absorb this current:
Regulating an output voltage of 1.8V, the maximum value
of R1 should be 32k. Note that at output voltages above
2.4V no maximum value of R1 is necessary to absorb the
sense pin currents; however, R1 is still bounded by the
V
Soft-Start/Run Function
The RUN/SS pin is a multipurpose pin that provides a soft-
start function and a means to shut down the LTC1735.
Soft-start reduces surge currents from V
increasing the controller’s current limit I
can also be used for power supply sequencing.
Pulling the RUN/SS pin below 1.5V puts the LTC1735 into
shutdown. This pin can be driven directly from logic as
shown in Figure 5. The V
of RUN/SS voltage (refer to Typical Performance Charac-
teristics graphs on page 6). Releasing the RUN/SS pin
allows an internal 1.2 A current source to charge up the
external soft-start capacitor C
pulled all the way to ground there is a delay before starting
of approximately:
When the voltage on RUN/SS reaches 1.5V the LTC1735
begins operating with a current limit at approximately
25mV/R
from 1.5V to 3.0V, the internal current limit is increased
from 25mV/R
limit ramps up slowly, taking an additional 1.25s/ F to
reach full current. The output current thus ramps up
slowly, reducing the starting surge current required from
the input power supply.
OUT
OSENSE
I
R
t
SENSE
DELAY
1
resistive divider. The maximum current flowing out
(
MAX
OSENSE
SENSE
+
feedback current.
)
+ I
1 2
SENSE
1 5
. As the voltage on the RUN/SS pin increases
SENSE
24
.
is servoed to the 0.8V reference voltage, we
.
k
V
A
–
2 4
C
to 75mV/R
= (2.4V – V
.
SS
V V
0 8
IN
.
–
quiescent current is a function
V
1 25
OUT
.
SENSE
SS.
OUT
s F C
/
If RUN/SS has been
)/24k
. The output current
SS
LTC1735
TH(MAX)
IN
by gradually
. This pin
17
1735fc