LTC3728LXCUH#TR Linear Technology, LTC3728LXCUH#TR Datasheet - Page 22

• Manufacturer Part Number: LTC3728LXCUH#TR
• Description: IC CTRLR SW REG 2-PH SYNC 32-QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3728LXCUH#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 7 V
• Current - Output: 3A
• Frequency - Switching: 250kHz ~ 550kHz
• Voltage - Input: 4.5 ~ 28 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS INFORMATION
LTC3728L/LTC3728LX
Since V
we can choose R1 in Figure 2 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 for an output voltage above
2.4V, R1 has no maximum value necessary to absorb the
sense currents; however, R1 is still bounded by the V
feedback current.
Soft-Start/Run Function
The RUN/SS1 and RUN/SS2 pins are multipurpose pins
that provide a soft-start function and a means to shut down
the LTC3728L/LTC3728LX. Soft-start reduces the input
power source’s surge currents by gradually increasing the
controller’s current limit (proportional to V
can also be used for power supply sequencing.
An internal 1.2μA current source charges up the C
pacitor
1.5V, the particular controller is permitted to start operating.
As the voltage on RUN/SS increases from 1.5V to 3.0V,
the internal current limit is increased from 25mV/R
to 75mV/R
taking an additional 1.25s/μF to reach full current. The
output current thus ramps up slowly, reducing the start-
ing surge current required from the input power supply.
If RUN/SS has been pulled all the way to ground there is
a delay before starting of approximately:
By pulling both RUN/SS pins below 1V, the IC is put into
low current shutdown (I
can be driven directly from logic, as shown in Figure 7.
Diode, D1, in Figure 7 reduces the start delay but allows
C
22
SS
for V
R1
t
t
DELAY
IRAMP
to ramp up slowly providing the soft-start function.
(MAX)
.
When the voltage on RUN/SS1 (RUN/SS2) reaches
OUT
OSENSE
=
=
SENSE
= 24k
< 2.4V
1.2μA
3V 1.5V
1.5V
1.2μA
. The output current limit ramps up slowly,
is servoed to the 0.8V reference voltage,
2.4V – V
C
SS
0.8V
C
= 1.25s / μF
SS
(
Q
OUT
= 1.25s / μF
= 20μA). The RUN/SS pins
(
)
C
SS
)
C
ITH
SS
). This pin
OSENSE
SS
SENSE
ca-
Fault Conditions: Overcurrent Latchoff
The RUN/SS pins also provide the ability to latch off the
controller(s) when an overcurrent condition is detected.
The RUN/SS capacitor, C
and limit the inrush current. After the controller has been
started and been given adequate time to charge up the
output capacitor and provide full load current, the RUN/SS
capacitor is used for a short-circuit timer. If the regulator’s
output voltage falls to less than 70% of its nominal value
after C
sumption that the output is in an overcurrent condition. If
the condition lasts for a long enough period as determined
by the size of the C
the controller will be shut down until the RUN/SS pin volt-
age is recycled. If the overload occurs during start-up, the
time can be approximated by:
If the overload occurs after start-up the voltage on C
begin discharging from the Zener clamp voltage:
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor to the RUN/SS pin, as shown
in Figure 7. This resistance shortens the soft-start period
and prevents the discharge of the RUN/SS capacitor
during an over current condition. Tying this pull-up resistor
to V
Why should you defeat overcurrent latchoff? During the
prototyping stage of a design, there may be a problem
Each RUN/SS pin has an internal 6V Zener clamp (see the
Functional Diagram).
t
t
LO1
LO2
IN
, as in Figure 7, defeats overcurrent latchoff.
SS
≈ [C
≈ [C
= 2.7 • 10
reaches 4.1V, C
SS
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
SS
Figure 7. RUN/SS Pin Interfacing
(6 – 3.5)]/(1.2μA) = 2.1 • 10
(4.1 – 1.5 + 4.1 – 3.5)]/(1.2μA)
3.3V OR 5V
6
SS
(C
and the specifi ed discharge current,
SS
)
SS
D1
SS
begins discharging on the as-
, is used initially to turn on
V
IN
R
SS
*
RUN/SS
C
SS
6
3728 F07
(C
SS
)
SS
3728lxfe
will