參數(shù)資料
型號(hào)： LTC3728LIUH-1#TR
廠(chǎng)商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： 3 A DUAL SWITCHING CONTROLLER, 590 kHz SWITCHING FREQ-MAX, PQCC32
封裝： 5 X 5 MM, PLASTIC, MO-220WHHD, QFN-32
文件頁(yè)數(shù)： 11/32頁(yè)
文件大?。?/td> 475K
代理商： LTC3728LIUH-1#TR
LTC3728L-1
19
3728l1fc
APPLICATIONS INFORMATION
stage is biased with internal resistors from an internal 2.4V
source as shown in the Functional Diagram. This requires
that current either be sourced or sunk from the SENSE
pins depending on the output voltage. If the output voltage
is below 2.4V current will ow out of both SENSE pins to
the main output. The output can be easily preloaded by
the VOUT resistive divider to compensate for the current
comparator’s negative input bias current. The maximum
current owing out of each pair of SENSE pins is:
ISENSE+ + ISENSE– = (2.4V – VOUT)/24k
Since VOSENSE is servoed to the 0.8V reference voltage,
we can choose R1 in Figure 2 to have a maximum value
to absorb this current.
R1(MAX) = 24k
0.8V
2.4V – VOUT
for VOUT < 2.4V
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 VOSENSE
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-1. Soft-start reduces the input power source’s
surge currents by gradually increasing the controller’s
current limit (proportional to VITH). This pin can also be
used for power supply sequencing.
An internal 1.2μA current source charges up the CSScapaci-
tor.WhenthevoltageonRUN/SS1(RUN/SS2)reaches1.5V,
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/RSENSE to
75mV/RSENSE. The output current limit ramps up slowly,
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:
tDELAY =
1.5V
1.2
μA
CSS = 1.25s / μF
()C
SS
tIRAMP =
3V 1.5V
1.2
μA
CSS = 1.25s / μF
()C
SS
By pulling both RUN/SS pins below 1V, the IC is put into
low current shutdown (IQ = 20μA). The RUN/SS pins
can be driven directly from logic as shown in Figure 7.
Diode D1 in Figure 7 reduces the start delay but allows
CSS to ramp up slowly providing the soft-start function.
Each RUN/SS pin has an internal 6V zener clamp (See
Functional Diagram). Because the LTC3728L-1 is designed
for applications not requiring over current latchoff, no
pull-up resistor is required on the RUN/SS pin to defeat
latchoff. Refer to the LTC3728L/LTC3728LX datasheet if
this feature is required.
Fault Conditions: Current Limit and Current Foldback
The current comparators have a maximum sense volt-
age of 75mV resulting in a maximum MOSFET current
of 75mV/RSENSE. The maximum value of current limit
generally occurs with the largest VIN at the highest ambi-
Figure 7. RUN/SS Pin Interfacing
3.3V OR 5V
RUN/SS
D1
(a)
(b)
CSS
RUN/SS
CSS
3728L1 F07
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