LTC1735-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1735-1' PDF : 28 Pages View PDF
LTC1735-1
APPLICATIO S I FOR ATIO
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 +/ SENSE – Pins
The common mode input range of the current comparator
is from 0V to 1.1(INTVCC). Continuous linear operation is
guaranteed throughout this range allowing output volt-
ages 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 either to be sourced or sunk by these
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 VOUT resistive divider. The maxi-
mum current flowing out of the 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 3 to have a maximum value to
absorb this current:
R1(Max)
=
24k
0.8V
2.4V – VOUT
Regulating an output voltage of 1.8V, the maximum value
of R1 should be 32k. Note that for output voltages above
2.4V no maximum value of R1 is necessary to absorb the
sense currents; however, R1 is still bounded by the
VOSENSE feedback current.
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 LTC1735-1.
Soft-start reduces surge currents from VIN by gradually
increasing the controller’s current limit ITH(MAX). This pin
can also be used for power supply sequencing.
Pulling the RUN/SS pin below 1.5V puts the LTC1735-1
into a low quiescent current shutdown (IQ < 25µA). This
pin can be driven directly from logic as shown in Figures
4 and 5. Releasing the RUN/SS pin allows an internal
1.2µA current source to charge up the external soft-start
capacitor CSS. If RUN/SS has been pulled all the way to
ground there is a delay before starting of approximately:
( ) TDELAY
=
1.5V
1.2µA
CSS
=
1.25s/µF
CSS
When the voltage on RUN/SS reaches 1.5V the
LTC1735-1 begins operating with a current limit at ap-
proximately 25mV/RSENSE. As the voltage on RUN/SS
increases from 1.5V to 3V, the internal current limit is
increased from 25mV/RSENSE to 75mV/RSENSE. The out-
put current limit ramps up slowly, taking an additional
1.25s/µF to reach full current. Ramping the output cur-
rent slowly reduces the starting surge current
required from the input supply.
Diode D1 in Figure 4 and Figure 5 reduces the start delay
while allowing CSS to charge up slowly for the soft-start
function. This diode and CSS can be deleted if soft-start is
not needed. The RUN/SS pin has an internal 6V zener
clamp (see Functional Diagram).
3.3V OR 5V
RUN/SS
D1
CSS
RUN/SS
CSS
Figure 4. RUN/SS Pin Interfacing
1735-1 F04
3.3V OR 5V
D1
VIN
RUN/SS
RSS
CSS
INTVCC
RSS
D1 RUN/SS
CSS
(a)
(b)
1735-1 F05
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated
Fault Conditions: Overcurrent Latchoff
The RUN/SS pin also provides the ability to shut off the
controller and latchoff when an overcurrent condition is
detected. The RUN/SS capacitor CSS is used initially to
turn on and limit the inrush current of the controller. After
the controller has been started and given adequate time to
charge up the output capacitor and provide full load
16
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