Datasheet

LTC3728
21
3728fg
APPLICATIONS INFORMATION
reference by the error amplifi er. The output voltage is given
by the equation:
V
OUT
= 0.8V 1+
R2
R1






where R1 and R2 are defi ned in Figure 2.
SENSE
+
/SENSE
–
PINS
The common mode input range of the current comparator
sense pins is from 0V to (1.1)INTV
CC
. Continuous linear
operation is guaranteed throughout this range allowing
output voltage setting from 0.8V to 7.7V, depending upon
the voltage applied to EXTV
CC
. A differential NPN input
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 fl ow out of both SENSE pins to
the main output. The output can be easily preloaded by
the V
OUT
resistive divider to compensate for the current
comparator’s negative input bias current. The maximum
current fl owing out of each pair of SENSE pins is:
I
SENSE
+
+ I
SENSE
–
= (2.4V – V
OUT
)/24k
Since V
OSENSE
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 – V
OUT






for V
OUT
< 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 V
OSENSE
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 LTC3728. Soft-start reduces the input power source’s
surge currents by gradually increasing the controller’s
current limit (proportional to V
ITH
). This pin can also be
used for power supply sequencing.
An internal 1.2µA current source charges up the C
SS
capacitor.
When the voltage on RUN/SS1 (RUN/SS2)
reaches 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
SENSE
to 75mV/R
SENSE
. The output current limit ramps
up slowly, taking an additional 1.25s/F to reach full cur-
rent. The output current thus ramps up slowly, reducing
the starting 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:
t
D ELAY
=
1.5V
1.2μA
C
SS
= 1.25s / μF
()
C
SS
t
IRAMP
=
3V  1.5V
1.2μA
C
SS
= 1.25s / μF
()
C
SS
By pulling both RUN/SS pins below 1V, the LTC3728 is put
into low current shutdown (I
Q
= 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
C
SS
to ramp up slowly providing the soft-start function.
Each RUN/SS pin has an internal 6V Zener clamp (see
the Functional Diagram).