ltc3728legn-trpbf Linear Technology Corporation, ltc3728legn-trpbf Datasheet - Page 21

• Manufacturer Part Number: ltc3728legn-trpbf
• Description: Dual, 550khz, 2-phase Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
reference by the error amplifi er. The output voltage is given
by the equation:
where R1 and R2 are defi ned in Figure 2.
SENSE
The common mode input range of the current comparator
sense pins is from 0V to (1.1)INTV
operation is guaranteed throughout this range allowing
output voltage setting from 0.8V to 7.7V, depending upon
the voltage applied to EXTV
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
comparator’s negative input bias current. The maximum
current fl owing out of each pair of SENSE pins is:
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.
I
for V
R1
V
SENSE
OUT
OUT
(MAX)
+
OUT
OSENSE
/SENSE
= 0.8V 1+
+
resistive divider to compensate for the current
+ I
= 24k
< 2.4V
SENSE
is servoed to the 0.8V reference voltage,
–
PINS
–
2.4V – V
R2
= (2.4V – V
R1
0.8V
OUT
CC
. A differential NPN input
OUT
CC
)/24k
. Continuous linear
OSENSE
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
used for power supply sequencing.
An internal 1.2μA current source charges up the C
tor
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
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 LTC3728 is
put into low current shutdown (I
pins can be driven directly from logic as shown in Figure
7. Diode D1 in Figure 7 reduces the start delay but allows
C
Each RUN/SS pin has an internal 6V zener clamp (See
Functional Diagram).
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
SS
.
t
t
When the voltage on RUN/SS1 (RUN/SS2) reaches 1.5V,
DELAY
IRAMP
to ramp up slowly providing the soft-start function.
SENSE
=
=
1.2μA
3V 1.5V
1.5V
. The output current limit ramps up slowly,
1.2μA
C
SS
C
= 1.25s / μF
SS
(
SS
= 1.25s / μF
, is used initially to turn on
(
ITH
Q
). This pin can also be
= 20μA). The RUN/SS
)
C
LTC3728
SS
)
C
SS
SS
SENSE
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to