LTC3728_15 Datasheet, PDF (21/36 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
APPLICATIONS INFORMATION
reference by the error amplifier. The output voltage is given
by the equation:
V_OUT
=
0.8V



1+
R2 
R1
where R1 and R2 are defined 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 flow 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 flowing 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/μF 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_DELAY
=
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).
3728fg
21