LTC1708-PG_15 Datasheet, PDF (20/32 Pages) Linear Technology – Dual Adjustable 5-Bit VID High Efficiency, 2-Phase Current Mode Synchronous Buck DC/DC Controller

LTC1708-PG
APPLICATIO S I FOR ATIO
Between the ATTNOUT pin and ground is a variable resis-
tor, R1, whose value is controlled by the five input pins (VID0
to VID4). Another resistor, R2, between the ATTNIN and
the ATTNOUT pins completes the resistive divider. The
output voltage is thus set by the ratio of (R1 + R2) to R1.
The LTC1708 has remote sense capability. The top of the
internal resistive divider is connected to ATTNIN, and it is
referenced to the SGND pin. This allows a Kelvin connec-
tion for remotely sensing the output voltage directly across
the load, eliminating any PC board trace resistance errors.
Each VID digital input is pulled up by a 40k resistor in
series with a diode from VIDV_CC. Therefore, it must be
grounded to get a digital low input, and can be either
floated or connected to VIDV_CC to get a digital high input.
The series diode is used to prevent the digital inputs from
being damaged or clamped if they are driven higher than
VIDV_CC. The digital inputs accept CMOS voltage levels.
VIDV_CC is the supply voltage for the VID section. It is
normally connected to INTV_CC but can be driven from
other sources such as a 3.3V supply. If it is driven from
another source, that source MUST be in the range of 2.7V
to 5.5V and MUST be alive prior to enabling the LTC1708.
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_EAIN is servoed to the 0.8V reference voltage, we
can choose R1 in Figure 8 to have a maximum value to
absorb this current.
20
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_EAIN
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 LTC1708. 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
current. The output current thus ramps up slowly, reduc-
ing 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:
( ) ^tDELAY
=
1.5V
1.2µA
C_SS
=
1.25s / µF
C_SS
( ) ^tIRAMP
=
3V − 1.5V
1.2µA
C_SS
=
1.25s / µF
C_SS
By pulling both RUN/SS pins below 1V and/or pulling the
STBYMD pin below 0.2V, the LTC1708 is put into low
current shutdown (I_Q = 20µA). The RUN/SS pins can be
driven directly from logic as shown in Figure 8. Diode D1
in Figure 8 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 Functional
Diagram).
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