ADP3168JRUZ-REEL AD [Analog Devices], ADP3168JRUZ-REEL Datasheet - Page 10

• Manufacturer Part Number: ADP3168JRUZ-REEL
• Description: 6-Bit, Programmable 2-, 3-, 4-Phase Synchronous Buck Controller
• Manufacturer: AD [Analog Devices]

ADP3168
The positive input of the CSA is connected to the CSREF pin,
which is connected to the output voltage. The inputs to the
amplifier are summed together through resistors from the
sensing element (such as the switch node side of the output
inductors) to the inverting input, CSSUM. The feedback resistor
between CSCOMP and CSSUM sets the gain of the amplifier,
and a filter capacitor is placed in parallel with this resistor.
The gain of the amplifier is programmable by adjusting the
feedback resistor to set the load line required by the micro-
processor. The current information is then given as the differ-
ence of CSREF − CSCOMP. This difference signal is used
internally to offset the VID DAC for voltage positioning and
as a differential input for the current-limit comparator.
To provide the best accuracy for the current sensing, the CSA
was designed to have a low offset input voltage. Also, the
sensing gain is determined by external resistors so that it can
be made extremely accurate.
ACTIVE IMPEDANCE CONTROL MODE
For controlling the dynamic output voltage droop as a function
of output current, a signal proportional to the total output
current at the CSCOMP pin can be scaled to be equal to the
droop impedance of the regulator times the output current.
This droop voltage is then used to set the input control voltage
to the system. The droop voltage is subtracted from the DAC
reference input voltage directly to tell the error amplifier where
the output voltage should be. This differs from previous
implementations and allows enhanced feed-forward response.
CURRENT-CONTROL MODE AND THERMAL
BALANCE
The ADP3168 has individual inputs that are used for
monitoring the current in each phase. This information is
combined with an internal ramp to create a current-balancing
feedback system that has been optimized for initial current
balance accuracy and dynamic thermal balancing during
operation. This current-balance information is independent of
the average output current information used for positioning
described previously.
The magnitude of the internal ramp can be set to optimize
the transient response of the system. It also monitors the
supply voltage for feed-forward control for changes in the
supply. A resistor connected from the power input voltage to
the RAMPADJ pin determines the slope of the internal PWM
ramp. Detailed information about programming the ramp is
given in the Application Information section.
External resistors can be placed in series with individual phases,
for example, to create an intentional current imbalance so one
phase may have better cooling and can support higher currents.
Rev. B | Page 10 of 24
Resistors R
in Figure 11) can be used for adjusting thermal balance. It is
best to add these resistors during the initial design, so make
sure placeholders are provided in the layout.
To increase the current in any given phase, make R
phase larger. (Make R
change during balancing.) Increasing R
a substantial increase in phase current. Increase each R
by small amounts to achieve balance, starting with the coolest
phase first.
VOLTAGE CONTROL MODE
A high gain bandwidth voltage mode error amplifier is used for
the voltage-mode control loop. The control input voltage to the
positive input is set via the VID 6-bit logic code, according to
the voltages listed in Table 4. This voltage is also offset by the
droop voltage for active positioning of the output voltage as a
function of current, commonly known as active voltage
positioning. The output of the amplifier is the COMP pin,
which sets the termination voltage for the internal PWM ramps.
The negative input (FB) is tied to the output sense location with
a resistor, R
voltage at this point. A current source from the FB pin flowing
through R
the VID voltage. The no-load voltage is negative with respect to
the VID DAC. The main loop compensation is incorporated
into the feedback network between FB and COMP.
SOFT START
The power-on ramp-up time of the output voltage is set with a
capacitor and a resistor in parallel from the DELAY pin to
ground. The RC time constant also determines the current-limit
latch-off time, as explained in the following section. In UVLO
or when EN is a logic low, the DELAY pin is held at ground.
After the UVLO threshold is reached and EN is a logic high, the
DELAY capacitor is charged up with an internal 20 µA current
source. The output voltage follows the ramping voltage on the
DELAY pin, limiting the inrush current. The soft-start time
depends on the values of VID DAC and C
effect from R
for detailed information on setting C
When the PWRGD threshold is reached, the soft-start cycle is
stopped and the DELAY pin is pulled up to 3 V. This ensures
that the output voltage is at the VID voltage when the PWRGD
signals to the system that the output voltage is good. If EN is
taken low or VCC drops below UVLO, the DELAY capacitor is
reset to ground to be ready for another soft-start cycle. Figure 8
shows a typical start-up sequence for the ADP3168.
B
SW1
B,
is used for setting the no-load offset voltage from
DLY
and is used for sensing and controlling the output
through R
. Refer to the Application Information section
SW
= 0 for the hottest phase and do not
SW4
(see the typical applica-tion circuit
DLY
SW
.
to only 500 Ω makes
DLY
, with a secondary
SW
for that
SW
value