DESCRIPTION
Demonstration circuit 3126A is a quad power supply featuring
the LT®8686S. The LT8686S is a 42V quad channel
current mode monolithic synchronous step-down regulator.
The LT8686S combines two 42V capable 2A buck
regulators with two 8V capable 2A buck regulators. The
demo board is designed for 5V, 3.3V, 1.8V, and 1.2V outputs
from a nominal 12V input with switching frequency
set at 2MHz. The 5V and 3.3V converters are powered
from a wide range of 6V to 42V. The 1.8V and 1.2V converters
are powered from the 5V output by default or from
3V to 8V supply alternatively. The current capability is 2A
for all 4 outputs when running individually.
The DC3126A provides two 42V regulators that can be
combined to deliver up to 4A of output current using a
singular inductor. Similarly, the two 8V regulators can be
combined to deliver up to 4A of output current using a
singular inductor. The allowed channel combinations are
given in the data sheet.
The independent track/soft-start and power good for each
output simplify the complex design of quad-output power
converters. Each output can be independently disabled
into low quiescent current shutdown mode with its own
TRK/SS pin.
A user-selectable SYNC/MODE pin on the demo board
provides two primary modes of operation: pulse-skipping
mode and low ripple Burst Mode® operation, plus
the option of selecting frequency spread-spectrum for
each to improve the EMI/EMC performance. Burst Mode
delivers higher efficiency at light load than pulse-skipping
mode. In pulse-skipping mode, full switching frequency
is maintained to lower load currents than Burst Mode.
The SYNC/MODE pin can also be used to synchronize the
switching frequency to an external clock. The switching
frequency for all regulators can be programmed either via
an oscillator resistor or an external clock over a 350kHz
to 3MHz range. At all frequencies, a 180° phase shift is
maintained between channel 1 and channel 2, channel 3
and channel 4, reducing the input peak current and voltage
ripple.
The demo board has an EMI filter installed on the bottom
layer. The conducted and radiated EMI performance of
the board is shown on Figure 4. The red line in Figure 4
is CISPR25 Class 5 peak limit. The figure shows that the
circuit passes the test with a wide margin. To achieve EMI/
EMC performance as shown in Figure 4, the input EMI
filter is required, and the input voltage should be applied
at VEMI turret.
The LT8686S data sheet gives a complete description of
the part operation and application information. The data
sheet must be read in conjunction with this quick start
guide for DC3126A. The LT8686S is assembled in a 5mm
× 5mm LQFN package with exposed pads for low thermal
resistance. Proper board layout is essential for both low
EMI operation and best thermal performance.