DESCRIPTION
Evaluation circuit EVAL-LT8350-AZ is a 40V synchronous
buck-boost converter featuring the LT®8350. It drives
up to 2.5A load at 12V output when VIN is between 9V
and 40V and will run down to 3VIN with reduced output
current. EVAL-LT8350-AZ runs at 350kHz switching
frequency with spread spectrum frequency modulation
(SSFM) disabled. When enabled, SSFM spreads the
switching frequency of the LT8350 from fSW to fSW +
25% for reduced EMI emission.
The LT8350 has an operating input voltage range of
3V to 40V. It has internal, synchronous 42V MOSFETs
at buck-side and 20V MOSFETs at boost-side for high
efficiency and small size. It has an adjustable switching
frequency between 200kHz and 2MHz. The LT8350 can
be synchronized to an external source, programmed with
SSFM enabled for low EMI, or set to normal operation.
The LT8350’s integrated LOADTG high side PMOS driver
assists with disconnecting load when a fault is triggered.
LOADTG turns off the PMOS when FB < 0.25V or FB >
1.1V or ISP–ISN > 0.75V. LOADEN can be used directly
to turn off LOADTG and all power switches.
The LT8350 can regulate output current using ISP and
ISN. Maximum load current can be limited when LT8350
is used for a constant voltage regulator. It can be also used
for a LED driver or a battery charger where constant current
regulation is required. Output current can be adjusted
by placing a controllable DC voltage on the CTRL pin.
The output current can be monitored through the ISMON
output pin. ISMON can be used to improve load transient
response by injecting load current to VC. The load current
injection is described in the following sections.
Undervoltage lockout can be adjusted on EVAL-LT8350-AZ
with a few simple resistor choices.
Small ceramic input and output capacitors are used to
save space and cost. The board is designed with tiny, high
frequency capacitors placed near the VIN and VOUT pins
for a reduction in radiated EMI.
There is an inductor EMI filter and a small ferrite bead EMI
filter on the input and output of EVAL-LT8350-AZ. These
filters, combined with proper board layout and SSFM,
are effective in reducing EMI to pass CISPR25 class 5
conducted EMI. Please follow the recommended layout
and four-layer PCB thickness of EVAL-LT8350-AZ for low
EMI applications.
The LT8350 data sheet gives a complete description of
the part, operation, and applications information. The data
sheet must be read in conjunction with this demo manual
for EVAL-LT8350-AZ. The LT8350RV is assembled in a
32-lead laminate package with QFN footprint (LQFN) with
thermally enhanced exposed ground pads. Proper board
layout is essential for maximum thermal performance.