LTC3867_15 Datasheet, PDF (20/36 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3867
APPLICATIONS INFORMATION
Pre-Biased Output Start-Up
There may be situations that require the power supply to
start up with a pre-bias on the output capacitors. In this
case, it is desirable to start up without discharging that
output pre-bias. The LTC3867 can safely power up into a
pre-biased output without discharging it.
The LTC3867 accomplishes this by disabling both TG and
BG until the TK/SS pin voltage and the internal soft-start
voltage are above the V_FB pin voltage. When V_FB is higher
than TK/SS or the internal soft-start voltage, the error amp
output is railed low. The control loop would like to turn
BG on, which would discharge the output. Disabling BG
and TG prevents the pre-biased output voltage from being
discharged. When TK/SS and the internal soft-start both
cross 500mV or V_FB, whichever is lower, TG and BG are
enabled. If the pre-bias is higher than the OV threshold
however, the bottom gate is turned on immediately to pull
the output back into the regulation window.
Overcurrent Fault Recovery
When the output of the power supply is loaded beyond
its preset current limit, the regulated output voltage
will collapse depending on the load. The output may be
shorted to ground through a very low impedance path or
it may be a resistive short, in which case the output will
collapse partially, until the load current equals the preset
current limit. The controller will continue to source current
into the short. The amount of current sourced depends
on the I_LIM pin setting and the V_FB voltage as shown in
the Current Foldback graph in the Typical Performance
Characteristics section.
Upon removal of the short, the output soft starts using
the internal soft-start, thus reducing output overshoot. In
the absence of this feature, the output capacitors would
have been charged at current limit, and in applications
with minimal output capacitance this may have resulted
in output overshoot. Current limit foldback is not disabled
during an overcurrent recovery. The load must step below
the folded back current limit threshold in order to restart
from a hard short.
Thermal Protection
Excessive ambient temperatures, loads and inadequate
airflow or heat sinking can subject the chip, inductor,
FETs etc. to high temperatures. This thermal stress re-
duces component life and if severe enough, can result
in immediate catastrophic failure. To protect the power
supply from undue thermal stress, the LTC3867 has a
fixed chip temperature-based thermal shutdown and an
external inductor temperature-based thermal shutdown
that is adjustable. The internal thermal shutdown is set
for 160°C with 10°C of hysteresis. When the chip reaches
160°C, both TG and BG are disabled until the chip cools
down below 150°C.
In addition, the ITSD pin sources 20µA of current. By plac-
ing a Murata PRF18 series PTC thermistor between this
pin and ground, close to the inductor, the top and bottom
FET can be turned off when the inductor reaches a pre-set
temperature. The Murata PRF18 series PTC thermistors
have a typical resistance of 470Ω at room temperature.
Their temperature dependence is nonlinear. Over a fairly
narrow temperature range, the resistance changes a few
orders of magnitude. The LTC3867 trips when the PTC
resistance is at about 47k. The PRF18 series includes
thermistors with different trip points—select one based
on the shutdown temperature desired. Please refer to the
Murata data sheets for more details regarding the PRF18
series PTC thermistors.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant frequen-
cy current mode architectures by preventing sub-harmonic
oscillation at high duty cycles. It is accomplished internally
by adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this
results in a reduction of maximum inductor peak current
for duty cycles greater than 40%. However, the LTC3867
uses a scheme that counteracts this compensating ramp,
which allows the maximum inductor peak current to remain
unaffected throughout all duty cycles.
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