LTC3866 Datasheet, PDF (17/36 Pages) Linear Technology – Current Mode Synchronous Controller for Sub Milliohm DCR Sensing

LTC3866
Applications Information
50
45
40
CORRECTED
35
I_MAX
30
25 _RITEMP:
20 ^{RS = 22.6k}
R_P = 90.9k
15 NTC THERMISTOR:
UNCORRECTED
I_MAX
10
R_O = 100k
T_O = 25°C
5 ^{B = 4334}
NOMINAL I_MAX = 30A
0
–50 –25 0 25 50 75 100 125 150
INDUCTOR TEMPERATURE (°C)
3866 F05
Figure 5. Worst-Case I_MAX Versus Inductor Temperature Curve
with and without NTC Temperature Compensation
V_OUT
R_NTC
L1
SW1
3867 F08
Figure 6. Thermistor Location. Place the Thermistor Next to
the Inductor for Accurate Sensing of the Inductor Temperature,
But Keep the ITEMP Pin Away from the Switch Nodes and Gate
Drive Traces
For the most accurate temperature detection, place the
thermistor next to the output inductor as shown in Figure 6.
Care should be taken to keep the ITEMP sense line away
from switch nodes.
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 LTC3866 can safely power up into a
pre-biased output without discharging it.
The LTC3866 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,
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 ILIM 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 (Note 4). To protect the
power supply from undue thermal stress, the LTC3866
has a fixed chip temperature-based thermal shutdown.
The internal thermal shutdown is set for approximately
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.
3866fa
17