NB671, 24V, HIGH CURRENT SYNCHRONOUS STEP-DOWN CONVERTER
12μs delay; then the fault latch will be triggered---
Thermal Shutdown
latches HS off and LS on; the LS FET keeps on
until the inductor current goes zero.
Thermal shutdown is employed in the NB671.
The junction temperature of the IC is internally
monitored. If the junction temperature exceeds
the threshold value (typical 150ºC), the converter
shuts off. This is a non-latch protection. There is
about 25ºC hysteresis. Once the junction
temperature drops to about 125ºC, it initiates a
SS.
UVLO Protection
The NB671 has under-voltage lock-out protection
(UVLO). When the VCC voltage is higher than
the UVLO rising threshold voltage, the part will
be powered up. It shuts off when the VIN voltage
is lower than the UVLO falling threshold voltage.
This is non-latch protection. The part is disabled
when the VCC voltage falls below 4.6V. If an
application requires a higher under-voltage
lockout (UVLO), use the EN pin as shown in
Figure 9 to adjust the input voltage UVLO by
using two external resistors. It is recommended
to use the enable resistors to set the UVLO
falling threshold (VSTOP) above 4.6V. The rising
threshold (VSTART) should be set to provide
enough hysteresis to allow for any input supply
variations.
Output Discharge
NB671 discharges the output when EN is low, or
the controller is turned off by the protection
functions (UVP & OCP, OCP, OVP, UVLO, and
thermal shutdown). The part discharges outputs
using an internal 6Ω MOSFET which is
connected to VOUT and GND. The external low-
side MOSFET is not turned on for the output
discharge operation to avoid the possibility of
causing negative voltage at the output.
Figure 9—Adjustable UVLO
NB671 Rev. 1.0
1/14/2013
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