NB675L –24V, HIGH CURRENT SYNCHRONOUS BUCK CONVERTER
NOT RECOMMENDED FOR NEW DESIGNS, REFER TO NB685A
connected to the source terminal of the bottom
MOSFET.
UVLO Protection
The NB675L 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 4.5V. 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.
Since the comparison is done during the high
side MOSFET OFF and low side MOSFET ON
state, the OC trip level sets the valley level of the
inductor current. Thus, the load current at over-
current threshold, IOC, can be calculated as
follows:
I
inductor
IOC = I_limit +
(13)
2
In an over-current condition, the current to the
load exceeds the current to the output capacitor;
thus the output voltage tends to fall off.
Eventually, it will end up with crossing the under
voltage protection threshold and shutdown.
VTT Over-Current Protection
The VTT LDO has an internally fixed current limit
of 1.8A for source and 1.6A for sink operation.
Once the VTT current reaches these limitations,
the current will be limited until the OC removed.
IN
NB675L
RUP
EN Comparator
VDDQ
Over/Under-Voltage
Protection
EN
(OVP/UVP)
NB675L monitors a resistor divided feedback
voltage to detect over and under voltage. When
the feedback voltage becomes higher than 115%
of the target voltage, the controller will enter
Dynamic Regulation Period. During this period,
the LS will off when the LS current goes to -1A,
this will then discharge the output and try to keep
it within the normal range. If the dynamic
regulation can not limit the increasing of the Vo,
once the feedback voltage becomes higher than
130% of the feedback voltage, the OVP
comparator output goes high and the circuit
latches as the high-side MOSFET driver OFF
and the low-side MOSFET turn on acting as an -
1A current source.
RDOWN
Figure 9—Adjustable UVLO
Thermal Shutdown
Thermal shutdown is employed in the NB675L.
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.
Output Discharge
When the feedback voltage becomes lower than
60% of the target voltage, the UVP comparator
output goes high if the UV still occurs after 4.5us
delay; then the fault latch will be triggered---
latches HS off and LS on; the LS FET keeps on
until the inductor current goes zero.
NB675L discharges all the outputs, including
VDDQ, VTTREF and VTT when EN2=low, or the
controller is turned off by the protection functions
(UVP & OCP, OCP, OVP, UVLO, and thermal
shutdown). The part discharge the outputs using
an internal 6Ω MOSFET.
NB675L Rev. 1.1
1/19/2020
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2023 MPS. All Rights Reserved.
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