RT6248B
Operation
The RT6248B is a high-efficiency, synchronous step-down
DC-DC converter that can deliver up to 8A output current
from a 4.5V to 18V input supply. The RT6248B adopts
ACOT
TM
control mode, which can reduce the output
capacitance and provide ultrafast transient responses, and
allow minimal component sizes without any additional
external compensation network.
Input Under-Voltage Lockout
In addition to the EN pin, the RT6248B also provides enable
control through the VIN pin. It features an under-voltage
lockout (UVLO) function that monitors the internal linear
regulator (VCC). If V
EN
rises above V
ENH
first, switching
will still be inhibited until the VIN voltage rises above V
UVLO
.
It is to ensure that the internal regulator is ready so that
operation with not-fully-enhanced internal MOSFET
switches can be prevented. After the device is powered
up, if the input voltage VIN goes below the UVLO falling
threshold voltage (V
UVLO
− ΔV
UVLO
), this switching will be
inhibited; if VIN rises above the UVLO rising threshold
(V
UVLO
), the device will resume switching.
Soft-Start
The soft-start function is used to prevent large inrush
currents while the converter is being powered up. The
RT6248B provides an internal soft-start feature for inrush
control. During the start-up sequence, the internal capacitor
is charged by an internal current source I
SS
to generate a
soft-start ramp voltage as a reference voltage to the PWM
comparator. The device will initiate switching and the
output voltage will smoothly ramp up to its targeted
regulation voltage only after this ramp voltage is greater
than the feedback voltage V
FB
to ensure the converters
have a smooth start-up. The typical soft-start time is
0.4ms.
Over-Current Limit
The RT6248B current limit is (8A, 10A, 12A) it is a cycle-
by-cycle
“valley”
type, measuring the inductor current
through the synchronous rectifier during the off-time while
the inductor current ramps down. If output voltage drops
below the output under-voltage protection level, the
RT6248B will stop switching to avoid excessive heat.
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
Output Over-Voltage Protection (OVP) and Under-
Voltage Protection (UVP)
The RT6248B includes output under-voltage protection
(UVP) against over-load or short-circuited condition by
constantly monitoring the feedback voltage V
FB
. If V
FB
drops below the under-voltage protection trip threshold
(typically 60% of the internal reference voltage), the UV
comparator will go high to turn off both the internal high-
side and low-side MOSFET switches.
Power Good
The PGOOD pin is an open-drain output and is connected
to an external pull-up resistor. It is controlled by a
comparator, which the feedback signal V
FB
is fed to. If
V
FB
is above 90% of the internal reference voltage, the
PGOOD pin will be in high impedance and V
PGOOD
will be
held high. Otherwise, the PGOOD output will be pulled
low.
Over-Temperature Protection (Thermal Shutdown)
The RT6248B includes an over-temperature protection
(OTP) circuitry to prevent overheating due to excessive
power dissipation. The OTP will shut down switching
operation when junction temperature exceeds a thermal
shutdown threshold T
SD
. Once the junction temperature
cools down by a thermal shutdown hysteresis (ΔT
SD
), the
IC will resume normal operation with a complete soft-start.
www.richtek.com
4
DS6248B-01 May 2018
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
