RT8097B
Operation
The RT8097B is a synchronous low voltage step-down
converter that can support the input voltage range from
2.7V to 6V and the output current can be up to 2A. The
RT8097B uses a constant on-time, current mode
architecture. In normal operation, the high side P-
MOSFET is turned on when the switch controller is set
by the comparator and is turned off when the Ton
comparator resets the switch controller.
Over Current Protection (OCP)
The RT8097B provides over current protection by
detecting low side MOSFET valley inductor current. If
the sensed valley inductor current is over the current
limit threshold (3.1A typ.), the OCP will be triggered.
When OCP is tripped, the RT8097B will keep the over
current threshold level then cause the UV protection.
Thermal Shutdown (OTP)
Low side MOSFET peak current is measured by internal
RSENSE. The error amplifier EA adjusts COMP voltage
by comparing the feedback signal (VFB) from the output
voltage with the internal 0.6V reference. When the load
current increases, it causes a drop in the feedback
voltage relative to the reference, then the COMP
voltage rises to allow higher inductor current to match
the load current.
The device implements an internal thermal shutdown
function when the junction temperature exceeds 150°C.
The thermal shutdown forces the device to stop
switching when the junction temperature exceeds the
thermal shutdown threshold. Once the die temperature
decreases below the hysteresis of 20°C, the device
reinstates the power up sequence.
Maximum Duty Cycle
UV Comparator
The maximum duty cycle (70%, min.) can be calculated
by minimum off time (180ns, max.), dead time (60ns,
max.) and switching frequency (1.2MHz, max.).
If the feedback voltage (VFB) is lower than threshold
voltage 0.2V, the UV comparator's output will go high
and the switch controller will turn off the high side
MOSFET. The output under voltage protection is
designed to operate in Hiccup mode for RT8097BH,
Latch mode for RT8097BL.
DMAX = 1 - ( tOFF_MIN + tD ) x fSW
Where tOFF_MIN is minimum off time, tD is dead time and
fSW is switching frequency.
If input voltage and output voltage are closed, RT8097B
operates at high duty cycle. Once the operational duty
cycle is larger than the maximum duty cycle (70%, min.),
RT8097B keeps minimum off time (180ns, max.) and
deadtime (60ns, max.), then the output voltage starts to
drop. The input voltage at which the devices enter
dropout changes depending on the input voltage, output
voltage, switching frequency, load current, and the
efficiency of the design.
PGOOD Comparator
When the feedback voltage (VFB) is higher than
threshold voltage 0.54V, the PGOOD open drain output
will be high impedance. The internal PG MOSFET is
typical 10Ω. The PGOOD signal delay time from EN is
about 2ms.
Enable Comparator
A logic-high enables the converter; a logic-low forces
the IC into shutdown mode.
t
OFF_MIN
180ns
Soft-Start (SS)
High-Side
MOSFET
An internal current source charges an internal capacitor
to build the soft-start ramp voltage. The VFB voltage will
track the internal ramp voltage during soft-start interval.
The typical soft-start time is 1.2ms.
Deadtime
30ns
Deadtime
30ns
Low-Side
MOSFET
Copyright © 2021 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS8097B-09 June 2021
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