RT7273
Operation
Overall
The RT7273 is a 3-CH synchronous high voltage Buck
Converter that can support the input voltage range from
4.5V to 18V and the output current up to 3A/2A/2A
separately. The RT7273 uses an adjustable constant
frequency, current-mode architecture. In normal operation,
the high-side N-MOSFET is turned on when the Switch
Controller is set by the Oscillator and is turned off when
the current comparator resets the Switch Controller. While
the high-side N-MOSFET is turned off, the low-side
N-MOSFET is turned on.
High-side N-MOSFET peak current is measured by internal
R
SENSE
. The Current Signal is where Slope Compensator
works together with sensing voltage of R
SENSE
. The error
amplifier EA adjusts COMP voltage by comparing the
feedback signal from the output voltage with the internal
0.8V reference. When the load current increases, it causes
a drop in the feedback voltage relative to the reference,
the COMP voltage then rises to allow higher inductor
current to match the load current.
UV and PGOOD Comparator
If the feedback voltage (V
FB
) is higher than 0.72V and lower
than 0.872V, the two comparators' output will go low and
trigger Switch Controller to generate PGOOD signal for
this channel. However, the whole chip PGOOD signal will
go high only if all three channels' PGOOD conditions are
established. If V
FB
is lower than UV threshold, the UV
comparator's output will go high and the Switch Controller
will turn off the high-side N-MOSFET. The output under-
voltage protection is designed to operate in hiccup mode.
This function is only available after soft-start finished.
Oscillator
The frequency of internal oscillator can be adjusted by
the external resistor at ROSC pin in the range between
300kHz and 2.2MHz. It can also be synchronized by an
external clock in the range between 200kHz and 2.2MHz
from SYNC pin.
Enable Comparator
The internal 1.5μA pull-up current to EN pin can be used
to set the power sequence of each channel by connecting
a capacitor to EN pin. Internal 5kΩ resistor and Zener
diode are used to clamp the input signal to 3V. Thus, the
EN pin can also be connected to VIN through a 100kΩ
resistor.
Soft-Start
An internal current source (6μA) charges an external
capacitor connected to SS pin to build the soft-start ramp
voltage. The V
FB
voltage will track the soft-start ramp voltage
during soft-start interval. The typical soft-start time is 2ms.
Over-Current Limit
Each channel can set its own over-current limit by external
resistor. It is recommended that the over-current limit level
should be 1.5 times larger than the maximum loading
current.
Copyright
©
2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7273-04
October 2014
www.richtek.com
5
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
