®
RT7273
3-CH, 18V, Synchronous Step-Down Converter
General Description
The RT7273 features three synchronous wide input range
high efficiency Buck converters. The converters are
designed to simplify its application while giving the designer
the option to optimize their usage according to the target
application.
The converters can operate in 5V, 9V or 12V systems
and have integrated power transistors. The output voltage
can be set externally using a resistor divider to any value
between 0.8V and the input supply minus 1V. Each
converter features an enable pin that allows a delayed
start-up for sequencing purposes, a soft-start pin that
allows adjustable soft-start time by choosing the soft-
start capacitor, and a current limit pin (RLIMx) to adjust
current limit by selecting an external resistor. The COMP
pin allows optimizing transient versus dc accuracy
response with a simple RC compensation.
The switching frequency of the converters can either be
set with an external resistor connected to ROSC pin or
be synchronized to an external clock connected to SYNC
pin if needed. The switching converters are designed to
operate from 300kHz to 2.2MHz. The converters operate
with 180
°
phase between CH 1 and CH 2, CH 3 (CH 2 and
CH 3 ran in phase) to minimize the input filter requirements.
The RT7273 also features a low power mode enabled by
an external signal, which allows for a reduction on the
input power supplied to the system when the host
processor is in stand-by (low activity) mode.
Features
Wide Input Supply Voltage Range : 4.5V to 18V
Output Range : 0.8V to (VIN
−
1V)
Fully Integrated Triple-Buck
½
Maximum Current 3.5A/2.5A/2.5A
½
Continuous Operation 3A/2A/2A
High Efficiency
Switching Frequency
300kHz to 2.2MHz Set by External Resistor
External Synchronization Pin for Oscillator
External Enable/Sequencing Pins
Adjustable Cycle-By-Cycle Current Limit Set by
External Resistor
Soft-Start
Current Mode Control with Simple Compensation
Circuit
Power Good Indicator
Discontinuous Operating Mode at Light Load when
LOWP = High
RoHS Compliant and Halogen Free
½
Simplified Application Circuit
V
IN
VINx
VINR
RT7273
LX1
FB1
LX2
FB2
LX3
GND
FB3
V
OUT3
V
OUT2
V
OUT1
ENx
SYNC
PGOOD
LOWP
SSx
RLIMx
ROSC
Copyright
©
2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7273-04
October 2014
www.richtek.com
1
RT7273
Applications
Marking Information
RT7273GQW : Product Number
Set Top Boxes
Blu-ray DVD
DVR
DTV
Car Audio/Video
Security Camera
RT7273
GQW
YMDNN
YMDNN : Date Code
Pin Configurations
RT7273
Package Type
QW : WQFN-40L 6x6 (W-Type)
(Exposed Pad-Option 2)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
½
RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
RLIM3
SS3
COMP3
FB3
SYNC
ROSC
FB1
COMP1
SS1
RLIM1
EN3
BOOT3
VIN3
LX3
LX3
GND
VINR
VINR
VINR
GND
40 39 38 37 36 35 34 33 32 31
1
2
3
4
5
6
7
8
9
10
11 12 13 14 15 16 17 18 19 20
41
30
29
28
27
26
25
24
23
22
21
Ordering Information
(TOP VIEW)
GND
GND
VCC
PVCC
PGOOD
GND
LOWP
FB2
COMP2
SS2
RLIM2
Functional Pin Description
Pin No.
1
2
3
4
5
6
7
8
9
Pin Name
RLIM3
SS3
COMP3
FB3
SYNC
ROSC
FB1
COMP1
SS1
Pin Function
Current Limit Setting for CH 3. Connect a resistor from RLIM3 to GND to set
the peak current limit on the output inductor.
Soft-Start Time Setting for CH 3. Connect a capacitor to this pin and GND
for soft-start time setting.
Compensation Node for CH 3. COMP is used to compensate the regulation
control loop. Connect a series RC network from COMP to GND. In some
cases, an additional capacitor from COMP to GND is required.
Feedback Voltage Input for CH 3.
Synchronous Clock Input. Connect to GND if not used.
Oscillator Setting. Connect a resistor from ROSC to GND to set the
switching frequency.
Feedback Voltage Input for CH 1.
Compensation Node for CH 1. COMP is used to compensate the regulation
control loop. Connect a series RC network from COMP to GND. In some
cases, an additional capacitor from COMP to GND is required.
Soft-Start Time Setting for CH 1. Connect a capacitor to this pin and GND
for soft-start time setting.
Copyright
©
2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
EN1
BOOT1
VIN1
LX1
LX1
LX2
LX2
VIN2
BOOT2
EN2
½
WQFN-40L 6x6
DS7273-04
October 2014
RT7273
Pin No.
10
11
12
13
14, 15
16, 17
18
19
20
21
22
23
24
25
26, 30, 31, 35,
41 (Exposed Pad)
27
28
29
32, 33, 34
36, 37
38
39
40
Pin Name
RLIM1
EN1
BOOT1
VIN1
LX1
LX2
VIN2
BOOT2
EN2
RLIM2
SS2
COMP2
FB2
LOWP
GND
PGOOD
PVCC
VCC
VINR
LX3
VIN3
BOOT3
EN3
Pin Function
Current Limit Setting for CH 1. Connect a resistor from RLIM to GND to set
the peak current limit on the output inductor.
Enable Control Input for CH 1. A low level signal on this pin disables it. If this
pin is left open, a weak internal pull-up to VCC will allow automatic enables.
Bootstrap Supply for High-Side Gate Driver of CH 1. Connect a 0.1F
ceramic capacitor from this pin to LX1.
Power Input for CH 1 and Connected to High-Side MOSFET Drain. Place a
10F ceramic capacitor close to this pin.
Switch Node of CH 1.
Switch Node of CH 2.
Power Input for CH 2. Place a 10F ceramic capacitor close to this pin.
Bootstrap Supply for High-Side Gate Driver of CH 2. Connect a 0.1F
ceramic capacitor from this pin to LX2.
Enable Control Input for CH 2. A low level signal on this pin disables it. If this
pin is left open, a weak internal pull-up to VCC will allow automatic enables.
Current Limit Setting for CH 2. Connect a resistor from RLIM2 to GND to set
the peak current limit on the output inductor.
Soft-Start Time Setting for CH 2. Connect a capacitor to this pin and GND
for soft-start time setting.
Compensation Node for CH 2. COMP is used to compensate the regulation
control loop. Connect a series RC network from COMP to GND. In some
cases, an additional capacitor from COMP to GND is required.
Feedback Voltage Input for CH 2.
Discontinuous Operation Mode Input (Active High).
Ground. The exposed pad must be connected to GND and soldered to a
large PCB copper plane for maximum power dissipation.
Power Good Indicator Output with Open-Drain.
5V Power Supply Output. Connect a capacitor 10F between this pin and
GND.
4.6V Power Supply Output. Connect a capacitor 3.3F between this pin and
GND.
Supply Voltage Input for Internal Control Circuit.
Switch Output for CH 3.
Power Input for CH 3. Place a 10F ceramic capacitor close to this pin.
Bootstrap Supply for High-Side Gate Driver of CH 3. Connect a 0.1F
ceramic capacitor from this pin to LX3.
Enable Control Input for CH 3. A low level signal on this pin disables it. If this
pin is left open, a weak internal pull-up to VCC will allow automatic enables.
Copyright
©
2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7273-04
October 2014
www.richtek.com
3
RT7273
Function Block Diagram
Whole Chip Function Block Diagram
VINR
Internal
Regulator
PVCC
VCC
BOOT1
CH 1
Step-Down
Converter
LX1
FB1
COMP1
ROSC
SYNC
VIN2
EN2
RLIM2
SS2
LOWP
GND
VIN3
EN3
RLIM3
SS3
PGOOD
CH 3
Step-Down
Converter
Power Good
OSC
VCC PVCC
BOOT2
CH 2
Step-Down
Converter
LX2
FB2
COMP2
VCC PVCC
BOOT3
LX3
FB3
COMP3
VIN1
EN1
RLIM1
SS1
Each Channel Function Block Diagram
VCC
VIN
Slope Comp
+
Oscillator
+
-
R
SENSE
PVCC
BOOT
0.872V
-
PGOOD
Comparator
0.72V
V
CC
6µA
SS
V
CC
1.5µA
EN
5k
+
+
+
-
UV & PGOOD
Comparator
Switch
Controller
SW
0.8V
+
+
EA
-
-
Current
Comparator
GND
OC
3V
1.4V
-
Enable
Comparator
COMP
RLIM
FB
Copyright
©
2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS7273-04
October 2014
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
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