DS2701 00

®
RT2701
Dual-Phase PWM Controller for GPU Core Power Supply
General Description
Features
The RT2701 is a dual-phase synchronous Buck PWM
controller with integrated drivers which are optimized for
high performance graphic microprocessor and computer
applications. The IC integrates a PWM controller, two 12V
MOSFET drivers with internal bootstrap diodes, as well
as output current monitoring and protection functions into
the WQFN-24L 4x4 package. The RT2701 adopts DCR
and RDS(ON) current sensing. Over current protection is
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Dual-Phase PWM Controller
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Two Embedded MOSFET Drivers and Embedded
Switching Boot Diode
Dynamic Auto Phase Control with Adjustable
Threshold
Cross-talk Jitter Suspend (CJSTM)
Remote GND Detection for High Accuracy
Automatic Diode Emulation Mode/Or Ultrasonic
Mode at Light Load
Lossless RDS(ON) Current Sensing for Current Balance
Lossless DCR Current Sensing for AVP & OCP
Reference Voltage Output with 1% Accuracy
External Reference Input with Soft-Start (RISS)
Embedded One-Bit VID Control
Adjustable OCP Threshold
Adjustable Switching Frequency
Reference Tracking UVP/OVP Protection
Shoot Through Protection and Short Pulse Free
Technology
RoHS Compliant and Halogen Free
accomplished through continuous inductor DCR current
sensing, while RDS(ON) current sensing is used for accurate
channel current balance. Using both methods of current
sampling utilizes the best advantages of each technique.
The RT2701 also features an one-bit VID control operation
in which the feedback voltage is regulated and tracks
external input reference voltage. Other features include
adjustable operating frequency, external compensation and
enable/shutdown functions.
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Ordering Information
RT2701
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Package Type
QW : WQFN-24L 4x4 (W-Type)
(Exposed Pad-Option 1)
Applications
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Lead Plating System
G : Green (Halogen Free and Pb Free)
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Note :
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Richtek products are :
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`
Middle to High End GPU Core Power
High End Desktop PC Memory Core Power
Low Voltage, High Current DC/DC Converter
Voltage Regulator Modules
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
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Suitable for use in SnPb or Pb-free soldering processes.
Simplified Application Circuit
RT2701
VIN
GPIO
VCC
VID
PHASE1
PHASE2
MOSFET
L1
VVDD
L2
MOSFET
OCP
PS
PGND
RMPSET
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2701-00 May 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT2701
Pin Configurations
Marking Information
03= : Product Code
RSET
VID
BOOT2
UGATE2
PHASE2
LGATE2
(TOP VIEW)
03=YM
DNN
YMDNN : Date Code
24 23 22 21 20 19
VSET
VREF
EN/MSEL
RMPSET
COMP
FB
1
18
2
17
3
16
PGND
4
15
25
5
14
13
6
8
9 10 11 12
VRTN
TON
OCP
CSN
CSP
PS
7
VCC
VDD
LGATE1
PHASE1
UGATE1
BOOT1
WQFN-24L 4x4
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
VSET
Output Voltage Setting. Connect a voltage divider from VREF to VSET to set the
output voltage.
2
VREF
Reference Voltage Output (2V). The RT2701 generates a 2V reference voltage
from VREF to VRTN.
Enable Control Input and Mode Selection. This pin is a tri-state input. Pull up this pin
to be higher than 4.2V, the controller operates in DEM mode. Pull up this pin to
between 1.2V to 3V, the controller operates in ASM mode. Pull down this pin to
GND, the controller will shutdown.
Internal Ramp Slew Rate Setting. Connect a resistor (RRMP) from RMPSET to GND
to the ramp slew rate. The value of RRMP must be set equal to RTON.
3
EN/MSEL
4
RMPSET
5
COMP
Compensation Node. This pin is the output node of the error amplifier.
6
FB
7
VRTN
8
TON
9
OCP
Feedback Voltage Input. This pin is the negative input node of the error amplifier.
Remote Differential Feedback, Invert Input. This pin is the negative node of the
differential remote voltage sensing.
Switching Frequency Setting. Connect a resistor (RTON ) from TON to VIN to set the
switching frequency. The value of R TON must be set equal to RRMP .
OCP Level Setting. Connect a resistor from OCP to GND to set the current limit
threshold.
10
CSN
Negative Input of Current Sensing.
11
CSP
Positive Input of Current Sensing.
12
PS
Dynamic Phase Control Input. Connect a resistor from PS to GND to set the auto
down phase threshold.
13
BOOT1
Bootstrap Supply for High Side MOSFET Driver of Phase1.
14
UGATE1
High Side Gate Driver of Phase1. Connect this pin to the Gate of high side
MOSFET.
15
PHASE1
Return node of Phase1 High Side Driver. Connect this pin to the Source of high side
MOSFET together with the drain of low side MOSFET and the inductor.
16
LGATE1
Low Side Gate Driver of Phase1. Connect this pin to the Gate of low side MOSFET.
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is a registered trademark of Richtek Technology Corporation.
DS2701-00 May 2013
RT2701
Pin No.
Pin Name
Pin Function
17
VDD
Regulator Power for Internal Circuit. The regulated voltage provides power supply
for all low voltage circuits.
18
VCC
Supply Voltage Input. Connect this pin to GND by a ceramic cap larger than 1μF.
19
LGATE2
Low Side Gate Driver of Phase2. Connect this pin to the Gate of low side MOSFET.
20
PHASE2
21
UGATE2
22
BOOT2
23
VID
24
RSET
Return node of Phase2 High Side Driver. Connect this pin to the Source of high side
MOSFET together with the Drain low side of MOSFET and the inductor.
High Side Gate Driver of Phase2. Connect this pin to the Gate of high side
MOSFET.
Bootstrap Supply for High Side MOSFET Driver of Phase2.
Programming Output Voltage Control. When VID pin is logic high, internal
N-MOSFET that connected to RSET pin is turn on.
Output Voltage Setting. Connect a resistor from RSET pin to VSET pin, the output
voltage can be switched two levels by driving VID pin.
Power Ground. The exposed pad must be soldered to a large PCB and connected
to PGND for maximum power dissipation.
25
PGND
(Exposed Pad)
Function Block Diagram
VID
RSET
VREF
VCC
Reference
Output Gen.
Internal
Regulator&BG
VDD
Power On Reset
& Central Logic
VSET
UV Trip Point
+
-
OV Trip Point
Control & Protection Logic
+
-
Boot-Phase
Detection 1
Ramp
Gen
RMPSET
Boot-Phase
Detection 2
VSETA
VRTN
Soft-Start
+
FB
ERROR
AMP
COMP
EN/MSEL
EN/Mode
Select
+
+
+
+
+
LPF
BOOT1
UGATE1
PHASE1
TON
Gen 1
PWM
CMP
PWM1
+
+
To Power
on Reset
To driver Logic
ZCD
To Power on
Reset
PHASE1 To driver Logic
LGATE1
Driver
Logic
TON
Gen 2
LGATE2
PGND
VIN
Detection
TON
S/H
GM
+
S/H
GM
+
Current
Balance
PS
APS
CSP
CSN
OCP
+
-
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2701-00 May 2013
BOOT2
UGATE2
PHASE2
PWM2
AOC
Isum
Phase
shedding
OCP
+
1/2
++
To Protection Logic
is a registered trademark of Richtek Technology Corporation.
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3
RT2701
Operation
The RT2701 integrates a PWM controller, two 12V
MOSFET drivers with internal bootstrap diodes, as well
as output current monitoring and protection functions.
Power On Reset
The Power On Reset (POR) circuit monitors the supply
voltage of the controller (VCC). When VCC exceeds the
POR rising threshold, the controller will be enabled. If
VCC falls below the POR falling threshold during normal
operation, all MOSFETs stop switching. There is a
hysteresis between the POR rising threshold and falling
threshold to prevent noise mis-trigger.
Soft-Start
Current Balance
The RT2701 implements internal current balance
mechanism in the current loop. The RT2701 senses each
phase current signal and compares it with the average
current. If the sensed current of any particular phase is
higher than average current, the on-time of this phase will
be adjusted to be shorter.
OCP
Once the sensed total current exceeds the current limit
threshold, the driver will be forced to turn off the gate drivers
for high side power MOSFETs. Until the OCP situation is
removed.
An internal soft-start function is used to prevent large
inrush current while converter is powered-up. The FB
voltage will track the internal soft-start voltage during softstart interval. During the soft-start period, the controller
will operate in dual-phase mode to ensure enough charge
for output loads.
Over Voltage Protection
EN/Mode Select
Under Voltage Protection
The RT2701 supports DEM (Diode Emulation Mode) and
ASM (Audio Skipping Mode) operation which can be
enabled by EN/MSEL pin. When the EN/MSEL pin is
pulled up above 4.2V, the controller will operate in DEM
and reduce the switching frequency at light load conditions
for saving power loss. If the EN/MSEL voltage is between
1.2V and 3V, the controller will operate in ASM. In ASM
operation, the minimum switching frequency is limited to
30kHz to avoid acoustic noises. If the pin is pulled to
GND, the RT2701 will be shut down.
The voltage on CSN pin is also monitored for Under Voltage
Protection (UVP). If the output voltage is lower than the
UVP threshold, the controller will turn off both high side
and low side MOSFETs. When the UVP is triggered, the
RT2701 will enter hiccup mode and continuously try to
restart until the UVP situation is removed.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
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The RT2701 monitors the output voltage via the CSN pin
for Over Voltage Protection (OVP). Once the output voltage
exceeds the OVP threshold, the controller will turn off
high side MOSFETs and turn on low side MOSFETs to
protect the load until the OVP situation is removed.
is a registered trademark of Richtek Technology Corporation.
DS2701-00 May 2013
RT2701
Absolute Maximum Ratings
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(Note 1)
VDD, VSEN, COMP, VSET, VREF, EN/MSEL, PS, OCP, CSN,
CSP, RSET, VID, RMPSET to PGND ------------------------------------------------------------ −0.3V to 6V
VCC, TON to PGND --------------------------------------------------------------------------------- −0.3V to 15V
VRTN to PGND ---------------------------------------------------------------------------------------- −0.3V to 0.3V
BOOTx to PHASEx ---------------------------------------------------------------------------------- −0.3V to 15V
PHASEx to PGND
DC -------------------------------------------------------------------------------------------------------- −3V to 15V
<20ns --------------------------------------------------------------------------------------------------- −5V to 30V
UGATEx to PHASEx
DC -------------------------------------------------------------------------------------------------------- −0.3V to BOOTx − PHASEx
<20ns --------------------------------------------------------------------------------------------------- −5V to (BOOTx − PHASEx + 5V)
LGATEx to PGND
DC -------------------------------------------------------------------------------------------------------- −0.3V to PVCC+ 0.3V
<20ns --------------------------------------------------------------------------------------------------- −5V to (VCC + 5V)
Power Dissipation, PD @ TA = 25°C
WQFN-24L 4x4 --------------------------------------------------------------------------------------- 1.923W
Package Thermal Resistance (Note 2)
WQFN-24L 4x4, θJA ---------------------------------------------------------------------------------- 52°C/W
WQFN-24L 4x4, θJC --------------------------------------------------------------------------------- 7°C/W
Junction Temperature -------------------------------------------------------------------------------- 150°C
Lead Temperature (Soldering, 10 sec.) ---------------------------------------------------------- 260°C
Storage Temperature Range ----------------------------------------------------------------------- −65°C to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Model) ------------------------------------------------------------------------- 2kV
Recommended Operating Conditions
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(Note 4)
Supply Voltage, VCC -------------------------------------------------------------------------------- 4.5V to 13.2V
Junction Temperature Range ----------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range ----------------------------------------------------------------------- −40°C to 85°C
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2701-00 May 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
RT2701
Electrical Characteristics
(VCC = 12V, No Load, TA = −40°C to 85°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Supply Input
Supply Current
IVCC + IPVCC EN = 3.3V, Not Switching
--
3.5
5
mA
Shutdown Current
ICC + IPVCC
EN = 0V
--
--
600
μA
VVCC_th
VCC Rising
--
4.2
4.5
V
--
0.3
--
V
Power On Reset
VCC POR Threshold
Power On Reset Hysteresis VVCC_hys
Reference
Reference Output
VREF
(No Load, Active Mode )
1.98
2
2.02
V
Reference Input Range
VSET
VSET pin (this max. voltage will affect
VCOMP max.)
0.5
--
2
V
Initial Soft-Start time
tb
Initially, VOUT = 0.1V to 1.2V
--
1.5
--
ms
Reference Change Delay
Time
tc
--
300
--
μs
VOSEA
−8
--
8
mV
RL = 47kΩ
--
80
--
dB
CLOAD = 5pF
--
10
--
MHz
--
5
--
V/μs
0.5
--
2
V
--
250
--
μA
Start Up Delay
Error Amplifier
Input Offset Voltage
DC Gain
Gain Bandwidth Product
GBW
Slew Rate
SR
Output Voltage Range
VCOMP
CLOAD = 10pF (Gain = −4,
Rf = 47k, VOUT = 0.5V to 3V)
RL = 47kΩ (max. depend on VSET
max.)
MAX Source Current
IOUTEA
VCOMP = 2V
Current Sense Amplifier (for Droop and OCP and Phase Shedding)
Input Offset Voltage
VOSCS
−2
--
2
mV
Impedance at Neg. Input
R CSN
1
--
--
MΩ
Impedance at Pos Input
R CSP
1
--
--
MΩ
Maximum Input Range
VCSP − VCSN
--
--
65
mV
TON Setting
On-Time Setting
tON
IRTON = 62μA
315
350
385
ns
VOVABS
With Respect to VOUT(MAX)
2.1
2.2
--
V
VREL_OV
With Respect to VOUT
--
138
--
%
Protection
Absolute Over Voltage
Protection Threshold
Relative Over Voltage
Protection Threshold
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is a registered trademark of Richtek Technology Corporation.
DS2701-00 May 2013
RT2701
Parameter
Symbol
Under Voltage Protection Threshold VUV
Current Source by OCP Pin
Test Conditions
Min
Typ
Max
Unit
Measured at VSENS with Respect
to Unloaded Output Voltage (UOV)
--
50%
--
%
7.2
8
8.8
μA
--
--
0.5
V
ASM Mode
1.2
--
3
DEM Mode
4.5
--
--
EN = 0V
−1
--
5
μA
--
8
--
μA
--
500
--
ns
1
2
3.5
Ω
0.7
1.4
2.5
Ω
--
20
--
Ω
IOCP
Logic Inputs
EN Threshold Voltage
VIL
EN Pin Mode Select Voltage
Leakage Current of EN
Low Level (SD) (Hysteresis)
V
Auto Phase Control
Current Source by PSI Pin
IPS
Maximum Duty Cycle
UGATE Min. Off Time
Gate Driver
Upper Driver Sink
R UGATEsk
VUGATEx − VPHASEx = 0.1V,
IUGATEx = 50mA
Lower Driver Sink
R LGATEsk
VLGATEx = 0.1V, ILGATEx = 50mA
Internal Boost Charging Switch
On-Resistance
R BOOT
PVCC to BOOTx
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in
the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may
affect device reliability.
Note 2. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2701-00 May 2013
is a registered trademark of Richtek Technology Corporation.
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7
RT2701
Typical Application Circuit
VIN
12V
RT2701
17 VDD
C8
10µF
C4 Optional
C5
1.2nF
R18
11k
VRTN
R19
15k
EN/MODE
BOOT1 13
UGATE1 14
1
PHASE1 15
VSET
R4
Optional
R22 56k
VIN
2 VREF
24 RSET
R21 43k
9
OCP
12 PS
R16 160k
4 RMPSET
R20 160k
8 TON
R17 100
VCC 18
3 EN/MSEL
VID 23
BOOT2 22
UGATE2 21
PHASE2 20
PGND
FB
6
VRTN 7
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8
C9
0.1µF
Q1
L1
R5 0
Q2
GPIO
R9 0
C14
0.1µF
Q3
R7
NC
C12
NC
VIN
C13
10µF
/16V x 5
R8 0
Q4
LGATE2 19
CSP 11
CSN 10
C6
10µF/16V x 5
R6 0
LGATE1 16
COMP 5
25 (Exposed pad)
R3 1
C7
10µF
C2
1.5nF
C1
2.2nF
R2
R1
3.9k
2k
R12
NC
C15
NC
0.36µH
/0.8m
R10
9.1k
VOUT
1.1V
C10
820µF
/2.5V x 4
C11
10µF
/6.3V x 10
L2
0.36µH/0.8m
R11
9.1k
R13 NC
C3 0.1µF
R14
100
R15
100
VCC_SNS
VSS_SNS
is a registered trademark of Richtek Technology Corporation.
DS2701-00 May 2013
RT2701
Typical Operating Characteristics
Efficiency vs. Load Current
Efficiency vs. Load Current
100
100
90
90
80
Phase 2 Active
70
Efficiency (%)
Efficiency (%)
80
60
50
40
30
20
70
60
50
40
30
20
10
10
VIN = VCC = 12V, VOUT = 1.1V
0
0
5
10
15 20 25 30 35 40
VIN = VCC = 12V, VOUT = 1.1V
0
0.01
45 50 55 60
0.1
Load Current (A)
TON vs. Temperature
2.04
355
2.03
2.02
VREF (V)
345
TON (ns)
10
VREF vs. Temperature
360
350
340
335
330
2.01
2.00
1.99
1.98
325
1.97
320
VIN = VCC = 12V, No Load
315
VIN = VCC = 12V, No Load
1.96
-50
-25
0
25
50
75
100
125
-50
-25
0
25
50
75
Temperature (°C)
Temperature (°C)
Inductor Current vs. Output Current
Power On from EN
35
100
125
VIN = VCC = 12V, IOUT = 50A
30
Inductor Current (A)
1
Load Current (A)
VEN
(10V/Div)
25
Phase 1
Phase 2
20
VOUT
(1V/Div)
15
10
UGATE1
(50V/Div)
5
UGATE2
(50V/Div)
VIN = VCC = 12V
0
20
25
30
35
40
45
50
55
60
Time (1ms/Div)
Output Current (A)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2701-00 May 2013
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RT2701
Power On from VCC
Power Off from EN
VIN = VCC = 12V, IOUT = 50A
VIN = VCC = 12V, IOUT = 50A
VEN
(10V/Div)
V CC
(10V/Div)
VOUT
(1V/Div)
VOUT
(1V/Div)
UGATE1
(50V/Div)
UGATE1
(50V/Div)
UGATE2
(50V/Div)
UGATE2
(50V/Div)
Time (1ms/Div)
Time (1ms/Div)
Power Off from VCC
Dynamic Output Voltage Control
VSET = 0.78V to 1.15V, IOUT = 40A
VIN = VCC = 12V, IOUT = 50A
V CC
(10V/Div)
VSET
(1V/Div)
VOUT
(1V/Div)
VOUT
(1V/Div)
UGATE1
(50V/Div)
UGATE1
(50V/Div)
UGATE2
(50V/Div)
UGATE2
(50V/Div)
Time (1ms/Div)
Time (200μs/Div)
Dynamic Output Voltage Control
Load Transient Response
VSET = 1.15V to 0.78V, IOUT = 40A
VIN = VCC = 12V
VSET
(1V/Div)
VOUT
(500mV/Div)
VOUT
(1V/Div)
IOUT
(50A/Div)
UGATE1
(50V/Div)
UGATE1
(50V/Div)
UGATE2
(50V/Div)
UGATE2
(50V/Div)
Time (200μs/Div)
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Time (10μs/Div)
is a registered trademark of Richtek Technology Corporation.
DS2701-00 May 2013
RT2701
OVP
Load Transient Response
VIN = VCC = 12V, IOUT = 25A
VIN = VCC = 12V
VOUT
(500mV/Div)
VOUT
(1V/Div)
IOUT
(50A/Div)
UGATE1
(20V/Div)
UGATE1
(50V/Div)
UGATE2
(50V/Div)
LGATE1
(10V/Div)
Time (10μs/Div)
Time (20μs/Div)
UVP
Short Circuit
VIN = VCC = 12V
VIN = VCC = 12V, IOUT = 50A
VOUT
(1V/Div)
VOUT
(1V/Div)
UGATE1
(20V/Div)
IL1
(20A/Div)
LGATE1
(10V/Div)
IL2
(20A/Div)
Time (10μs/Div)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2701-00 May 2013
Time (10ms/Div)
is a registered trademark of Richtek Technology Corporation.
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RT2701
Application Information
The RT2701 is a dual-phase synchronous Buck PWM
controller with integrated drivers which is optimized for
high-performance graphic microprocessor and computer
applications. A COT (Constant-On-Time) PWM controller
and two 12V MOSFET drivers with internal bootstrap
diodes are integrated so that the external circuit can be
easily designed and the component count can be reduced.
The IC also adopts lossless DCR and RDS(ON) current
sensing. Dynamic phase control and current limit are
accomplished through continuous inductor DCR current
sensing, while RDS(ON) current sensing is used for accurate
channel current balance.
Dynamic mode transition function with various operating
states, which include dual-phase, single phase, diode
emulation and audio skipping modes is supported. These
different operating states make the system efficiency as
high as possible.
A one-bit VID control operation in which the feedback
voltage is regulated and tracks external input reference
voltage is provided. The RT2701 also features complete
fault protection functions including over voltage, under
voltage and current limit.
DEM/ASM Mode Selection
DEM (Diode Emulation Mode) and ASM (Audio Skipping
Mode) operation can be enabled by driving the tri-state
EN/MSEL pin to a logic high level. The RT2701 can switch
operation into DEM when EN/MSEL pin is pulled up to
above 4.2V. In DEM operation, the RT2701 automatically
reduces the operation frequency at light load conditions
for saving power loss. If EN/MSEL is pulled between 1.2V
to 3V, the controller will switch operation into ASM. In
ASM operation, the minimum switching frequency is
limited to 30kHz to avoid the acoustic noise. Finally, if
the pin is pulled to GND, the RT2701 will shutdown.
Power On Reset
The POR (power on reset) circuit monitors the supply
voltage of the controller (VCC). When VCC exceeds the
POR rising threshold, the controller will be enabled. During
soft-start period, the output voltage will first boot to around
1V, and directly ramp to the set level. If VCC falls below
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12
the POR falling threshold during normal operation, all
MOSFETs stop switching and the controller resets. The
POR rising and falling threshold has a hysteresis to prevent
noise mis-trigger.
Soft-Start
The RT2701 provides soft-start function. The soft-start
function is used to prevent large inrush current while
converter is being powered-up. The FB voltage will track
the internal soft-start voltage during soft-start interval.
Therefore, the duty cycle of the UGATE signal at power
up as well as the input current limited. During the softstart period, the controller will be in dual-phase operation
by default to ensure enough charge during start-up.
One-Bit VID and Dynamic Output Voltage Control
The output voltage is determined by the applied voltage
on the VSET pin. The RT2701 generates a 2V reference
voltage from VREF to VRTN. As shown in Figure 1,
connecting a resistive divider from the VREF pin to the
VSET pin can set the output voltage according to the
equation below :
VOUT = 2V × ⎛⎜ R2 ⎞⎟
⎝ R1 + R2 ⎠
The RT2701 also features a one-bit VID control through
an internal N-MOSFET also shown in Figure 1. Connecting
a resistor (R3) from RSET pin to VSET pin, the output
voltage can be switched between two levels by controlling
the VID pin. When the VID pin is logic high, the internal NMOSFET turns on to set the output voltage to a lower
level. The output voltage can be calculated as below :
⎡ (R2//R3) ⎤
VOUT = 2V × ⎢
⎥
⎣ R1 + (R2//R3) ⎦
The available setting range of the VSET voltage is from
0.5V to 2V.
One-Bit VID and Dynamic Output Voltage Control
For the RT2701, it can be set lower than 10mV/μs by
CVSET as shown in Figure 1. That is, assume the ΔVOUT =
300mV, R1=11kΩ, R2 = R3 = 27kΩ, the desired slew
rate at falling is SRF = 10mV/μs, and the CVSET can be
calculated by the formula below :
ΔVOUT
C VSET =
= 1nF
5 × (R1 // R2 // R3 ) × SRF
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RT2701
And then, the rising slew rate SRR will be
SRR =
ΔVOUT
= 7.67mV/μs
5 × (R1 // R2 ) × C VSET
VREF
REF Generator
(2V)
R1
RDC : DCR of inductor
RLL : Load line resistance
The value of RTON can be selected using Figure 3 and the
value of RRMP must be set equal to RTON.
The current through RTON should be set between 30μA to
280μA.
VSET
CVSET
R2
R3
GPIO
TON
CCRCOT
On-Time
Computer
RSET
VID
RTON
R1
VIN
C1
RMPSET
RRMP
On-Time
Figure 1. Output Voltage Setting with One Bit VID
Control
Figure 2. On-Time Setting with RC Filter
Frequency vs. RTON
Switching Frequency Setting
700
Switching frequency is a trade-off between efficiency and
converter size. Higher operation frequency allows the use
of smaller components. This is common in ultra portable
devices where the load currents are lower and the
controller is powered from a lower voltage supply. On the
other hand, lower frequency operation offers higher overall
efficiency at the expense of component size and board
space. Figure 2 shows the On-Time Setting Circuit.
Connect a resistor (RTON) from TON to VIN and a resistor
(R RMP) from RMPSET to GND to set the switching
frequency according to the formula below :
650
RTON =
VIN − VSET
×
fS × C × VREF
Frequency (kHz)1
600
550
500
450
400
350
300
250
200
150
0
50
100
150
200
250
300
RTON (Ω)
Ω
Figure 3. Frequency vs. RTON
VSET + IL × (RDS(ON)_L-MOS + RDC − RLL )
VIN + IL × (RDS(ON)_L-MOS − RDS(ON)_H-MOS )
Where
fS : Switching frequency
RTON : TON setting resistor
C : Capacitance for on time compute (13.7pF)
VREF : Reference voltage for on time compute
IL : Inductor current
RDS(ON)_L-MOS : RDS(ON) of Low Side MOSFET
RDS(ON)_H-MOS : RDS(ON) of High Side MOSFET
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2701-00 May 2013
Current Sense Setting (with Temperature
Compensation)
The RT2701 uses continuous inductor current sensing to
make the controller less noise sensitive. Low offset
amplifiers are used for loop control and over current
detection. The CSP and CSN denote the positive and
negative input of the current sense amplifier of any phase.
Since the DCR of the inductor is temperature dependent,
it affects the down phase threshold, OCP threshold and
output voltage accuracy, especially at heavy load.
Temperature compensation is recommended for the
lossless inductor DCR current sense method. Figure 4
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13
RT2701
shows a simple but effective way to compensate the unwanted temperature variations of the inductor DCR by using
an NTC thermistor.
VOUT
L1
PHASE1
RS
L2
PHASE2
RP
RS
CSP
CSN
⎛
RS
L× ⎜ 2 +
⎜
REQU_25°C
⎝
CX =
RS × DCR25°C
RNTC
CX
The RT2701 observes the voltage VX, across the CSP and
CSN pins for inductor current information. To design VX
without regard to the temperature coefficient, refer to the
formula below :
RS
2+
R
DCRTH
EQU_TH
(1)
=
RS
DCRTL
2+
REQU_TL
where R EQU_TH is equal to R P + R NTC // R X at high
temperature and REQU_TL is equal to RP + RNTC // RX at low
temperature. Usually, RX is set to equal RNTC (25°C). RP
and RX are selected to linearize the NTC's temperature
characteristic. For a given NTC and RP, the design is to
first obtain RS and then CX. Usually, set RX = RNTC. To
solve (1), RS must first be obtained as below :
2 (α − 1)
1
REQU_TH
−
(2)
α
REQU_TL
Where α is equal to DCRTH/DCRTL
The standard formula for the resistance of the NTC
thermistor as a function of temperature is given by :
⎧ ⎡⎛
β ⎜
1
⎞ −⎛ 1 ⎞ ⎤ ⎫
⎟ ⎜
⎟
⎨ ⎢
⎥⎬
RNTC, T°C = R25°C × e⎩ ⎣⎝ T + 273 ⎠ ⎝ 278 ⎠ ⎦ ⎭
⎞
⎟⎟
⎠
(5)
Loop Compensation
Figure 4. Inductor DCR Sensing
RS =
(4)
CX can be obtained by below formula,
COUT
RX
+
VX
-
DCRT°C = DCR25°C x [1 + 0.00393 x ( T − 25) ]
where the 0.00393 is the temperature coefficient of copper.
DCR
DCR
To calculate DCR value at different temperatures, can use
the equation below :
Optimized compensation of the RT2701 allows for best
possible load step response of the regulator's output. A
type-I compensator with a single pole and single zero is
adequate for a proper compensation. Figure 5 shows the
compensation circuit. Prior design procedure shows how
to determine the resistive feedback components of the
error amplifier gain, C1 and C2 must be calculated for the
compensation. The target is to achieve the constant
resistive output impedance over the widest possible
frequency range. The pole frequency, fP, of the compensator
must be set to compensate the output capacitor ESR
zero :
1
2π × RC × C
fP =
(6)
where C is the capacitance of the output capacitor, and
RC is the ESR of output capacitor. C2 can be calculated
as follows :
RC × C
(7)
R2
The zero of compensator has to be placed at half of the
switching frequency to filter the switching related noise,
such that,
1
(8)
C1 =
R1× π × fS
C2 =
(3)
where R25°C is the thermistor's nominal resistance at room
temperature, β (beta) is the thermistor's material constant
in Kelvins, and T is the thermistor's actual temperature in
Celsius.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
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14
is a registered trademark of Richtek Technology Corporation.
DS2701-00 May 2013
RT2701
VIN
UGATE1
L1 DCR
PHASE1
Driver
Logic
PS
LGATE1
COUT
VIN
L1
RX
L2
UGATE2
PHASE2
+
CMP
-
VOUT
DCR
RX
LGATE2
CX
RX
CSN
COMP
FB
+
VREF
VRTN
C2
C1
R2
R1
VSEN
VRTN
Figure 5. Compensation Circuit
Dynamic Phase Number Control
The RT2701 controls the operation phase number according
to the total current. Figure 6 shows the dynamic phase
number control circuit. By connecting a resistor (RPS) from
the PS pin to GND, the phase transition threshold can be
set. The formula is :
RPS =
DCR × ISUM × 5
1μ
where ISUM is the sum of the inductor valley current. For
example, if DCR is 0.74mΩ, and the desired up phase
threshold is 15A, the value of RPS will be
−3
RPS = 0.74 × 10 × 15 × 5 = 55.5kΩ
1× 10−6
Once the total inductor valley current is higher than the
threshold, the controller will transit to dual-phase operation.
when the total current becomes lower than the setting
threshold minus around 5A hysteresis, the active phase
number will return to single phase. If the PS pin is set
floating, the controller will force to dual-phase operation.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2701-00 May 2013
Active
Phase
Number
COUT
CSN
gm
+
VCX
CX
C3
+
GM
-
CMP
-
RPS
CSP
CSP
+
GM
-
+
DCR
RX
L2 DCR
VPS
Figure 6. Dynamic Phase Number Control Circuit
Current Balance
The RT2701 implements internal current balance
mechanism in the current loop. The RT2701 senses per
phase current signal and compares it with the average
current. If the sensed current of any particular phase is
higher than average current, the on-time of this phase will
be adjusted to be shorter.
Current Limit Setting
The RT2701 includes a built-in current limit protection
function. Figure 7 shows the protection circuit. The current
limit threshold is adjusted by an external resistor, ROC, at
the OCP pin. The value of ROC can be set according to
the following formula :
ROC =
DCR × ISUM × 6
8μ
where ISUM is the desired current limit threshold. Once
the sensed total current exceeds the current limit
threshold, the driver will be forced to turn off UGATE until
the OCP situation is removed.
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15
RT2701
Output Capacitor Selection
OCP
L1
L2
DCR
-
CMP
OCP
+
ROC
DCR
RX
RX
VOC
COUT
CX
CSN
CSP
gm
+
VCX
Figure 7. Over Current Protection Circuit
Over Voltage Protection
The RT2701 monitors the output voltage via the CSN pin
for Over Voltage Protection (OVP). Once the output voltage
exceeds the OVP threshold, OVP is triggered. The RT2701
will turn on low side MOSFETs and turn off high side
MOSFETs to protect the load until the OVP situation is
removed. A 4μs delay is used in the OVP detection circuit
to prevent false trigger.
Under Voltage Protection
The voltage on CSN pin is also monitored for under voltage
protection. If the output voltage is lower than the UVP
threshold, UVP will be triggered. The RT2701 will then
turn off both high side and low side MOSFETs. When
UVP is triggered, the RT2701 will enter hiccup mode and
continuously try to restart until the UVP situation is
cleared.
Inductor Selection
The switching frequency and ripple current determine the
inductor value as follows :
L(MIN) =
VIN − VOUT
IRIPPLE(MAX)
× TON
where TON is the UGATE turn on period.
Higher inductance results in lower ripple current and higher
efficiency but brings slower load transient response. Thus,
more output capacitors may be required. The lower DC
resistance can reduce power loss. The core must be large
enough and not to be saturated at the peak inductor
current.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
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16
Output capacitors are used to maintain high performance
for the output beyond the bandwidth of the converter itself.
Two different kinds of output capacitors can be found, bulk
capacitors closely located to the inductors and ceramic
output capacitors close to the load. The latter are for
mid-frequency decoupling with especially small ESR and
ESL values while the bulk capacitors have to provide
enough stored energy to overcome the low-frequency
bandwidth gap between the regulator and the GPU.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction to ambient
thermal resistance.
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
WQFN-24L 4x4 package, the thermal resistance, θJA, is
52°C/W on a standard JEDEC 51-7 four-layer thermal test
board. The maximum power dissipation at TA = 25°C can
be calculated by the following formula :
PD(MAX) = (125°C − 25°C) / (52°C/W) = 1.923W for
WQFN-24L 4x4 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 8 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
is a registered trademark of Richtek Technology Corporation.
DS2701-00 May 2013
RT2701
Maximum Power Dissipation (W)1
2.0
Four-Layer PCB
Layout Considerations
Careful PC board layout is critical to achieving low
switching losses and clean, stable operation. The
switching power stage requires particular attention. If
possible, mount all of the power components on the top
side of the board with their ground terminals flushed
against one another. Follow these guidelines for optimum
PC board layout :
1.6
1.2
0.8
0.4
`
0.0
0
25
50
75
100
125
`
Ambient Temperature (°C)
Figure 8. Derating Curve of Maximum Power Dissipation
`
`
`
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2701-00 May 2013
Keep the high current paths short, especially at the
ground terminals.
Keep the power traces and load connections short. This
is essential for high efficiency.
When trade-offs in trace lengths must be made, it’s
preferable to allow the inductor charging path to be made
longer than the discharging path.
Place the current sense components close to the
controller. CSP and CSN connections for current limit
and voltage positioning must be made using Kelvin sense
connections to guarantee the current sense accuracy.
The PCB trace from the sense nodes should be
paralleled back to the controller.
Route high speed switching nodes away from sensitive
analog areas (COMP, FB, CSP, CSN, etc...)
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17
RT2701
Outline Dimension
D2
D
SEE DETAIL A
L
1
E
E2
e
b
A3
Symbol
D2
E2
1
2
DETAIL A
Pin #1 ID and Tie Bar Mark Options
A
A1
1
2
Note : The configuration of the Pin #1 identifier is optional,
but must be located within the zone indicated.
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
0.700
0.800
0.028
0.031
A1
0.000
0.050
0.000
0.002
A3
0.175
0.250
0.007
0.010
b
0.180
0.300
0.007
0.012
D
3.950
4.050
0.156
0.159
Option 1
2.400
2.500
0.094
0.098
Option 2
2.650
2.750
0.104
0.108
E
3.950
4.050
0.156
0.159
Option 1
2.400
2.500
0.094
0.098
Option 2
2.650
2.750
0.104
0.108
e
L
0.500
0.350
0.020
0.450
0.014
0.018
W-Type 24L QFN 4x4 Package
Richtek Technology Corporation
5F, No. 20, Taiyuen Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot
assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be
accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
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DS2701-00 May 2013