DS7249 00

®
RT7249
3A, 18V Synchronous Step-Down Converter with
Adjustable Current Limited Power Switch
General Description
Features
The RT7249 is a high efficiency, monolithic synchronous
step-down DC/DC converter with a power switch. The Buck
converter can deliver up to 3A output current from 4.5V to
18V input supply. The RT7249's current-mode architecture
and external compensation allow the transient response
to be optimized over a wide input range and loads. Cycleby-cycle current limit provides protections against shorted
output and soft-start eliminates input current surge during
start-up. The RT7249 also provides under-voltage protection
and thermal shutdown. PWM frequency is adjustable by
the ROSC pin and Power Switch current limit can also be
adjusted by the RLIM pin. The low current (<3μA) in
shutdown mode provides output disconnection, enabling
easy power management in battery-powered systems. The
RT7249 is available in the WQFN-16L 4x4 package.
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Marking Information
1A= : Product Code
1A=YM
DNN
YMDNN : Date Code
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Buck Regulator
` 4.5V to 18V Input Voltage Range
` 3A Output Current
` Internal N-Channel MOSFETs
` Current Mode Control
` Adjustable PWM Frequency
` Adjustable Output from 0.8V to 15V
` Adjustable Soft-Start Time
` Stable with Ceramic Output Capacitors
` Cycle-by-Cycle Current Limit
` Input Under-Voltage Lockout
` Output Under-Voltage Protection
` Thermal Shutdown
Power Switch
` ±15% Current-Limit Accuracy at 1.2A
` Adjustable Current Limit : 75mA to 2580mA
` Meet USB Current-Limiting Requirements
` Reverse Input-Output Voltage Protection
` Built-in Soft-Start
` 120mΩ
Ω High-Side MOSFET
` Operating Range : 2.5 V to 5.5 V
RoHS Compliant and Halogen Free
Simplified Application Circuit
RT7249
VIN
SW_IN
VIN
VOUT
BOOT
FB
SW_IN
Enable
EN_SW
Enable
EN
RLIM
LX
SW_OUT
SW_OUT
COMP
ROSC
PGND
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS7249-00
June 2014
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT7249
Applications
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Pin Configurations
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
`
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
`
Suitable for use in SnPb or Pb-free soldering processes.
(TOP VIEW)
PGND
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VIN
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Package Type
QW : WQFN-16L 4x4 (W-Type)
V5V
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RT7249
Wireless AP/Router
Set-Top-Boxes
Industrial and Commercial Low Power Systems
LCD Monitors and TVs
Green Electronics/Appliances
Point of Load Regulation of High-Performance DSPs
USB Bus/Self Powered Hubs
USB Peripheral Ports
ACPI Power Distribution
Battery Power Equipment
3G/3.5G Data Card
EN
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Ordering Information
16
15
14
13
SS
1
12
BOOT
COMP
2
11
LX
ROSC
3
10
FB
RLIM
4
9
SW_IN
PGND
5
6
7
8
EN_SW
FAULT
AGND
SW_OUT
17
WQFN-16L 4x4
Function Pin Description
Pin No.
Pin Name
Pin Function
1
SS
Soft-Start Time Setting. SS controls the soft-start period. Connect a capacitor
from SS to GND to set the soft-start period. An internal current source (6μA)
charges 0.1μF capacitor and sets the soft-start period to 13.5ms. If SS is floating,
the SS charge current will decrease to 1/128μA and charge 30pF capacitor to set
the soft-start period to 4ms.
2
COMP
Compensation Node. 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.
3
ROSC
Switching Frequency Setting.
4
RLIM
Current Limit Setting. Switch current limit threshold can be set by an external
resistor. Current limit value is from 75mA to 2580mA. The value of 10kΩ ≤ RILIM ≤
210kΩ is recommended.
5
EN_SW
Enable Control Input for Power Switch.
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS7249-00
June 2014
RT7249
Pin No.
Pin Name
Pin Function
6
FAULT
Active-Low Open-Drain Output. Asserted during over current, over
temperature, or reverse voltage conditions.
7
AGND
Analog Ground.
8
SW_OUT
Power Switch Output.
9
SW_IN
Power Switch Input. Supply voltage range is from 2.5V to 5.5V.
10
FB
Feedback Voltage Input. This pin is connected to the converter output. It is
used to set the output of the converter to regulate the desired value via an
resistive divider.
11
LX
Switch Node. Output of the internal high-side MOSFET. Connect this pin to
external low-side N-MOSFET, inductor and bootstrap capacitor.
12
BOOT
Bootstrap Supply for High-Side Gate Driver. Connect a 1μF ceramic
capacitor between the BOOT and LX pins.
13,
17 (Exposed Pad)
PGND
Power Ground. The exposed pad must be soldered to a large PCB and
connected to PGND for maximum thermal dissipation.
14
VIN
Power Input. Supply voltage range is from 4.5V to 18V. Must bypass with a
suitable large ceramic capacitor.
15
V5V
BG Driver Bias Supply. Decouple with a 1μF X5R/X7R ceramic capacitor
between the V5V and GND pins.
16
EN
Enable Control Input for Buck Converter. A logic-high enables the converter;
a logic-low forces the device into shutdown mode.
Function Block Diagram
VIN
Internal
Regulator
Oscillator
Slope Comp
Shutdown
VCC
Comparator VA
1.2V
-
5k
EN
Foldback
Control
+
0.4V
Lockout
Comparator
1.7V
Current Sense
Amplifier
+
+
UV
Comparator
+
BOOT
LX
Control
+
Current
Comparator
3V
PGND
AGND
SS Control
SS
FB
COMP
ROSC
SW_IN
V5V
VA
-
0.8V
+
+EA
-
Reverse Voltage
Comparator
+
-
Switch Well
Current
Sense
UVLO
EN_SW
Drive
CS
SW_OUT
5ms Deglitch
Current
Limit
FAULT
Thermal
Sense
RLIM
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS7249-00
June 2014
10ms Deglitch
is a registered trademark of Richtek Technology Corporation.
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3
RT7249
Operation
Buck
Enable
The RT7249 is a current-mode synchronous step-down
converter with adjustable frequency from 300kHz to
1.4MHz. In normal operation, the high-side N-MOSFET is
turned on when the S-R latch is set by the oscillator and
is turned off when the current comparator resets the S-R
latch. While the high-side N-MOSFET is turned off, the
low-side N-MOSFET is turned on to conduct the inductor
current until next cycle begins.
The Buck converter is turned on when the EN pin is higher
than 2V. When the EN pin is lower than 0.4V, the converter
will enter shutdown mode and reduce the supply current
to 1μA.
Switch
The RT7249 has a single P-MOSFET high-side power
switch with active high enable input, optimized for selfpowered and bus-powered Universal Serial Bus (USB)
applications.
The switch's low R DS(ON) meets USB voltage drop
requirements and a flag output is available to indicate fault
conditions to the local USB controller.
Error Amplifier
The error amplifier adjusts its output voltage by comparing
the feedback signal (VFB) with the internal 0.8V reference.
When the load current increases, it causes a drop in the
feedback voltage relative to the reference. The error
amplifier's output voltage then rises to allow higher inductor
current to match the load current.
Oscillator
The internal oscillator provides adjustable frequency from
300kHz to 1.4MHz with an external resistor. When ROSC
is short to Ground, the frequency is set to 300kHz. When
ROSC is floating or connected to V5V, the frequency is
set to 600kHz. When ROSC is connected to a 54kΩ
resistor, the frequency is set to 500kHz.
Foldback Control
When VFB is lower than 0.4V, the switching frequency will
be decreased up to half of oscillation frequency. If VFB is
lower than 0.2V, the switching frequency will be decreased
up to 1/4 of oscillation frequency.
Soft-Start (SS)
An internal current source (6μA) charges an extra capacitor
to build the soft-start ramp voltage (VSS). The VFB voltage
will track the internal ramp voltage during soft-start interval.
If SS is floating, the soft-start period is 4ms.
Enable SW
The switch is turned on when the EN pin is higher than
2V. When the EN pin is lower than 0.4V, the switch will
enter shutdown mode.
Current Limit and Short-Circuit Protection
When a heavy load or short-circuit situation occurs while
the switch is enabled, large transient current may flow
through the device. The RT7249 includes a current limit
circuitry to prevent the devices from damaging by these
large current. The RT7249 provides an adjustable current
limit threshold from 120mA to 2.6A (typ.) via an external
resistor, RILIM, between 10kΩ and 50kΩ. However, if the
ILIM pin is connected to VIN, the current limit threshold
will be 75mA (typ.). Once the current limit threshold is
exceeded, the device enters latch off and turns off the
switch.
Internal Regulator
The regulator provides low voltage power to supply the
internal control circuits and the bootstrap power for highside gate driver.
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS7249-00
June 2014
RT7249
Absolute Maximum Ratings
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(Note 1)
Supply Voltage, VIN ----------------------------------------------------------------------------------------------Switch Voltage, LX ------------------------------------------------------------------------------------------------< 10ns ----------------------------------------------------------------------------------------------------------------Supply Input Voltage, SW_IN ----------------------------------------------------------------------------------RLIM, EN_SW, FAULT, SW_OUT Pin Voltages ------------------------------------------------------------VBOOT − VLX ---------------------------------------------------------------------------------------------------------Other Pins -----------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
WQFN-16L 4x4 ----------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
WQFN-16L 4x4, θJA -----------------------------------------------------------------------------------------------WQFN-16L 4x4, θJC ----------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -----------------------------------------------------------------------Junction Temperature ---------------------------------------------------------------------------------------------Storage Temperature Range ------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Model) ---------------------------------------------------------------------------------------
Recommended Operating Conditions
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−0.3V to 20V
−0.3V to (VIN + 0.3V)
−5V to 20V
−0.3V to 6V
−0.3V to 5.5V
−0.3V to 6.3V
−0.3V to 6V
2.74W
36.5°C/W
3.3°C/W
260°C
150°C
−65°C to 150°C
2kV
(Note 3)
Supply Voltage, VIN ----------------------------------------------------------------------------------------------Supply Voltage, SW_IN ------------------------------------------------------------------------------------------Junction Temperature Range ------------------------------------------------------------------------------------Ambient Temperature Range -------------------------------------------------------------------------------------
4.5V to 18V
2.5V to 5.5V
−40°C to 125°C
−40°C to 85°C
Electrical Characteristics
Switching Buck Regulator SPEC
(VIN = 12V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
μA
Shutdown Current
VEN = 0V
--
0.5
3
Quiescent Current
VEN = 3V, VFB = 0.9V, No Load Not
Switching
--
0.8
1.2
VEN = 3V, No Load Switching
--
7
--
0.792
0.8
0.808
V
--
700
--
μA/V
Feedback Voltage
VFB
4.5V ≤ VIN ≤ 18V
Error Amplifier
Transconductance
GEA
ΔIC = ±10μA
Switch
On-Resistance
High-Side
RDS(ON)1
--
85
--
Low-Side
RDS(ON)2
--
72
--
VEN = 0V, VSW = 0V
--
0
10
High-Side
Min. Duty Cycle
--
5
--
Low-Side
From Drain to Source
--
1.5
--
High-Side Switch Leakage
Current
Switch Current
Limit
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS7249-00
June 2014
mA
mΩ
μA
A
is a registered trademark of Richtek Technology Corporation.
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5
RT7249
Parameter
Symbol
COMP to Current Sense
Transconductance
Test Conditions
GCS
Oscillation Frequency
Min
Typ
Max
Unit
--
5
--
A/V
300
--
1400
kHz
Oscillation Frequency1
fOSC1
ROSC Short to Ground
255
300
345
kHz
Oscillation Frequency2
fOSC2
ROSC Float or Connect to V5V
510
600
690
kHz
Oscillation Frequency3
fOSC3
ROSC Connect to R = 54kΩ
425
500
575
kHz
Oscillation Frequency4
fOSC4
ROSC Connect to R = 220kΩ
1190
1400
1610
kHz
Maximum Duty Cycle
DMAX
VFB = 0.7V, ROSC Short to Ground
--
93
--
%
Minimum On Time
tON
--
100
--
ns
Line Regulation–DC
IOUT = 2A
--
0.5
--
%/V
Load Regulation–DC
IOUT = 0.3A − 2.7A
--
0.5
--
%/A
EN Input Voltage
Logic-High
VIH
2
--
--
Logic-Low
VIL
--
--
0.4
VIN Rising
4
4.25
4.5
V
VIN Falling
3.68
3.93
4.18
V
Hysteresis
--
320
--
mV
VUVLO
Input Under-Voltage Lockout
Threshold
ΔVUVLO
V
V5V Voltage
V5V
4.8
5
5.2
V
Soft-Start Charging Current
ISS
--
6
--
μA
Internal Soft-Start Period
tSS
--
4
--
ms
Thermal Shutdown
TSD
--
150
--
°C
Min
Typ
Max
Unit
VIH
2
--
--
VIL
--
--
0.4
Rising
--
2.4
--
V
Falling
--
2.2
--
V
Hysteresis
--
200
--
mV
Static Drain-Source On-State
RDS(ON)
Resistance
ISW = 0.5A, SW_IN = 5V
--
90
--
ISW = 0.5A, SW_IN = 2.5V
--
125
--
Turn-On Delay Time
tD_ ON
SW_IN = 5V, CL = 1μF, RL = 100Ω
--
0.66
--
ms
Turn-Off Delay Time
tD_OFF
--
1.6
--
ms
Output Rising Time
tr
--
1.1
1.5
ms
Output Falling Time
tf
--
1.2
1.5
ms
SS Pin Open
Power Switch SPEC
(VSW_IN = 3.6V, TA = 25°C, 10kΩ ≤ RILIM ≤ 210kΩ unless otherwise specified)
Parameter
EN_SW Input Voltage
Symbol
Under-Voltage Lockout
Threshold
Test Conditions
V
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
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mΩ
is a registered trademark of Richtek Technology Corporation.
DS7249-00
June 2014
RT7249
Parameter
Current-Limit Threshold and
Short-Circuit Current, Out
Connect to GND
Response Time to Short
Circuit
Symbol
IOS
tIOS
Test Conditions
Min
Typ
Max
RILIM = 10kΩ
2420
2580
2740
RILIM = 15kΩ
1595
1700
1805
RILIM = 20kΩ
1215
1295
1375
RILIM = 49.9kΩ
468
520
572
RILIM = 210kΩ
110
130
150
ILIM Shorted to IN
50
75
100
(Note 5)
--
2
--
μs
--
10
--
mV
= 1mA
Unit
mA
FAULT Output Low Voltage
FAULT
FAULT Deglitch
Over Current Condition
--
10
--
ms
Discharge Resistance
VSW_IN = 5V, EN_SW = 0V
--
100
--
Ω
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.
Note 5. Guaranteed by design.
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS7249-00
June 2014
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
7
RT7249
Typical Application Circuit
L1
4.7µH
RT7249
VIN
4.5V to 18V
SW_IN
CIN
10µF
CSW_IN
10µF
14 VIN
9 SW_IN
5 EN_SW
Enable
16 EN
Enable
RRLIM
20k
RROSC
4
LX 11
10
FB
SW_OUT 8
PGND
COMP
SS
AGND
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8
COUT
22µF x 2
CSW_OUT
10µF
CFF
SW_OUT
R1
40.2k
VOUT
5V/3A
R2
7.68k
2
CP
NC
1
FAULT 6
V5V 15
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
CBOOT
1µF
13, 17 (Exposed Pad)
RLIM
3 ROSC
7
BOOT
12
CSS
100nF
RFAULT
100k
RCOMP
10k
CCOMP
4.7nF
CV5V
1µF
is a registered trademark of Richtek Technology Corporation.
DS7249-00
June 2014
RT7249
Typical Operating Characteristics
Efficiency vs. Load Current
Output Voltage vs. Input Voltage
100
5.20
90
VIN = 8V
VIN = 12V
VIN = 17V
70
5.16
Output Voltage(V)
Efficiency (%)
80
Buck Converter
60
50
40
30
5.12
5.08
IOUT
IOUT
IOUT
IOUT
5.04
20
10
0A
1A
2A
3A
VOUT = 5V
Buck Converter, VOUT = 5V
5.00
0
0
0.5
1
1.5
2
2.5
6
3
8
10
Load Current (A)
12
14
16
Output Voltage vs. Load Current
5.20
Buck Converter
Buck Converter
5.15
Output Voltage (V)
0.815
0.810
0.805
0.800
VIN = 8V
VIN = 12V
VIN = 17V
0.795
5.10
VIN = 17V
VIN = 12V
VIN = 8V
5.05
5.00
4.95
VOUT = 5V
VOUT = 5V
0.790
4.90
-50
-25
0
25
50
75
100
125
0
0.5
1
Temperature (°C)
Switching Frequency vs. Output Current
2
2.5
3
Switching Frequency vs. Ambient Temperature
650
Switching Frequency (kHz)1
Buck Converter
Switching Frequency (kHz)1
1.5
Load Current (A)
650
645
640
635
630
625
620
615
Buck Converter
630
610
VIN = 8V
VIN = 12V
VIN = 17V
590
570
IOUT = 0A, RRSOC Float
VOUT = 5V, IOUT = 0A, RRSOC Float
550
610
0
0.5
1
1.5
2
2.5
Output Current (A)
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS7249-00
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Input Voltage(V)
Reference Voltage vs. Temperature
0.820
Reference Voltage (V)
=
=
=
=
June 2014
3
-50
-25
0
25
50
75
100
125
Ambient Temperature (°C)
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RT7249
Current Limit vs. Input voltage
Switching Frequency vs. RSOC
8
Buck Converter
1400
Buck Converter
7
Inductor Current (A)
Switching Frequency (kHz)1
1600
1200
1000
800
600
400
6
5
4
3
200
VIN = 12V, VOUT = 5V, IOUT = 0A
VIN = 4.5V to 17V, VOUT = 3.3V
0
2
0
20
40
60
80 100 120 140 160 180 200 220
4
RSOC (kΩ)
Power Switching Current Limit (mA)1
Power Switching Current Limit (A)1
Power Switch
1.8
1.6
1.4
1.2
VIN = 5V
VIN = 3.3V
VIN = 2.5V
1.0
0.8
0.6
0.4
0.2
6
7
8
9 10 11 12 13 14 15 16 17 18
Input Voltage (V)
Power Switching Current Limit vs. Temperature
2.0
5
SW_IN = 2.5V to 5V, RLIM = 20kΩ
0.0
Power Switching Current Limit vs. RLIM
3.0
Power Switch
2.5
2.0
1.5
1.0
0.5
SW_IN = 5V
0.0
-50
-25
0
25
50
75
100
125
0
20
40
60
80 100 120 140 160 180 200 220
Temperature (°C)
RLIM (kΩ)
RDS(ON) vs. Temperature
nFault Delay vs. Temperature
12
Power Switch
RDS(ON) (m Ω) 1
nFault Delay (ms)
10
8
VIN = 2.5V
VIN = 3.3V
VIN = 5V
6
4
2
SW_IN = 2.5V to 5V
0
-50
-25
0
25
50
75
100
Temperature (°C)
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10
125
300
280
260
240
220
200
180
160
140
120
100
80
60
40
20
0
Power Switch
VIN = 2.5V
VIN = 3.3V
VIN = 5V
SW_IN = 2.5V to 5V
-50
-25
0
25
50
75
100
125
Temperature (°C)
is a registered trademark of Richtek Technology Corporation.
DS7249-00
June 2014
RT7249
Load Transient Response
Load Transient Response
Buck Converter
Buck Converter
VOUT
(200mV/Div)
VOUT
(100mV/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 5V, IOUT = 0A to 3A
VIN = 12V, VOUT = 5V, IOUT = 1.5A to 3A
Time (250μs/Div)
Time (250μs/Div)
Output Ripple Voltage
Output Ripple Voltage
VOUT
(10mV/Div)
VOUT
(10mV/Div)
VSW
(10V/Div)
VSW
(10V/Div)
IL
(1A/Div)
VIN = 12V, VOUT
Buck Converter,
= 5V, IOUT = 1.5A
IL
(2A/Div)
Time (1μs/Div)
Time (1μs/Div)
Power On from VIN
Power Off from VIN
Buck Converter
Buck Converter
VIN
(5V/Div)
VIN
(5V/Div)
VOUT
(5V/Div)
VOUT
(5V/Div)
IL
(2A/Div)
IL
(2A/Div)
VIN = 12V, VOUT = 5V, IOUT = 3A
Time (25ms/Div)
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS7249-00
June 2014
Buck Converter,
VIN = 12V, VOUT = 5V, IOUT = 3A
VIN = 12V, VOUT = 5V, IOUT = 3A
Time (25ms/Div)
is a registered trademark of Richtek Technology Corporation.
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RT7249
Power Off from EN
Power On from EN
VEN
(2V/Div)
VEN
(2V/Div)
VOUT
(5V/Div)
VOUT
(5V/Div)
IL
(2A/Div)
Buck Converter, VIN = 12V,
VOUT = 5V, IOUT = 3A
IL
(2A/Div)
Buck Converter, VIN = 12V,
VOUT = 5V, IOUT = 3A
Time (10ms/Div)
Time (2.5ms/Div)
Power Switching On from SWIN
Power Switching Off from SWIN
Power Switch
Power Switch
SWIN
(2V/Div)
VIN
(2V/Div)
SWOUT
(5V/Div)
VOUT
(5V/Div)
I SWOUT
(1A/Div)
VIN = 12V, VOUT = 5V, IOUT = 3A
IL
(1A/Div)
Time (5ms/Div)
Time (100ms/Div)
Power Switching Current Limit
nFalut
SWIN
(5V/Div)
SWIN
(5V/Div)
SWOUT
(5V/Div)
SWOUT
(5V/Div)
ILX
(2A/Div)
I SWOUT
(2A/Div)
I SWOUT
(1A/Div)
VIN = 12V, VOUT = 5V, IOUT = 3A
VIN = 12V, VOUT = 5V, VOUT = SWIN, ISWOUT = short
Time (1ms/Div)
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nFalut
(5A/Div)
VIN = 12V, VOUT = 5V, VOUT = SWIN, ISWOUT = short
Time (1ms/Div)
is a registered trademark of Richtek Technology Corporation.
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June 2014
RT7249
Application Information
(Buck Converter)
Soft-Start
Output Voltage Setting
The RT7249 provides a soft-start function. The soft-start
function is used to prevent large inrush current while
converter is being powered-up. The soft-start timing can
be adjusted by the external capacitor between SS and
GND. An internal current source ISS (6μA) charges an
external capacitor to build a soft-start ramp voltage. The
VFB voltage will track the internal ramp voltage during softstart interval. The typical soft-start time is calculated as
follows :
0.8 × CSS
Soft-Start time tSS =
, if CSS capacitor
ISS
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
VOUT
R1
FB
RT7249
R2
GND
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
VOUT
= VREF ⎛⎜ 1+ R1 ⎞⎟
⎝ R2 ⎠
where VREF is the reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 1μF low ESR ceramic capacitor between the
BOOT and LX pins. This capacitor provides the gate driver
voltage for the high-side MOSFET. It is recommended to
add an external bootstrap diode between an external 5V
and the BOOT pin for efficiency improvement when input
voltage is lower than 5.5V or duty ratio is higher than 65%
.The bootstrap diode can be a low cost one such as IN4148
or BAT54. The external 5V can be a 5V fixed input from
system or a 5V output of the RT7249. Note that the external
boot voltage must be lower than 5.5V.
5V
BOOT
0.1µF
RT7249
SW
Figure 2. External Bootstrap Diode
is 0.1μF, then soft-start time =
0.8 × 0.1μ
≒ 13.5ms
6μ
Chip Enable Operation
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shut down the device. During shutdown
mode, the RT7249's quiescent current drops to lower than
3μA. Driving the EN pin high (>2V, <18V) will turn on the
device again. For external timing control, the EN pin can
also be externally pulled high by adding a REN resistor
and CEN capacitor from the VIN pin (see Figure 3).
EN
VIN
REN
EN
RT7249
CEN
GND
Figure 3. Enable Timing Control
An external MOSFET can be added to implement digital
control on the EN pin when no system voltage above 2.5V
is available, as shown in Figure 4. In this case, a 100kΩ
pull-up resistor, REN, is connected between VIN and the
EN pin. MOSFET Q1 will be under logic control to pull
down the EN pin.
VIN
EN
REN
100k
EN
Q1
RT7249
GND
Figure 4. Digital Enable Control Circuit
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
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June 2014
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RT7249
Under-Voltage Protection
Hiccup Mode
For the RT7249, it provides Hiccup Mode Under-Voltage
Protection (UVP). When the VFB voltage drops below 0.4V,
the UVP function will be triggered to shut down switching
operation. If the UVP condition remains for a period, the
RT7249 will retry automatically. When the UVP condition
is removed, the converter will resume operation. The UVP
is disabled during soft-start period.
Hiccup Mode
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
the highest efficiency operation. However, it requires a
large inductor to achieve this goal.
For the ripple current selection, the value of ΔIL = 0.24(IMAX)
will be a reasonable starting point. The largest ripple
current occurs at the highest VIN. To guarantee that the
ripple current stays below the specified maximum, the
inductor value should be chosen according to the following
equation :
⎡ VOUT ⎤ ⎡
VOUT ⎤
L =⎢
× ⎢1 −
⎥
⎥
f
I
V
×
Δ
L(MAX) ⎦ ⎣
IN(MAX) ⎦
⎣
VOUT
(2V/Div)
The inductor's current rating (caused a 40°C temperature
rising from 25°C ambient) should be greater than the
maximum load current and its saturation current should
be greater than the short circuit peak current limit. Please
see Table 1 for the inductor selection reference.
ILX
(1A/Div)
IOUT = Short
Time (25ms/Div)
Figure 5. Hiccup Mode Under-Voltage Protection
Over Temperature Protection
The RT7249 features an Over-Temperature Protection
(OTP) circuitry to prevent from overheating due to
excessive power dissipation. The OTP will shut down
switching operation when junction temperature exceeds
150°C. Once the junction temperature cools down by
approximately 20°C, the converter will resume operation.
To maintain continuous operation, the maximum junction
temperature should be lower than 125°C.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ΔIL increases with higher VIN
and decreases with higher inductance.
V
V
ΔIL = ⎡⎢ OUT ⎤⎥ × ⎡⎢1− OUT ⎤⎥
f
×
L
VIN ⎦
⎣
⎦ ⎣
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14
Table 1. Suggested Inductors for Typical
Application Circuit
Component
Supplier
Series
Dimensions
(mm)
TDK
VLF10045
10 x 9.7 x 4.5
TDK
TAIYO
YUDEN
SLF12565
12.5 x 12.5 x 6.5
NR8040
8x8x4
CIN and COUT Selection
The input capacitance, C IN, is needed to filter the
trapezoidal current at the Source of the high-side MOSFET.
To prevent large ripple current, a low ESR input capacitor
sized for the maximum RMS current should be used. The
approximate RMS current equation is given :
V
VIN
IRMS = IOUT(MAX) OUT
−1
VIN
VOUT
This formula has a maximum at VIN = 2VOUT, where IRMS =
IOUT / 2. This simple worst case condition is commonly
used for design because even significant deviations do
not offer much relief. Choose a capacitor rated at a higher
temperature than required. Several capacitors may also
be paralleled to meet size or height requirements in the
design. For the input capacitor, two 10μF low ESR ceramic
capacitors are suggested. The selection of COUT is
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June 2014
RT7249
determined by the required ESR to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key
for COUT selection to ensure that the control loop is stable.
Loop stability can be checked by viewing the load transient
response as described in a later section. The output ripple,
ΔVOUT, is determined by :
1
⎤
ΔVOUT ≤ ΔIL ⎡⎢ESR +
8fCOUT ⎥⎦
⎣
The output ripple will be the highest at the maximum input
voltage since ΔIL increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirement. Higher values,
lower cost ceramic capacitors are now becoming available
in smaller case sizes. Their high ripple current, high voltage
rating and low ESR make them ideal for switching regulator
applications. However, care must be taken when these
capacitors are used at input and output. When a ceramic
capacitor is used at the input and the power is supplied
by a wall adapter through long wires, a load step at the
output can induce ringing at the input, VIN. At worst, a
sudden inrush of current through the long wires can
potentially cause a voltage spike at VIN large enough to
damage the part.
Application Information (Power Switching)
The RT7249 is a P-MOSFET included high-side power
switch with active high enable input, optimized for self
powered and bus powered Universal Serial Bus (USB)
applications. The switch's low RDS(ON) meets USB voltage
drop requirements and a flag output is available to indicate
nFault conditions to the local USB controller.
Current Limit and Short-Circuit Protection
When a heavy load or short-circuit situation occurs while
the switch is enabled, large transient current may flow
through the device. The RT7249 includes a current limit
circuitry to prevent the MOSFET switch and the hub
downstream ports from damaging due to large currents.
The RT7249 provides an adjustable current limit threshold
between 130mA and 1.295A (typ.) via an external resistor,
RILIM, between 20kΩ and 210kΩ. However, if the RLIM
pin is connected to VIN, the current limit threshold will be
75mA (typ.). Once the current limit threshold is exceeded,
the device enters latch mode.
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS7249-00
June 2014
nFault Flag
The RT7249 provides a FAULT signal pin which is an
open-drain N-MOSFET output. This open-drain output
goes low when current exceeds current limit threshold,
VOUT − VIN exceeds reverse voltage trip level. The FAULT
output is capable of sinking a 1mA load to typically 180mV
above ground. The FAULT pin requires a pull-up resistor
; this resistor should be large in value to reduce energy
drain. A 100kΩ pull-up resistor works well for most
applications. In case of an over-current condition, FAULT
will be asserted only after the flag response delay time,
tD, has elapsed. This ensures that FAULT is asserted
upon valid over-current conditions and that erroneous error
reporting is eliminated. For example, false over-current
conditions may occur during hot-plug events when
extremely large capacitive loads are connected, which
induces a high transient inrush current that exceeds the
current limit threshold. The FAULT response delay time,
tD, is typically 7.5ms.
Supply Filter/Bypass Capacitor
A 10μF low-ESR ceramic capacitor connected from VIN
to GND and located close to the device is strongly
recommended to prevent input voltage drooping during
hotplug events. However, higher capacitor values may be
used to further reduce the voltage droop on the input.
Without this bypass capacitor, an output short may cause
sufficient ringing on the input (from source lead inductance)
to destroy the internal control circuitry. Note that the input
transient voltage must never exceed 6V as stated in the
Absolute Maximum Ratings.
Output Filter Capacitor
A low-ESR 150μF aluminum electrolytic capacitor or 22μF
ceramic capacitor connected between VOUT and GND is
recommended to meet the USB standard maximum droop
requirement for the hub, VBUS. Standard bypass methods
should be used to minimize inductance and resistance
between the bypass capacitor and the downstream
connector to reduce EMI and decouple voltage droop
caused by hot-insertion transients in downstream cables.
Ferrite beads in series with VBUS, the ground line and the
0.1μF bypass capacitors at the power connector pins are
recommended for EMI and ESD protection. The bypass
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RT7249
Chip Enable Input
The RT7249 will be disabled when the EN pin is in a logic
low condition. During this condition, the internal circuitry
and MOSFET are turned off, reducing the supply current
to 1μA typically. The maximum guaranteed voltage for a
logic-low at the EN pin is 0.66V. A minimum guaranteed
voltage of 1.1V at the EN pin will turn off the RT7249.
Floating the input may cause unpredictable operation. EN
should not be allowed to go negative with respect to GND.
Under-Voltage Lockout
Under-Voltage Lockout (UVLO) prevents the MOSFET
switch from turning on until input voltage exceeds
approximately 2.3V. If input voltage drops below
approximately 2.1V, UVLO turns off the MOSFET switch
and FAULT will be asserted accordingly. The under-voltage
lockout detection functions only when the switch is
enabled.
PD(MAX) = (125°C − 25°C) / (36.5°C/W) = 2.74W for
WQFN-16L 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 6 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
Maximum Power Dissipation (W)1
capacitor itself should have a low dissipation factor to allow
decoupling at higher frequencies.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 6. Derating Curve of Maximum Power Dissipation
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-16L 4X4 package, the thermal resistance, θJA, is
36.5°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 :
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is a registered trademark of Richtek Technology Corporation.
DS7249-00
June 2014
RT7249
Outline Dimension
D
SEE DETAIL A
D2
L
1
E
E2
e
b
A
A1
1
1
2
2
DETAIL A
Pin #1 ID and Tie Bar Mark Options
A3
Note : The configuration of the Pin #1 identifier is optional,
but must be located within the zone indicated.
Dimensions In Millimeters
Dimensions In Inches
Symbol
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
0.250
0.380
0.010
0.015
D
3.950
4.050
0.156
0.159
D2
2.000
2.450
0.079
0.096
E
3.950
4.050
0.156
0.159
E2
2.000
2.450
0.079
0.096
e
L
0.650
0.500
0.026
0.600
0.020
0.024
W-Type 16L QFN 4x4 Package
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st 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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June 2014
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