RT5797A - Farnell

RT5797A
3A, 1MHz, 5.5V CMCOT Synchronous Step-Down Converter
General Description
Features
The RT5797A is a high efficiency synchronous stepdown DC/DC converter. Its input voltage range is from
2.7V to 5.5V and provides an adjustable regulated
output voltage from 0.6V to 3.4V while delivering up to
3A of output current.





Constant-On-time (CMCOT) operation with internal
compensation allows the transient response to be
optimized over a wide range of loads and output
capacitors. The RT5797A is available in the WDFN-8L
2x2 package.
Ordering Information
for
Best
Transient Response, Robust Loop Stability with
Low-ESR (MLCC) COUT
The internal synchronous low on-resistance power
switches increase efficiency and eliminate the need for
an external Schottky diode. The Current Mode
Efficiency Up to 95%
RDS(ON) 100m HS / 70m LS
VIN Range 2.7V to 5.5V
VREF 0.6V with 2% Accuracy at 25C
CMCOT ™ Control Loop Design


Soft-Start 1.2ms
Power Saving in Light Load
Applications



STB, Cable Modem, & xDSL Platforms
LCD TV Power Supply & Metering Platforms
General Purpose Point of Load (POL)
Pin Configurations
RT5797A
FB
PG
VIN
PGND
Lead Plating System
G : Green (Halogen Free and Pb Free)
UVP Option
H : Hiccup
L : Latched-Off
1
2
3
4
PGND
(TOP VIEW)
Package Type
QW : WDFN-8L 2x2 (W-Type)
9
8
7
6
5
SGND
EN
LX
NC
WDFN-8L 2x2
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.
Simplified Application Circuit
RT5797A
LX
VIN
VIN
CIN
February 2015
VOUT
EN
R1
FB
PG
SGND PGND
R2
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L
COUT
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RT5797A
Marking Information
RT5797ALGQW
2U : Product Code
W : Date Code
RT5797AHGQW
2V : Product Code
W : Date Code
2UW
2VW
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
FB
Feedback Voltage Input. An external resistor divider from the output to GND,
tapped to the FB pin, sets the output voltage.
2
PG
Power Good Indicator. The output of this pin is an open-drain with external pull-up
resistor. PG is pulled up when the FB voltage is within 90%, otherwise it is LOW.
3
VIN
Supply Voltage Input. The RT5797A operates from a 2.7V to 5.5V input.
4, 9
PGND
(Exposed Pad)
Power Ground. The exposed pad must be soldered to a large PCB and connected
to PGND for maximum power dissipation.
5
NC
No Internal Connection.
6
LX
Switch Node.
7
EN
Enable Control Input.
8
SGND
Signal GND.
Function Block Diagram
EN
VIN
UVLO
Shut Down
Control
OTP
-
FB
VREF
Error
Amplifier
Ton
Comparator
+
+
RC
CCOMP
-
Logic
Control
LX
VIN
Driver
Current
Limit
Detector
LX
GND
+
Current
Sense
-
LX
PG
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is a registered trademark of Richtek Technology Corporation.
DS5797A-00
February 2015
RT5797A
Operation
The RT5797A is a synchronous low voltage step-down
converter that can support the input voltage range from
2.7V to 5.5V and the output current can be up to 3A.
The RT5797A uses a constant on-time, current mode
architecture. In normal operation, the high side PMOSFET is turned on when the switch controller is set
by the comparator and is turned off when the Ton
Enable Comparator
comparator resets the switch controller.
track the internal ramp voltage during soft-start interval.
The typical soft-start time is 1.2ms.
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.
UV Comparator
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 RT5797AH,
Latch mode for RT5797AL.
PGOOD Comparator
A logic-high enables the converter; a logic-low forces
the IC into shutdown mode.
Soft-Start (SS)
An internal current source charges an internal capacitor
to build the soft-start ramp voltage. The VFB voltage will
Over Current Protection (OCP)
The RT5797A provides over current protection by
detecting low side MOSFET valley inductor current. If
the sensed valley inductor current is over the current
limit threshold (3.7A typ.), the OCP will be triggered.
When OCP is tripped, the RT5797A will keep the over
current threshold level then cause the UV protection.
Thermal Shutdown (OTP)
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.
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 10. The PGOOD signal delay time from EN is
about 2ms.
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RT5797A
Absolute Maximum Ratings
(Note 1)

Supply Input Voltage -------------------------------------------------------------------------------------------0.3V to 6.5V

LX Pin Switch Voltage----------------------------------------------------------------------------------------- 0.3V to (VIN + 0.3V)
<20ns -------------------------------------------------------------------------------------------------------------- 4.5V to 7.5V

Power Dissipation, PD @ TA = 25C
WDFN-8L 2x2 ----------------------------------------------------------------------------------------------------2.19W

Package Thermal Resistance
(Note 2)
WDFN-8L 2x2, JA ----------------------------------------------------------------------------------------------45.5C/W

Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------260C

Junction Temperature ------------------------------------------------------------------------------------------40C to 150C

Storage Temperature Range ---------------------------------------------------------------------------------65C to 150C

ESD Susceptibility
(Note 3)
HBM (Human Body Model) -----------------------------------------------------------------------------------2kV
Recommended Operating Conditions
(Note 4)

Supply Input Voltage -------------------------------------------------------------------------------------------2.7V to 5.5V

Ambient Temperature Range---------------------------------------------------------------------------------40C to 85C

Junction Temperature Range --------------------------------------------------------------------------------40C to 125C
Electrical Characteristics
(VIN = 3.6V, TA = 25C, unless otherwise specified)
Parameter
Symbol
Min
Typ
Max
Unit
2.7
--
5.5
V
0.588
0.6
0.612
V
VFB = 3.3V
--
--
1
A
Active ,VFB = 0.63V, Not Switching
--
22
--
Shutdown
--
--
1
Switching Leakage Current
--
--
1
A
Switching Frequency
--
1
--
MHz
Input Voltage
VIN
Feedback Reference Voltage
VREF
Feedback Leakage Current
IFB
DC Bias Current
Test Conditions
A
Switch On Resistance, Low
RNMOS
ISW = 0.3A
--
70
--
m
Switch On Resistance, High
RPMOS
ISW = 0.3A
--
100
--
m
Valley Current Limit
ILIM
3.03
3.7
4.6
A
Under-Voltage Lockout
Threshold
VUVLO
VDD Rising
--
2.25
2.5
V
VDD Falling
--
2
--
V
--
150
--
°C
Over-Temperature Threshold
Enable Input
Voltage
Logic-High
VIH
1.5
--
--
Logic-Low
VIL
--
--
0.4
Rising
--
90
--
Falling
--
85
--
PG Pin Threshold
(relative to VOUT)
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RT5797A
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
--
--
100

--
1.2
--
ms
Minimum Off Time
--
120
--
ns
Output Discharge Switch On
Resistance
--
1.8
--
k
PG Open-Drain Impedance
(PG = low)
Soft-Start Time
TSS
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. JA is measured at TA = 25C on a high effective thermal conductivity four-layer test board per JEDEC 51-7.
Note 3. Devices are ESD sensitive. Handling precaution recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
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February 2015
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RT5797A
Typical Application Circuit
VIN
3
CIN
22μF
7
2
RT5797A
6
LX
VIN
L
EN
CFF*
R1
VOUT
COUT
22μF x 2
1
FB
PG
SGND PGND
8
4, 9 (Exposed Pad)
R2
*CFF : Optional for performance fine-tune
Table 1. Suggested Component Values
VOUT (V)
R1 (k)
R2 (k)
CIN (F)
L (H)
COUT (F)
3.3
90
20
22
1.5
22 x2
1.8
100
50
22
1.5
22 x2
1.5
100
66.6
22
1.5
22 x2
1.2
100
100
22
1.5
22 x2
1.05
100
133
22
1.5
22 x2
1
100
148
22
1.5
22 x2
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RT5797A
Typical Operating Characteristics
Efficiency vs. Output Current
100
90
90
70
VIN = 5V, VOUT = 3.3V
80
VIN = 3.3V, VOUT = 1.2V
70
Efficiency (%)
80
Efficiency (%)
Efficiency vs. Output Current
100
60
50
40
30
50
40
30
20
10
10
0
0.5
1
1.5
2
2.5
VIN = 3.3V, VOUT = 1.2V
60
20
0
VIN = 5V, VOUT = 3.3V
0
0.001
3
0.01
Output Voltage vs. Output Current
1
3.40
1.26
3.38
Output Current (V)
1.24
1.22
1.20
1.18
1.16
3.36
3.34
3.32
3.30
3.28
1.14
VIN = 3.3V
VIN = 5V
1.12
3.26
0
0.5
1
1.5
2
2.5
3
0
0.5
Output Current (A)
1
1.5
2
2.5
3
Output Voltage (A)
Output Voltage vs. Input Voltage
Output Voltage vs. Input Voltage
1.26
3.40
3.38
Output Voltage (V)
1.24
Output Voltage (V)
10
Output Voltage vs. Output Current
1.28
Output Voltage (V)
0.1
Output Current (A)
Output Current (A)
IOUT = 0A
1.22
1.20
1.18
IOUT = 2A
1.16
IOUT = 0A
3.36
3.34
3.32
IOUT = 2A
3.30
3.28
3.26
1.14
3.24
VIN = 2.5V to 5.5V, V OUT = 1.2V
1.12
2.5
3
3.5
4
4.5
5
Input Voltage (V)
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February 2015
5.5
VIN = 4.5V to 5.5V, V OUT = 3.3V
3.22
4.5
4.6
4.7
4.8
4.9
5
5.1
5.2
5.3
5.4
5.5
Input Voltage (V)
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RT5797A
Switching Frequency vs. Input Voltage
1.5
0.64
1.4
Switcing Frequency (MHz)
Reference Voltage (V)
Reference Voltage vs. Input Voltage
0.65
0.63
0.62
0.61
0.60
0.59
0.58
0.57
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.56
0.6
0.55
0.5
2.5
3
3.5
4
4.5
5
IOUT = 0.6A
2.5
5.5
3
3.5
4.5
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
5
5.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
VEN = 0V
VEN = 0V
0.0
0.0
2.5
3
3.5
4
4.5
5
-50
5.5
-25
0
25
50
75
100
125
Temperature (°C)
Input Voltage (V)
Quiescent Current vs. Input Voltage
Quiescent Current vs. Temperature
30
40
35
25
Quiescent Current (µA)
Quiescent Current (µA)
4.5
Shutdown Currrent vs. Temperature
5.0
Shutdown Current (μA)1
Shutdown Current (µA)1
Shutdown Current vs. Input Voltage
5.0
0.5
4
Input Voltage (V)
Input Voltage (V)
20
15
10
5
30
VIN = 5V
25
20
VIN = 3.3V
15
10
5
VFB = 0.63V, LX no switch
0
0
2.5
3
3.5
4
4.5
5
Input Voltage (V)
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5.5
-50
-25
0
25
50
75
100
125
Temperature (°C)
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RT5797A
Current Limit vs. Temperature
5
4
4
Inductor Current (A)
Inductor Current (A)
Current Limit vs. Input Voltage
5
3
2
1
3
2
1
VOUT = 1.2V
VOUT = 1.2V
0
0
2.5
3
3.5
4
4.5
5
5.5
-50
-25
Input Voltage (V)
25
50
75
100
125
Temperature (°C)
UVLO vs. Temperature
Enable Voltage vs. Temperature
2.5
1.4
2.4
1.2
Enable Voltage (V)
2.3
Input Voltage (V)
0
Turn On
2.2
2.1
2.0
1.9
Turn Off
1.8
1.0
Enable On
0.8
Enable Off
0.6
0.4
1.7
0.2
1.6
VIN = 3.3V
VEN = 3.3V
1.5
0.0
-50
-25
0
25
50
75
100
125
0
25
50
75
100
Load Transient Response
Load Transient Response
125
VOUT
(100mV/Div)
IOUT
(2A/Div)
VIN = 3.3V, VOUT = 1.2V, IOUT = 0A to 3A
Time (100μs/Div)
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-25
Temperature (°C)
VOUT
(100mV/Div)
IOUT
(2A/Div)
-50
Temperature (°C)
February 2015
VIN = 3.3V, VOUT = 1.2V, IOUT = 1A to 3A
Time (100μs/Div)
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RT5797A
Voltage Ripple
Voltage Ripple
VIN = 3.3V, VOUT = 1.2V, IOUT = 1A
VIN = 5V, VOUT = 3.3V, IOUT = 1A
VOUT
VOUT
(10mV/Div)
(10mV/Div)
VLX
(2V/Div)
VLX
(2V/Div)
Time (1μs/Div)
Time (1μs/Div)
Power On from EN
Power Off from EN
VEN
(2V/Div)
VEN
(2V/Div)
VPGOOD
(2V/Div)
VPGOOD
(2V/Div)
VOUT
VOUT
(1V/Div)
(1V/Div)
IOUT
IOUT
(1A/Div)
(1A/Div)
VIN = 3.3V, VOUT = 1.2V, IOUT = 0A
VIN = 3.3V, VOUT = 1.2V, IOUT = 0A
Time (500μs/Div)
Time (10ms/Div)
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RT5797A
Application Information
The RT5797A is a single-phase step-down converter. It
provides single feedback loop constant on-time, current
mode control with fast transient response. An internal
0.6V reference allows the output voltage to be precisely
regulated for low output voltage applications. A fixed
switching frequency (1MHz) oscillator and internal
compensation are integrated to minimize external
component count. Protection features include over
current protection, under voltage protection and over
temperature protection.
Output Voltage Setting
Connect a resistive voltage divider at the FB between
VOUT and GND to adjust the output voltage. The output
voltage is set according to the following equation :

VOUT = VREF  1  R1
R2

where VREF is the feedback reference voltage 0.6V
(typ.).
VOUT
R1
During soft-start, the internal soft-start capacitor
becomes charged and generates a linear ramping up
voltage across the capacitor. This voltage clamps the
voltage at the FB pin, causing PWM pulse width to
increase slowly and in turn reduce the input surge
current. The internal 0.6V reference takes over the loop
control once the internal ramping-up voltage becomes
higher than 0.6V.
Over Voltage Protection (OVP)
The RT5797AL provide Over Voltage Protection
function when output voltage over 120%. The IC will be
into Latch-off mode.
UVLO Protection
The RT5797A has input Under Voltage Lockout
protection (UVLO). If the input voltage exceeds the
UVLO rising threshold voltage (2.25V typ.), the
converter resets and prepares the PWM for operation.
If the input voltage falls below the UVLO falling
threshold voltage during normal operation, the device
will stop switching. The UVLO rising and falling
threshold voltage has a hysteresis to prevent noisecaused reset.
FB
R2
GND
Figure 1. Setting VOUT with a Voltage Divider
Chip Enable and Disable
The EN pin allows for power sequencing between the
controller bias voltage and another voltage rail. The
RT5797A remains in shutdown if the EN pin is lower
than 400mV. When the EN pin rises above the VEN trip
point, the RT5797A begins a new initialization and softstart cycle.
Internal Soft-Start
The RT5797A provides an internal soft-start function to
prevent large inrush current and output voltage
overshoot when the converter starts up. The soft-start
(SS) automatically begins once the chip is enabled.
Input Capacitor Selection
High quality ceramic input decoupling capacitor, such
as X5R or X7R, with values greater than 22F are
recommended for the input capacitor. The X5R and
X7R ceramic capacitors are usually selected for power
regulator capacitors because the dielectric material has
less capacitance variation and more temperature
stability.
Voltage rating and current rating are the key parameters
when selecting an input capacitor. Generally, selecting
an input capacitor with voltage rating 1.5 times greater
than the maximum input voltage is a conservatively safe
design.
The input capacitor is used to supply the input RMS
current, which can be approximately calculated using
the following equation :
IIN_RMS = ILOAD 
VOUT 
V
 1  OUT 
VIN 
VIN 
The next step is selecting a proper capacitor for RMS
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RT5797A
current rating. One good design uses more than one
capacitor with low equivalent series resistance (ESR) in
parallel to form a capacitor bank.
to the average inductor current.
The input capacitance value determines the input ripple
voltage of the regulator. The input voltage ripple can be
approximately calculated using the following equation :
must be large enough not to saturate at the peak
inductor current (IPEAK) :
VIN
IOUT(MAX) VOUT 
V
=

 1  OUT 
CIN  fSW
VIN 
VIN 
Output Capacitor Selection
The output capacitor and the inductor form a low pass
filter in the Buck topology. In steady state condition, the
ripple current flowing into/out of the capacitor results in
ripple voltage. The output voltage ripple (VP-P) can be
calculated by the following equation :
1

VP_P = LIR  ILOAD(MAX)   ESR +
8  COUT  fSW 

When load transient occurs, the output capacitor
supplies the load current before the controller can
respond. Therefore, the ESR will dominate the output
voltage sag during load transient. The output voltage
undershoot (VSAG) can be calculated by the following
equation :
VSAG = ILOAD  ESR
For a given output voltage sag specification, the ESR
value can be determined.
Another parameter that has influence on the output
voltage sag is the equivalent series inductance (ESL).
The rapid change in load current results in di/dt during
transient. Therefore, the ESL contributes to part of the
voltage sag. Using a capacitor with low ESL can obtain
better transient performance. Generally, using several
capacitors connected in parallel can have better
transient performance than using a single capacitor for
the same total ESR.
Find a low loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. The core
IPEAK = ILOAD(MAX) +  LIR  ILOAD(MAX) 
 2

The calculation above serves as a general reference.
To further improve transient response, the output
inductor can be further reduced. This relation should be
considered along with the selection of the output
capacitor.
Inductor saturation current should be chosen over IC’s
current limit.
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) / JA
where TJ(MAX) is the maximum junction temperature, TA
is the ambient temperature, and JA 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, JA, is layout
dependent. For WDFN-8L 2x2 package, the thermal
resistance, JA, is 45.5C/W on a standard 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) / (45.5C/W) = 2.19W for
WDFN-8L 2x2 package
Inductor Selection
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as shown
below :
L=
fSW
VOUT   VIN  VOUT 
 LIR  ILOAD(MAX)  VIN
The maximum power dissipation depends on the
operating ambient temperature for fixed TJ(MAX) and
thermal resistance, JA. The derating curve in Figure 2
allows the designer to see the effect of rising ambient
temperature on the maximum power dissipation.
where LIR is the ratio of the peak-to-peak ripple current
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RT5797A
Maximum Power Dissipation (W)1
2.5
Four-Layer PCB
2.0
1.5
1.0
0.5
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 2. Derating Curve of Maximum Power
Dissipation
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS5797A-00
February 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
13
RT5797A
Outline Dimension
Symbol
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
0.200
0.300
0.008
0.012
D
1.950
2.050
0.077
0.081
D2
1.000
1.250
0.039
0.049
E
1.950
2.050
0.077
0.081
E2
0.400
0.650
0.016
0.026
e
L
0.500
0.300
0.020
0.400
0.012
0.016
W-Type 8L DFN 2x2 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.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
www.richtek.com
14
is a registered trademark of Richtek Technology Corporation.
DS5797A-00
February 2015