RT6214A/B
3A, 18V, 500kHz, ACOTTM Step-Down Converter
General Description
Features
The RT6214A/B is a high-efficiency, monolithic
synchronous step-down DC/DC converter that can
deliver up to 3A output current from a 4.5V to 18V input
supply. The RT6214A/B adopts ACOT architecture to
allow the transient response to be improved and keep
in constant frequency. Cycle-by-cycle current limit
provides protection against shorted outputs and
soft-start eliminates input current surge during start-up.
Fault conditions also include output under voltage
protection and thermal shutdown.
Ordering Information
Applications
RT6214A/B
Package Type
J6F : TSOT-23-6 (FC)
UVP Option
H : Hiccup
0.8V 2% Voltage Reference
Internal Start-Up from Pre-biased Output Voltage
Compact Package: TSOT-23-6 pin
High / Low Side Over-Current Protection and
Hiccup
Output Voltage Range : 0.8V to 6.5V
Set-Top Boxes
Portable TVs
Access Point Routers
Lead Plating System
G : Green (Halogen Free and Pb Free)
Integrated 100m/50m MOSFETs
4.5V to 18V Supply Voltage Range
500kHz Switching Frequency
ACOT Control
DSL Modems
LCD TVs
Marking Information
RT6214AHGJ6F
PSM/PWM
A : PSM Mode
B : PWM Mode
1G= : Product Code
DNN : Date Code
1G=DNN
Note :
Richtek products are :
RT6214BHGJ6F
RoHS compliant and compatible with the current
requirements of IPC/JEDEC J-STD-020.
1F= : Product Code
DNN : Date Code
1F=DNN
Suitable for use in SnPb or Pb-free soldering processes.
Simplified Application Circuit
RT6214A/B
BOOT
VIN
VIN
CIN
CBOOT
L
VOUT
LX
Enable
EN
R1
GND
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November
2015
CFF
COUT
FB
R2
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RT6214A/B
Pin Configurations
(TOP VIEW)
BOOT EN
6
FB
5
4
2
3
GND LX VIN
TSOT-23-6 (FC)
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
GND
System Ground. Provides the ground return path for the control circuitry and
low-side power MOSFET.
2
LX
Switch Node. LX is the switching node that supplies power to the output and
connect the output LC filter from LX to the output load.
3
VIN
Power Input. Supplies the power switches of the device.
4
FB
Feedback Voltage Input. This pin is used to set the desired output voltage via
an external resistive divider. The feedback voltage is 0.8V typically.
5
EN
Enable Control Input. Floating this pin or connecting this pin to logic high can
enable the device and connecting this pin to GND can disable the device.
6
BOOT
Bootstrap Supply for High-Side Gate Driver. Connect a 100nF or greater
capacitor from LX to BOOT to power the high-side switch.
Function Block Diagram
BOOT
VIN
VIN
PVCC
Reg
Minoff
PVCC
VIBIAS
VREF
UGATE
OC
Control
LX
Driver
LGATE
UV
GND
GND LX
PVCC
Ripple
Gen.
EN
+
EN
VIN
+
LX
-
Comparator
On-Time
LX
FB
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DS6214A/B-01
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2015
RT6214A/B
Operation
The RT6214A/B is a synchronous step-down converter
UVLO Protection
with advanced constant on-time control mode. Using
the ACOTTM control mode can reduce the output
capacitance and provide fast transient response. It can
minimize the component size without additional
external compensation network.
To protect the chip from operating at insufficient supply
voltage, the UVLO is needed. When the input voltage
of VIN is lower than the UVLO falling threshold voltage,
the device will be lockout.
Thermal Shutdown
Current Protection
The inductor current is monitored via the internal
switches cycle-by-cycle. Once the output voltage drops
under UV threshold, the RT6214A/B will enter hiccup
mode.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS6214A/B-01
November
2015
When the junction temperature exceeds the OTP
threshold value, the IC will shut down the switching
operation. Once the junction temperature cools down
and is lower than the OTP lower threshold, the
converter will autocratically resume switching.
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RT6214A/B
Absolute Maximum Ratings
(Note 1)
Supply Input Voltage --------------------------------------------------------------------------------- 0.3V to 20V
Switch Node Voltage, LX ---------------------------------------------------------------------------- 0.3V to (VIN + 0.3V)
< 10ns ---------------------------------------------------------------------------------------------------- 5V to 25V
BOOT Pin Voltage ------------------------------------------------------------------------------------ (VLX – 0.3V) to (VIN + 6.3V)
Other Pins ----------------------------------------------------------------------------------------------- 0.3V to 6V
Power Dissipation, PD @ TA = 25C
TSOT-23-6 (FC) --------------------------------------------------------------------------------------- 1.667W
Package Thermal Resistance
(Note 2)
TSOT-23-6 (FC), JA --------------------------------------------------------------------------------- 60C/W
TSOT-23-6 (FC), JC --------------------------------------------------------------------------------- 8C/W
Lead Temperature (Soldering, 10 sec.) ---------------------------------------------------------- 260C
Junction Temperature -------------------------------------------------------------------------------- 150C
Storage Temperature Range ----------------------------------------------------------------------- 65C to 150C
ESD Susceptibility
(Note 3)
HBM (Human Body Model) ------------------------------------------------------------------------- 2kV
Recommended Operating Conditions
(Note 4)
Supply Input Voltage --------------------------------------------------------------------------------- 4.5V to 18V
Ambient Temperature Range----------------------------------------------------------------------- 40C to 85C
Junction Temperature Range ---------------------------------------------------------------------- 40C to 125C
Electrical Characteristics
(VIN = 12V, TA = 25C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
4.5
--
18
Unit
Supply Voltage
VIN Supply Input Operating
Voltage
VIN
Under-Voltage Lockout
Threshold
VUVLO
Under-Voltage Lockout
Threshold Hysteresis
VUVLO
V
3.6
3.9
4.2
--
340
--
mV
Supply Current
Supply Current (Shutdown)
ISHDN
VEN = 0V
--
--
5
µA
Supply Current (Quiescent)
IQ
VEN = 2V, VFB = 0.85V
--
0.5
--
mA
--
1000
--
µS
1.38
1.5
1.62
--
0.18
--
Soft-Start
Soft-Start Time
Enable Voltage
Enable Voltage Threshold
VEN Rising
Enable Voltage Hysteresis
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V
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November
2015
RT6214A/B
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
0.784
0.8
0.816
V
--
100
--
--
50
--
4
4.5
--
A
fOSC
--
500
--
kHz
Maximum Duty Cycle
DMAX
--
90
--
%
Minimum On Time
tON(MIN)
--
60
--
Minimum Off Time
tOFF(MIN)
--
240
--
UVP Detect
45
50
55
Hysteresis
--
10
--
Feedback Threshold Voltage
VFB_TH
4.5V ≤ VIN ≤ 18V
High-Side On-Resistance
RDS(ON)_H
VBOOT − VLX = 4.8V
Low-Side On-Resistance
RDS(ON)_L
Feedback Threshold Voltage
Internal MOSFET
mΩ
Current Limit
Current Limit
ILIM
Valley Current
Switching Frequency
Switching Frequency
On-Time Timer Control
nS
Output Under Voltage Protections
UVP Trip Threshold
%
Thermal Shutdown
Thermal Shutdown Threshold
TSD
--
150
--
Thermal Shutdown Hysteresis
TSD
--
20
--
°C
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 = 25C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. The first
layer of copper area is filled. JC is measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
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November
2015
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RT6214A/B
Typical Application Circuit
RT6214A/B
3
VIN
CIN
22μF
BOOT
LX
5
Enable
VIN
6
2
L
2.2μH
EN
GND
1
FB
CBOOT
0.1μF
VOUT
R1
12k
CFF
Open
COUT
44μF
4
Rt*
10k
R2
24k
* Note : When CFF is added, it is necessary to add Rt = 10k between feedback network and chip FB pin.
Table 1. Suggested Component Values (VIN = 12V)
VOUT (V)
R1 (k)
R2 (k)
L (H)
COUT (F)
CFF (pF)
1.05
10
32.4
2.2
44
--
1.2
20.5
41.2
2.2
44
--
1.8
40.2
32.4
3.3
44
--
2.5
40.2
19.1
3.3
44
22 to 68
3.3
40.2
13
4.7
44
22 to 68
5
40.2
7.68
4.7
44
22 to 68
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RT6214A/B
Typical Operating Characteristics
Efficiency vs. Output Current
Output Voltage vs. Output Current
100
1.40
90
1.35
Output Voltage (V)
Efficiency (%)
80
70
VIN = 4.5V
60
VIN = 12V
50
VIN = 18V
40
30
1.30
VIN = 4.5V
VIN = 12V
1.25
VIN = 18V
1.20
20
1.15
10
VOUT = 1.2V
0
0.001
VOUT = 1.2V
1.10
0.01
0.1
1
10
0
0.5
1
Output Current (A)
1.5
2
2.5
3
Output Current (A)
Reference Voltage vs. Temperature
EN Threshold vs. Temperature
0.820
1.60
Rising
1.50
0.810
EN Threshold (V)
Reference Voltage (V)
0.815
0.805
0.800
0.795
0.790
1.40
1.30
Falling
1.20
0.785
VIN = 12V, IOUT = 1A
0.780
VOUT = 1.2V, IOUT = 0A
1.10
-50
-25
0
25
50
75
100
125
-50
Output Voltage vs. Temperature
0
25
50
75
100
125
Load Transient
1.220
1.210
Output Voltage (V)
-25
Temperature (°C)
Temperature (°C)
VOUT
(50mV/Div)
1.200
VIN = 12V, VOUT = 1.2V,
IOUT = 0A to 3A , L = 2.2H
VIN = 4.5V
1.190
VIN = 12V
1.180
VIN = 18V
1.170
VOUT = 1.2V, IOUT = 1A
1.160
-50
-25
0
25
50
75
100
Temperature (°C)
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November
2015
125
IOUT
(1A/Div)
Time (100s/Div)
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RT6214A/B
Load Transient
VOUT
(50mV/Div)
Output Ripple Voltage
VSW
(6V/Div)
VIN = 12V, VOUT = 1.2V,
IOUT = 3A , L = 2.2H
IOUT
(1A/Div)
VOUT
(20mV/Div)
VIN = 12V, VOUT = 1.2V,
IOUT = 1.5A to 3A , L = 2.2H
Time (100s/Div)
Time (1s/Div)
Power On then Short
Power On from EN
VIN
(5V/Div)
VOUT
(2V/Div)
VIN = 12V, VOUT = 5V, IOUT = 3A
VIN = 12V, VOUT = 5V
VEN
(2V/Div)
VLX
(10V/Div)
VOUT
(5V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
Time (4ms/Div)
Time (1ms/Div)
Power Off from EN
Power On from VIN
VIN
(10V/Div)
VOUT
(2V/Div)
VEN
(5V/Div)
VIN
(10V/Div)
VLX
(2V/Div)
VIN = 12V, VOUT = 5V, IOUT = 3A
VEN
(5V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
Time (20s/Div)
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VIN = 12V, VOUT = 5V,
IOUT = 3A
Time (1ms/Div)
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RT6214A/B
Power Off from VIN
VIN
(10V/Div)
VOUT
(2V/Div)
VIN = 12V, VOUT = 5V,
IOUT = 3A
VEN
(5V/Div)
IOUT
(2A/Div)
Time (20s/Div)
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2015
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RT6214A/B
Application Information
Inductor Selection
Selecting an inductor involves specifying its inductance
and also its required peak current. The exact inductor
value is generally flexible and is ultimately chosen to
obtain the best mix of cost, physical size, and circuit
efficiency. Lower inductor values benefit from reduced
size and cost and they can improve the circuit's
transient response, but they increase the inductor
ripple current and output voltage ripple and reduce the
efficiency due to the resulting higher peak currents.
Conversely, higher inductor values increase efficiency,
but the inductor will either be physically larger or have
higher resistance since more turns of wire are required
and transient response will be slower since more time
is required to change current (up or down) in the
inductor. A good compromise between size, efficiency,
and transient response is to use a ripple current (IL)
about 20% to 50% of the desired full output load
current. Calculate the approximate inductor value by
selecting the input and output voltages, the switching
frequency (f SW ), the maximum output current
(IOUT(MAX)) and estimating a IL as some percentage of
that current.
L=
VOUT VIN VOUT
VIN fSW IL
Once an inductor value is chosen, the ripple current
(IL) is calculated to determine the required peak
inductor current.
IL =
VOUT VIN VOUT
I
and IL(PEAK) = IOUT(MAX) L
VIN fSW L
2
To guarantee the required output current, the inductor
needs a saturation current rating and a thermal rating
that exceeds IL(PEAK). These are minimum requirements.
To maintain control of inductor current in overload and
short circuit conditions, some applications may desire
current ratings up to the current limit value. However,
the IC's output under-voltage shutdown feature make
this unnecessary for most applications.
IL(PEAK) should not exceed the minimum value of IC's
upper current limit level or the IC may not be able to
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meet the desired output current. If needed, reduce the
inductor ripple current (IL) to increase the average
inductor current (and the output current) while ensuring
that IL(PEAK) does not exceed the upper current limit
level.
For best efficiency, choose an inductor with a low DC
resistance that meets the cost and size requirements.
For low inductor core losses some type of ferrite core is
usually best and a shielded core type, although
possibly larger or more expensive, will probably give
fewer EMI and other noise problems.
Considering the Typical Operating Circuit for 1.2V
output at 3A and an input voltage of 12V, using an
inductor ripple of 0.9A (30%), the calculated inductance
value is :
L
1.2 12 1.2
2.4μH
12 500kHz 0.9A
The ripple current was selected at 0.9A and, as long as
we use the calculated 2.4H inductance, that should be
the actual ripple current amount. The ripple current and
required peak current as below :
IL =
1.2 12 1.2
= 0.9A
12 500kHz 2.4μH
and IL(PEAK) = 3A 0.9A = 3.45A
2
For the 2.4H value, the inductor's saturation and
thermal rating should exceed 3.45A. Since the actual
value used was 2.4H and the ripple current exactly
0.9A, the required peak current is 3.45A.
Input Capacitor Selection
The input filter capacitors are needed to smooth out the
switched current drawn from the input power source
and to reduce voltage ripple on the input. The actual
capacitance value is less important than the RMS
current rating (and voltage rating, of course). The RMS
input ripple current (IRMS) is a function of the input
voltage, output voltage, and load current :
IRMS = IOUT(MAX)
VOUT
VIN
VIN
1
VOUT
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2015
RT6214A/B
Ceramic capacitors are most often used because of
their low cost, small size, high RMS current ratings, and
robust surge current capabilities. However, take care
For the Typical Operating Circuit for 1.2V output and an
inductor ripple of 0.4A, with 2 x 22F output
capacitance each with about 5m ESR including PCB
when these capacitors are used at the input of circuits
supplied by a wall adapter or other supply connected
through long, thin wires. Current surges through the
inductive wires can induce ringing at the RT6214A/B
input which could potentially cause large, damaging
voltage spikes at VIN. If this phenomenon is observed,
some bulk input capacitance may be required. Ceramic
capacitors (to meet the RMS current requirement) can
be placed in parallel with other types such as tantalum,
electrolytic, or polymer (to reduce ringing and
overshoot).
trace resistance, the output voltage ripple components
are :
Choose capacitors rated at higher temperatures than
required. Several ceramic capacitors may be paralleled
to meet the RMS current, size, and height requirements
of the application. The typical operating circuit uses two
10F and one 0.1F low ESR ceramic capacitors on
VRIPPLE(ESR) = 0.9A 5m = 4.5mV
0.9A
= 5.11mV
8 44μF 500kHz
VRIPPLE = 4.5mV 5.11mV = 9.61mV
VRIPPLE(C) =
Feed-forward Capacitor (Cff)
The RT6214A/B are optimized for ceramic output
capacitors and for low duty cycle applications. However
for high-output voltages, with high feedback attenuation,
the circuit's response becomes over-damped and
transient response can be slowed. In high-output
voltage circuits (VOUT > 3.3V) transient response is
improved by adding a small “feed-forward” capacitor
the input.
(Cff) across the upper FB divider resistor (Figure 1), to
increase the circuit's Q and reduce damping to speed
Output Capacitor Selection
up the transient response without affecting the
steady-state stability of the circuit. Choose a suitable
The RT6214A/B are optimized for ceramic output
capacitors and best performance will be obtained using
them. The total output capacitance value is usually
determined by the desired output voltage ripple level
and transient response requirements for sag
(undershoot on positive load steps) and soar
(overshoot on negative load steps).
capacitor value that following below step.
Get the BW the quickest method to do transient
response form no load to full load. Confirm the
damping frequency. The damping frequency is BW.
Output Ripple
Output ripple at the switching frequency is caused by
the inductor current ripple and its effect on the output
BW
capacitor's ESR and stored charge. These two ripple
components are called ESR ripple and capacitive ripple.
Since ceramic capacitors have extremely low ESR and
relatively little capacitance, both components are
VOUT
similar in amplitude and both should be considered if
ripple is critical.
VRIPPLE = VRIPPLE(ESR) VRIPPLE(C)
VRIPPLE(ESR) = IL RESR
VRIPPLE(C) =
R1
FB
RT6214A/B
IL
Cff
R2
GND
8 COUT fSW
Figure 1. Cff Capacitor Setting
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2015
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RT6214A/B
Cff can be calculated base on below equation :
Cff
Output Voltage Setting
Set the desired output voltage using a resistive divider
1
2 3.1412 R1 BW 0.8
from the output to ground with the midpoint connected
to FB. The output voltage is set according to the
following equation :
Enable Operation (EN)
For automatic start-up the high-voltage EN pin can be
connected to VIN, through a 100k resistor. Its large
hysteresis band makes EN useful for simple delay and
VOUT = 0.8V x (1 + R1 / R2)
VOUT
timing circuits. EN can be externally pulled to VIN by
adding a resistor-capacitor delay (REN and CEN in
R1
FB
Figure 2). Calculate the delay time using EN's internal
threshold where switching operation begins.
An external MOSFET can be added to implement
digital control of EN when no system voltage above 2V
is available (Figure 3). In this case, a 100k 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. To prevent enabling circuit when VIN
is smaller than the VOUT target value or some other
desired voltage level, a resistive voltage divider can be
placed between the input voltage and ground and
connected to EN to create an additional input under
voltage lockout threshold (Figure 4).
EN
VIN
RT6214A/B
R2
GND
Figure 5. Output Voltage Setting
Place the FB resistors within 5mm of the FB pin.
Choose R2 between 10k and 100k to minimize
power consumption without excessive noise pick-up
and calculate R1 as follows :
R1
R2 (VOUT VREF )
VREF
For output voltage accuracy, use divider resistors with
1% or better tolerance.
External BOOT Bootstrap Diode
REN
EN
RT6214A/B
CEN
When the input voltage is lower than 5.5V it is
recommended to add an external bootstrap diode
GND
between VIN (or VINR) and the BOOT pin to improve
Figure 2. External Timing Control
VIN
improve efficiency. The bootstrap diode can be a low
REN
100k
EN
Q1
Enable
RT6214A/B
GND
Figure 3. Digital Enable Control Circuit
VIN
REN1
EN
REN2
RT6214A/B
GND
Figure 4. Resistor Divider for Lockout Threshold
Setting
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enhancement of the internal MOSFET switch and
cost one such as 1N4148 or BAT54.
External BOOT Capacitor Series Resistance
The internal power MOSFET switch gate driver is
optimized to turn the switch on fast enough for low
power loss and good efficiency, but also slow enough
to reduce EMI. Switch turn-on is when most EMI occurs
since VLX rises rapidly. During switch turn-off, LX is
discharged relatively slowly by the inductor current
during the dead time between high-side and low-side
switch on-times. In some cases it is desirable to reduce
EMI further, at the expense of some additional power
dissipation. The switch turn-on can be slowed by
placing a small (
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