®
RT6242A/B
12A, 18V, 500kHz, ACOTTM Synchronous Step-Down Converter
General Description
Features
The RT6242A/B is a synchronous step-down converter
with Advanced Constant On-Time (ACOTTM) mode control.
4.5V to 18V Input Voltage Range
12A Output Current
Ω
12mΩ
Ω Internal High-Side N-MOSFET and 5.4mΩ
Internal Low-Side N-MOSFET
Advanced Constant On-Time Control
Fast Transient Response
Support All Ceramic Capacitors
Up to 95% Efficiency
Adjustable Switching Frequency from 300kHz to
700kHz
Adjustable Output Voltage from 0.7V to 8V
Adjustable Soft-Start
Pre-bias Start-Up
Adjustable Current Limit from 6A to 16A
Cycle-by-Cycle Over Current Protection
Power Good Output
Input Under-Voltage Lockout
Hiccup Mode Under-Voltage Protection
Thermal Shutdown Protection
TM
The ACOT provides a very fast transient response with
few external components. The low impedance internal
MOSFET supports high efficiency operation with wide
input voltage range from 4.5V to 18V. The proprietary circuit
of the RT6242A/B enables to support all ceramic
capacitors. The output voltage can be adjustable between
0.7V and 8V. The soft-start is adjustable by an external
capacitor.
Ordering Information
RT6242A/B
Package Type
QUF : UQFN-16JL 3x3 (U-Type) (FC)
Lead Plating System
G : Green (Halogen Free and Pb Free)
UVP Option
H : Hiccup Mode UVP
L : Latched OVP & UVP
A : PSM
B : PWM
Applications
Richtek products are :
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
Note :
Suitable for use in SnPb or Pb-free soldering processes.
Industrial and Commercial Low Power Systems
Computer Peripherals
LCD Monitors and TVs
Green Electronics/Appliances
Point of Load Regulation for High-Performance DSPs,
FPGAs, and ASICs
Simplified Application Circuit
VIN
RT6242A/B
SW
VIN
VOUT
BOOT
EN Signal
Power Good
EN
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DS6242A/B-03 January 2016
FB
RT
PGOOD
RLIM
PVCC
SS GND
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1
RT6242A/B
Pin Configurations
Marking Information
RT6242AHGQUF
RLIM
EN
SW
SW
(TOP VIEW)
16
15
14
13
7D= : Product Code
SW
PVCC
3
10
SW
RT
4
9
BOOT
5
6
7
8
PGOOD
SW
11
GND
12
2
SS
1
FB
VIN
AGND
UQFN-16JL 3x3 (FC)
7D=YM
DNN
YMDNN : Date Code
RT6242ALGQUF
7C= : Product Code
7C=YM
DNN
YMDNN : Date Code
RT6242BHGQUF
78= : Product Code
78=YM
DNN
YMDNN : Date Code
RT6242BLGQUF
75= : Product Code
75=YM
DNN
YMDNN : Date Code
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
AGND
Analog Ground.
2
FB
Feedback Voltage Input. It is used to regulate the output of the converter to a set
value via an external resistive voltage divider. The feedback reference voltage is
0.7V typically.
3
PVCC
Internal Regulator Output. Connect a 1F capacitor to GND to stabilize output
voltage.
4
RT
An External Timing Resistor Adjusts the Switching Frequency of the Device.
5
SS
Soft-Start Time Setting. An external capacitor should be connected between this
pin and GND.
6
VIN
Power Input. The input voltage range is from 4.5V to 18V. Must bypass with a
suitably large (10F x 2) ceramic capacitor.
7
GND
Ground.
8
PGOOD
Power Good Indicator Open-Drain Output.
9
BOOT
Bootstrap. This capacitor is needed to drive the power switch's gate above the
supply voltage. It is connected between SW and BOOT pins to form a floating
supply across the power switch driver. A 0.1F capacitor is recommended for use.
10 to 14
SW
Switch Node. Connect this pin to an external L-C filter.
15
EN
16
RLIM
Enable Control Input. A logic-high enables the converter; a logic-low forces the IC
into shutdown mode reducing the supply current to less than 10A.
An External Resistor Adjusts the Current Limit of the Device.
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RT6242A/B
Function Block Diagram
BOOT
PVCC
EN
POR &
Reg
VBIAS
Min.
Off-Time
PVCC
VIN
VREF
OC
UGATE
Control
Driver
SW
LGATE
UV & OV
PVCC
6µA
SS
GND
Ripple
Gen.
FB
VIN
+
+
Comparator
ZC
0.9 VREF
FB
Comparator
+
PGOOD
-
On-Time
Operation
The RT6242A/B is a synchronous step-down converter
with advanced Constant On-Time control mode. Using the
ACOTTM control mode can reduce the output capacitance
and fast transient response. It can minimize the component
size without additional external compensation network.
Current Protection
The inductor current is monitored via the internal switches
cycle-by-cycle. Once the output voltage drops under UV
threshold, the RT6242A/B will enter hiccup mode.
UVLO Protection
Power Good
After soft-start has finished, the power good function will
be activated. The PGOOD pin is an open-drain output. If
the FB voltage is lower than 85% VREF, the PGOOD pin
will be pulled low.
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
PVCC
The regulator provides 5V power to supply the internal
control circuit. 1μF ceramic capacitor for decoupling and
stability is required.
Soft-Start
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.
In order to prevent the converter output voltage from
overshooting during the startup period, the soft-start
function is necessary. The soft-start time is adjustable
by an external capacitor.
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RT6242A/B
Absolute Maximum Ratings
(Note 1)
Supply Voltage, VIN -----------------------------------------------------------------------------------------------Switch Voltage, SW -----------------------------------------------------------------------------------------------BOOT to SW --------------------------------------------------------------------------------------------------------EN to GND ------------------------------------------------------------------------------------------------------------Other Pins Voltage -------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
UQFN-16JL 3x3 (FC) ----------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
UQFN-16JL 3x3 (FC), θJA -----------------------------------------------------------------------------------------UQFN-16JL 3x3 (FC), θJC ----------------------------------------------------------------------------------------Junction Temperature Range -------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Mode) ----------------------------------------------------------------------------------------
Recommended Operating Conditions
−0.3V to 20V
−0.3V to (VIN + 0.3V)
−0.3V to 6V
−0.3V to 6V
−0.3V to 6V
3.623W
27.6°C/W
5.6°C/W
150°C
260°C
−65°C to 150°C
2kV
(Note 4)
Supply Voltage, VIN ------------------------------------------------------------------------------------------------ 4.5V to 18V
Junction Temperature Range -------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range -------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VIN = 12V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Supply Current
Shutdown Current
ISHDN
VEN = 0V
--
1.5
10
A
Quiescent Current
IQ
VEN = 2V, VFB = 1V
--
0.8
1.2
mA
Logic-High
1.1
1.2
1.3
Hysteresis
--
0.2
--
Logic Threshold
EN Voltage
V
VREF Voltage
Feedback Threshold Voltage
VREF
4.5V VIN 18V
0.693
0.7
0.707
V
Feedback Input Current
IFB
VFB = 0.71V
0.1
--
0.1
A
VPVCC
6V VIN 18V, 0 < IPVCC 5mA
--
5
--
V
Line Regulation
6V VIN 18V, IPVCC = 5mA
--
--
20
mV
Load Regulation
0 IPVCC 5mA
--
--
100
mV
VIN = 6V, VPVCC = 4V
--
150
--
mA
PVCC Output
PVCC Output Voltage
Output Current
IPVCC
RDS(ON)
Switch
On-Resistance
High-Side
RDS(ON)_H
--
12
--
Low-Side
RDS(ON)_L
--
5.4
--
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RT6242A/B
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
13
16
--
A
--
150
--
C
--
200
--
ns
Current Limit
Current Limit
ILIM
RLIM = 66k
Thermal Shutdown
Thermal Shutdown Threshold
TSD
On-Time Timer Control
VIN = 12V, VOUT = 1.05V,
RRT = 150k
On-Time
tON
Minimum On-Time
tON(MIN)
--
60
--
ns
Minimum Off-Time
tOFF(MIN)
--
230
--
ns
VSS = 0V
5
6
7
A
Wake Up VIN
4
4.2
4.4
--
0.5
--
FB Rising
85
90
95
FB Falling
--
80
--
PGOOD = 0.1V
10
20
--
mA
115
120
125
%
OVP Propogation Delay
--
10
--
s
UVP Threshold
55
60
65
%
UVP Hysteresis
--
17
--
%
UVP Propogation Delay
--
250
--
s
RRT = 106k
600
700
800
RRT = 150k
430
500
570
RRT = 250k
250
300
350
Soft-Start
SS Charge Current
UVLO
UVLO Threshold
Hysteresis
V
Power Good
PGOOD Threshold
PGOOD Sink Current
%
OVP and UVP Protection
OVP Threshold
Switching Frequency
FS
kHz
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.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
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RT6242A/B
Typical Application Circuit
VIN
C1
10µF x 2
C2
0.1µF
BOOT
8 PGOOD
15
Enable
5
C5
10nF
C6
0.1µF
9
EN
FB 2
SS
PVCC 3
16 RLIM
RLIM
L1
1µH
RT6242A/B
10 to 14
6 VIN
SW
C4
1µF
VOUT
1.4V/12A
R1
20k
C3
C7
22µF x 3
R2
20k
VPVCC
RT 4
AGND
1
GND
7
RT
RLIM = 172k, OCP typical 6A
RLIM = 94k, OCP typical 11.4A
RLIM = 80k, OCP typical 13.3A
RLIM = 66k, OCP typical 16A
Table 1. Suggested Component Values
VOUT (V)
R1 (k)
R2 (k)
C3 (pF)
L1 (H)
C7 (F)
1
8.66
20
--
1
66
1.4
20
20
--
1
66
1.8
31.6
20
10
1
66
2.5
51.1
20
10
1.2
66
5
124
20
22
1.5
66
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RT6242A/B
Typical Operating Characteristics
Efficiency vs. Output Current
100
Efficiency vs. Output Current
100
RT6242A
90
80
80
70
VIN = 6V
VIN = 12V
VIN = 17V
60
50
Efficiency (%)
Efficiency (%)
RT6242B
90
40
30
20
VIN = 6V
VIN = 12V
VIN = 17V
70
60
50
40
30
20
10
10
VOUT = 1.2V, FS = 500kHz, L = 1μH
0
0.01
0.1
1
10
VOUT = 1.2V, FS = 500kHz, L = 1μH
0
100
0
1
2
3
Output Current (A)
8
9
10
RT6242B
1.28
1.26
Output Voltage (V)
Output Voltage (V)
7
Output Voltage vs. Input Voltage
1.26
1.24
1.22
IOUT = 0A
IOUT = 6A
IOUT = 9A
1.20
1.18
1.16
1.14
1.24
1.22
1.20
IOUT = 0A
IOUT = 6A
IOUT = 9A
1.18
1.16
1.14
1.12
1.12
VOUT = 1.2V
1.10
VOUT = 1.2V
1.10
4
6
8
10
12
14
16
18
4
6
8
10
Input Voltage (V)
12
14
16
18
Input Voltage (V)
Output Voltage vs. Output Current
1.30
Output Voltage vs. Output Current
1.30
RT6242A
1.28
RT6242B
1.28
1.26
1.26
Output Voltage (V)
Output Voltage (V)
6
1.30
RT6242A
1.28
5
Output Current (A)
Output Voltage vs. Input Voltage
1.30
4
1.24
1.22
VIN = 17V
VIN = 12V
VIN = 6V
1.20
1.18
1.16
1.14
1.24
1.22
1.20
VIN = 17V
VIN = 12V
VIN = 6V
1.18
1.16
1.14
1.12
VOUT = 1.2V
1.10
0
1
2
3
4
5
6
7
8
Output Current (A)
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10
1.12
VOUT = 1.2V
1.10
0
1
2
3
4
5
6
7
8
9
10
Output Current (A)
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RT6242A/B
Frequency vs. Temperature
700
650
650
600
600
Frequency (kHz)1
Frequency (kHz)1
Frequency vs. Input Voltage
700
550
500
450
400
350
550
500
450
400
350
VIN = 12V, VOUT = 1.2V
VOUT = 1.2V
300
300
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18
-50
-25
0
Input Voltage (V)
50
75
100
125
Temperature (°C)
Feedback Threshold vs. Temperature
Frequency vs. RRT Resistor
0.707
0.706
0.705
700
0.704
0.703
0.702
0.701
0.700
0.699
0.698
0.697
0.696
0.695
0.694
600
650
Frequency (kHz)1
Feedback Threshold (V)
25
VIN = 17V
VIN = 12V
VIN = 4.5V
550
500
450
400
350
VIN = 12V
300
-50
-25
0
25
50
75
100
125
100 115 130 145 160 175 190 205 220 235 250
Temperature (°C)
RRT (kΩ)
Load Transient Response
Load Transient Response
RT6242A
RT6242B
VOUT
(50mV/Div)
VOUT
(50mV/Div)
IOUT
(5A/Div)
IOUT
(5A/Div)
VIN = 12V, VOUT = 1.2V, IOUT = 0.1A to 12A
Time (100μs/Div)
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VIN = 12V, VOUT = 1.2V, IOUT = 0.1A to 12A
Time (100μs/Div)
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RT6242A/B
Load Transient Response
Load Transient Response
RT6242B
RT6242A
VOUT
(50mV/Div)
VOUT
(50mV/Div)
IOUT
(5A/Div)
IOUT
(5A/Div)
VIN = 12V, VOUT = 1.2V, IOUT = 6A to 12A
RT6242A
VIN = 12V, VOUT = 1.2V, IOUT = 6A to 12A
Time (100μs/Div)
Time (100μs/Div)
Output Ripple Voltage
Output Ripple Voltage
VIN = 12V, VOUT = 1.2V, IOUT = 50mA
VOUT
(50mV/Div)
RT6242B
VOUT
(10mV/Div)
VLX
(10V/Div)
VLX
(10V/Div)
ILX
(2A/Div)
ILX
(5A/Div)
Time (50μs/Div)
Time (1μs/Div)
Output Ripple Voltage
Output Ripple Voltage
RT6242A
RT6242B
VOUT
(10mV/Div)
VOUT
(10mV/Div)
VLX
(10V/Div)
VLX
(10V/Div)
ILX
(5A/Div)
VIN = 12V, VOUT = 1.2V, IOUT = 50mA
VIN = 12V, VOUT = 1.2V, IOUT = 12A
Time (1μs/Div)
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ILX
(5A/Div)
VIN = 12V, VOUT = 1.2V, IOUT = 12A
Time (1μs/Div)
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RT6242A/B
Power On from EN
RT6242A
VOUT
(1V/Div)
VOUT
(1V/Div)
VLX
(10V/Div)
VIN = 12V, VOUT = 1.2V, IOUT = 0.1A
VLX
(10V/Div)
ILX
(2A/Div)
ILX
(2A/Div)
VIN = 12V, VOUT = 1.2V,
IOUT = 0.1A
Time (2ms/Div)
Time (2ms/Div)
Power On from EN
Power Off from EN
VEN
(5V/Div)
RT6242B
VEN
(5V/Div)
RT6242B
VOUT
(1V/Div)
VOUT
(1V/Div)
ILX
(10A/Div)
RT6242A
VEN
(5V/Div)
VEN
(5V/Div)
VLX
(10V/Div)
Power Off from EN
VLX
(10V/Div)
VIN = 12V, VOUT = 1.2V,
IOUT = 10A
VIN = 12V, VOUT = 1.2V,
IOUT = 10A
Time (4ms/Div)
Time (4ms/Div)
UVP Short (Latch Mode)
UVP Short (Hiccup Mode)
VIN
(5V/Div)
VOUT
(1V/Div)
ILX
(10A/Div)
VIN
(5V/Div)
VIN = 12V, VOUT = 1.2V, IOUT = Short
VIN = 12V, VOUT = 1.2V, IOUT = Short
VOUT
(500mV/Div)
VLX
(10V/Div)
VLX
(10V/Div)
ILX
(10A/Div)
ILX
(10A/Div)
Time (2ms/Div)
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Time (10ms/Div)
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RT6242A/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 (ΔIL) about
15% to 40% of the desired full output load current.
Calculate the approximate inductor value by selecting the
input and output voltages, the switching frequency (fSW),
the maximum output current (IOUT(MAX)) and estimating a
ΔIL as some percentage of that current.
L=
VOUT VIN VOUT
VIN fSW IL
Once an inductor value is chosen, the ripple current (ΔIL)
is calculated to determine the required peak inductor
current.
VOUT VIN VOUT
IL =
VIN fSW L
I
IL(PEAK) = IOUT(MAX) L
2
I
IL(VALLY) = IOUT(MAX) L
2
Inductor saturation current should be chosen over IC's
current limit.
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 :
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DS6242A/B-03 January 2016
V
IRMS = IOUT(MAX) OUT
VIN
VIN
1
VOUT
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 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 RT6242A/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).
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 10μF
and one 0.1μF low ESR ceramic capacitors on the input.
Output Capacitor Selection
The RT6242A/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).
Output Ripple
Output ripple at the switching frequency is caused by the
inductor current ripple and its effect on the output
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 similar
in amplitude and both should be considered if ripple is
critical.
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RT6242A/B
VRIPPLE = VRIPPLE(ESR) VRIPPLE(C)
Soft-Start (SS)
VRIPPLE(ESR) = IL RESR
IL
VRIPPLE(C) =
8 COUT fSW
The RT6242A/B soft-start uses an external capacitor at
SS to adjust the soft-start timing according to the following
equation :
Feed-forward Capacitor (Cff)
t ms
The RT6242A/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 (Cff) across the upper FB
divider resistor (Figure 1), to increase the circuit's Q and
reduce damping to speed up the transient response without
affecting the steady-state stability of the circuit. Choose
a suitable 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.
VOUT
Cff
FB
RT6242A/B
Following below equation to get the minimum capacitance
range in order to avoid UV occur.
COUT VOUT 0.6 1.2
(ILIM Load Current) 0.8
T 6μA
CSS
VREF
T
Do not leave SS unconnected.
Enable Operation (EN)
For automatic start-up, the low-voltage EN pin must be
connected to VIN with a 100kΩ resistor. EN can be
externally pulled to VIN by adding a resistor-capacitor
delay (REN and CEN in Figure 2). Calculate the delay time
using EN's internal threshold where switching operation
begins (1.2V, typical).
EN
VIN
R2
GND
Figure 1. Cff Capacitor Setting
ISS μA
An external MOSFET can be added to implement digital
control of EN (Figure 3). 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. 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).
BW
R1
CSS nF 0.7
REN
CEN
EN
RT6242A/B
GND
Figure 2. External Timing Control
Cff can be calculated base on below equation :
Cff
1
2 3.1412 R1 BW 0.8
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DS6242A/B-03 January 2016
RT6242A/B
VIN
REN
100k
External BOOT Bootstrap Diode
EN
Q1
Enable
RT6242A/B
GND
Figure 3. Digital Enable Control Circuit
VIN
REN1
External BOOT Capacitor Series Resistance
EN
REN2
RT6242A/B
GND
Figure 4. Resistor Divider for Lockout Threshold Setting
Output Voltage Setting
Set the desired output voltage using a resistive divider
from the output to ground with the midpoint connected to
FB. The output voltage is set according to the following
equation :
VOUT = 0.7 x (1 + R1 / R2)
VOUT
R1
FB
RT6242A/B
When the input voltage is lower than 5.5V it is
recommended to add an external bootstrap diode between
VIN (or VINR) and the BOOT pin to improve enhancement
of the internal MOSFET switch and improve efficiency.
The bootstrap diode can be a low cost one such as 1N4148
or BAT54.
R2
GND
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 VSW
rises rapidly. During switch turn-off, SW 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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