®
RT8228A
Single Synchronous Buck PWM Controller
General Description
Features
The RT8228A PWM controller provides high efficiency,
excellent transient response, and high DC output accuracy
needed for stepping down high voltage batteries to
generate low voltage CPU core, I/O, and chipset RAM
supplies in notebook computers.
The constant on-time PWM control scheme handles wide
input/output voltage ratios with ease and provides 100ns
“instant-on” response to load transients while maintaining
a relatively constant switching frequency.
The RT8228A achieves high efficiency at a reduced cost
by eliminating the current sense resistor found in
traditional current mode PWMs. Efficiency is further
enhanced by its ability to drive very large synchronous
rectifier MOSFETs and enter diode emulation mode at
light load condition. The buck conversion allows this device
to directly step down high voltage batteries at the highest
possible efficiency. The Audio Skipping Mode (ASM)
setting maintains the switching frequency above 25kHz,
which eliminates noise in audio applications. The RT8228A
is intended for CPU core, chipset, DRAM, or other low
voltage supplies as low as 0.5V. The RT8228A is available
in a WQFN-12L 2x2 package.
Built in 1% 0.5V Reference Voltage
Adjustable 0.5V to 3.3V Output Range
Quick Load Step Response within 100ns
4700ppm/°
°C Programmable Current Limit by Low
Side RDS(ON) Sensing
4.5V to 26V Battery Input Range
Resistor Programmable Frequency
Internal Ramp Current Limit Soft-Start Control
Drives Large Synchronous Rectifier FETs
Integrated Boost Switch
Over/Under Voltage Protection
Thermal Shutdown
Power Good Indicator
RoHS Compliant and Halogen Free
Applications
Notebook Computers
CPU Core Supply
Chipset/RAM Supply as Low as 0.5V
Generic DC/DC Power Regulator
Pin Configurations
Richtek products are :
PHASE
UGATE
2
10
GND
13
3
4
5
6
FB
Note :
1
11
VCC
Lead Plating System
G : Green (Halogen Free and Pb Free)
Z : ECO (Ecological Element with
Halogen Free and Pb free)
LGATE
12
BOOT
Package Type
QW : WQFN-12L 2x2 (W-Type)
CS
RT8228A
TON
Ordering Information
GND
(TOP VIEW)
9
PGOOD
8
EN
MODE
7
WQFN-12L 2x2
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Copyright © 2021 Richtek Technology Corporation. All rights reserved.
DS8228A-08T00 October 2021
is a registered trademark of Richtek Technology Corporation.
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1
RT8228A
Marking Information
RT8228AGQW
RT8228AZQW
CQ : Product Code
CQ : Product Code
W : Date Code
CQW
W : Date Code
CQW
Part Status
Part No
Status
Package
Lead Plating System
RT8228AGQW Obsolete WQFN-12L 2x2 (W-Type) G : Green (Halogen Free and Pb Free)
RT8228AZQW Obsolete WQFN-12L 2x2 (W-Type)
Z : ECO (Ecological Element with Halogen Free and Pb
free)
The part status values are defined as below :
Active : Device is in production and is recommended for new designs.
Lifebuy : The device will be discontinued, and a lifetime-buy period is in effect.
NRND : Not recommended for new designs.
Preview : Device has been announced but is not in production.
Obsolete : Richtek has discontinued the production of the device.
Typical Application Circuit
VIN
4.5V to 26V
RTON
11
5
VDDP
R2
100k
TON
RT8228A
BOOT 4
VCC
C2
4.7µF
9 PGOOD
10 CS
PGOOD
UGATE 3
R5
0
C3
0.1µF
VOUT
1V
Q1
L1
Q2
* : Optional
R7*
C7*
R8
10k
C5* C6*
C1
220µF
FB 6
MODE
8 EN
GND
7
To 5V : DEM
To 2.5V : ASM
To GND : FCCM
R9
10k
12, 13 (Exposed Pad)
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2
R4
0
PHASE 2
1
LGATE
R6
Chip Enable
C4
10µF
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RT8228A-08 October 2021
RT8228A
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
LGATE
Gate Drive Output for Low Side External MOSFET.
2
PHASE
External Inductor Connection Pin for PWM Converter. It behaves as the current
sense comparator input for Low Side MOSFET RDS(ON) sensing and reference
voltage for on time generation.
3
UGATE
4
BOOT
5
VCC
6
FB
7
MODE
8
EN
VOUT Feedback Input. Connect FB to a resistive voltage divider from V OUT to
GND to adjust the output from 0.5V to 3.3V
Pull Down to GND for Forced CCM Mode.
Pull Up to 2.5V for Audio Skipping Mode (ASM).
Pull Up to 5V for Diode Emulation Mode (DEM).
PWM Chip Enable. Pull low to GND to disable the PWM.
9
PGOOD
Open Drain Power Good Indicator. High impedance indicates power is good.
10
CS
Current Limit Threshold Setting Input. Connect a setting resistor to GND and the
current limit threshold is equal to 1/10 of the voltage at this pin.
11
TON
On-time Setting. Connect a resistor between this pin and V IN.
Gate Drive Output for High Side External MOSFET.
Supply Input for High Side Driver. Connect through a capacitor to the floating
node (PHASE) pin.
Control Voltage Input. Provides the power for the buck controller, the low side
driver and the bootstrap circuit for high side driver. Bypass to GND with a 4.7F
ceramic capacitor.
12,
GND
13 (Exposed Pad)
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
Function Block Diagram
TRIG
On-time
Compute
1-SHOT
TON
PHASE
BOOT
R
- COMP
PSR
S
+
125% VREF
FB
70% VREF
OV
Latch
S1
Q
UV
Latch
S1
Q
-
PWM
Min. TOFF
Q
TRIG
VCC
DRV
MODE
-
PGOOD
+
POR
EN
10µA
SS
Timer
-
90% VREF
+
Thermal
Shutdown
CS
Copyright © 2021 Richtek Technology Corporation. All rights reserved.
RT8228A-08 October 2021
LGATE
GND
DEM/FCCM/ASM
VCC
UGATE
1-SHOT
+
125% VREF
DRV
PHASE
0.5V VREF
+
Q
+
-
X(-1/10)
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3
RT8228A
Absolute Maximum Ratings
(Note 1)
VCC to GND -------------------------------------------------------------------------------------------------------- –0.3V to 6V
FB, PGOOD, EN, CS, MODE to GND ----------------------------------------------------------------------- –0.3V to (VCC + 0.3V)
TON to GND -------------------------------------------------------------------------------------------------------- –0.3V to 32V
BOOT to PHASE ------------------------------------------------------------------------------------------------- –0.3V to 6.5V
PHASE to GND
DC -------------------------------------------------------------------------------------------------------------------- −0.3V to 32V
< 20ns --------------------------------------------------------------------------------------------------------------- −8V to 38V
UGATE to PHASE
DC -------------------------------------------------------------------------------------------------------------------- −0.3V to (VCC + 0.3V)
< 20ns --------------------------------------------------------------------------------------------------------------- −5V to 7.5V
LGATE to GND
DC -------------------------------------------------------------------------------------------------------------------- −0.3V to (VCC + 0.3V)
< 20ns --------------------------------------------------------------------------------------------------------------- −2.5V to 7.5V
Power Dissipation, PD @ TA = 25°C
WQFN-12L 2x2 ---------------------------------------------------------------------------------------------------- 0.606W
Package Thermal Resistance (Note 2)
WQFN-12L 2x2, θJA ---------------------------------------------------------------------------------------------- 165°C/W
Lead Temperature (Soldering, 10 sec.) ---------------------------------------------------------------------- 260°C
Junction Temperature -------------------------------------------------------------------------------------------- 150°C
Storage Temperature Range ------------------------------------------------------------------------------------ −65°C to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Mode) -------------------------------------------------------------------------------------- 2kV
MM (Machine Mode) --------------------------------------------------------------------------------------------- 200V
Recommended Operating Conditions
(Note 4)
Input Voltage, VIN ------------------------------------------------------------------------------------------------- 4.5V to 26V
Control Voltage, VCC --------------------------------------------------------------------------------------------- 4.5V to 5.5V
Junction Temperature Range ----------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range ----------------------------------------------------------------------------------- −40°C to 85°C
Copyright © 2021 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
RT8228A-08 October 2021
RT8228A
Electrical Characteristics
(VCC = 5V, VIN = 15V, VEN = 5V, VMODE = 5V, RTON = 500kΩ, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
--
0.5
1.25
mA
-----
-30
---
1
-1
1
A
A
A
A
VCC = 4.5V to 5.5V, DEM
495
500
505
mV
VFB = 0.5V
1
0.5
0.1
--
1
3.3
A
V
PWM Controller
VCC Quiescent Supply Current IQ
VCC Shutdown Current
TON Operating Current
TON Shutdown Current
CS Shutdown Current
FB Error Comparator
Threshold Voltage
FB Input Bias Current
Output Voltage Range
ISHDN
FB forced above the regulation
point, VEN = 5V
VCC Current, VEN = 0V
RTON = 500k
RTON = 500k
CS pull to GND
On-Time
VIN =15V, VPHASE = 1.25V,
VMODE = 0V
267
334
401
ns
Minimum Off-Time
VMODE = 0V, FB = 0.45V
250
400
550
ns
Current Sensing Threshold
CS Source Current
VCS = 0.5V to 2V
9
10
11
A
CS Source Current TC
On the basis of 25C
--
4700
--
ppm/C
Zero Crossing Threshold
VMODE > 1.8V, PHASE GND
10
--
5
mV
ASM Min Frequency
VMODE = 2.5V
--
25
--
kHz
Current Limit Threshold
UV Threshold
GND PHASE, VCS = 1V
UVP Detect, FB Falling Edge
85
60
100
70
115
80
mV
%
OVP Threshold
OVP Detect, FB Rising Edge
120
125
130
%
OV Fault Delay
VCC Power On Reset (POR)
Threshold
POR Threshold Hysteresis
Current Limit Ramp at
Soft-Start
UV Blank Time
FB forced above OV threshold
--
5
--
s
3.7
3.9
4.2
V
--
100
--
mV
--
900
--
s
--
4.5
--
ms
--
150
--
C
--
10
--
C
--
2.5
5
--
1.5
3
--
2.5
5
----
0.8
30
30
1.5
---
Protection Function
Thermal Shutdown
Rising Edge
Enable to current limit threshold =
50mV
From EN signal going high
TSD
Thermal Shutdown Hysteresis TSD
Driver On-Resistance
UGATE Driver Source
UGATE Driver Sink
LGATE Driver Source
LGATE Driver Sink
Dead Time
BOOT PHASE forced to 5V,
UGATE High State
BOOT PHASE forced to 5V,
RUGATEsk
UGATE Low State
RLGATEsr LGATE High State
RUGATEsr
RLGATEsk LGATE Low State
LGATE Rising (Phase = 1.5V)
UGATE Rising
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RT8228A-08 October 2021
ns
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RT8228A
Parameter
Symbol
Min
Typ
Max
Unit
--
--
80
Logic-High V IH
1.2
--
--
Logic-Low V IL
--
--
0.4
DEM Threshold
VCC 0.5
--
--
V
ASM Threshold
1.8
--
2.9
V
--
--
0.4
V
13
10
7
%
--
3
--
%
--
2.5
--
s
Internal Boost Charging
Switch on Resistance
EN Threshold
EN Threshold
Voltage
Test Conditions
V CC to BOOT, 10mA
V
Mode Threshold
FCCM Threshold
PGOOD (upper side threshold decided by OV threshold)
Measured at FB, with respect to
Trip Threshold (Falling)
reference
Trip Threshold Hysteresis
Fault Propagation Delay
Falling edge, FB forced
PGOOD trip threshold
below
Output Low Voltage
ISINK = 1mA
--
--
0.4
V
Leakage Current
High state, forced to 5V
--
--
1
A
Note 1. Stresses listed as the above “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings, 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 for extended
periods may remain possibility to affect device reliability.
Note 2. θJA is measured in the natural convection at TA = 25°C on a low effective thermal conductivity test board of JEDEC 513 thermal measurement standard.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
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RT8228A-08 October 2021
RT8228A
Typical Operating Characteristics
Efficiency vs. Load Current
Efficiency vs. Load Current
100
100
90
90
DEM Mode
60
50
40
CCM Mode
30
DEM Mode
80
70
Efficiency (%)
Efficiency (%)
80
70
60
50
40
CCM Mode
30
20
20
10
10
VIN = 8V, VOUT = 1V
0
0.001
0.01
0.1
1
VIN = 12V, VOUT = 1V
0
0.001
10
0.01
0.1
Switching Frequency vs. RTON Resistance
Efficiency vs. Load Current
900
90
800
Switching Frequency (kHz)1
100
Efficiency (%)
80
DEM Mode
60
50
40
30
CCM Mode
20
10
VIN = 20V, VOUT = 1V
0
0.001
700
600
500
400
300
200
100
CCM Mode, VIN = 12V, VOUT = 1V, No Load
0
0.01
0.1
1
10
100
Load Current (A)
300
450
350
400
350
300
250
200
150
100
0
10
12
14
16
18
20
22
24
Input Voltage (V)
Copyright © 2021 Richtek Technology Corporation. All rights reserved.
RT8228A-08 October 2021
500
600
700
800
VIN = 12V, VOUT = 1V
300
CCM Mode
250
200
150
100
50
DEM Mode
CCM Mode, VOUT = 1V, No Load
8
400
Switching Frequency vs. Load Current
400
Switching Frequency (kHz)1
Switching Frequency (kHz)1
Switching Frequency vs. Input Voltage
50
200
RTON Resistance (k Ω )
500
6
10
Load Current (A)
Load Current (A)
70
1
26
0
0.001
0.01
0.1
1
10
Load Current (A)
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RT8228A
Power On from EN
Switching Frequency vs. Load Current
Switching Frequency (kHz)1
400
VIN = 20V, VOUT = 1V
350
UGATE
(20V/Div)
300
250
CCM Mode
EN
(5V/Div)
200
150
VOUT
(500mV/Div)
PGOOD
(5V/Div)
100
50
0
0.001
DEM Mode
0.01
0.1
1
CCM Mode, VIN = 12V, VOUT = 1V, No Load
Time (1ms/Div)
10
Load Current (A)
Power On from EN
OVP
UGATE
(20V/Div)
UGATE
(20V/Div)
EN
(5V/Div)
VOUT
(500mV/Div)
VOUT
(500mV/Div)
PGOOD
(5V/Div)
LGATE
(5V/Div)
VIN = 12V, VOUT = 1V, No Load
DEM Mode, VIN = 12V, VOUT = 1V, No Load
Time (1ms/Div)
Time (200μs/Div)
UVP
Load Transient Response
UGATE
(50V/Div)
LGATE
(10V/Div)
VOUT_ac
(50mV/Div)
VOUT
(500mV/Div)
LGATE
(5V/Div)
IL
(10A/Div)
IOUT
(10A/Div)
UGATE
(20V/Div)
VIN = 12V, VOUT = 1V, No Load
Time (20μs/Div)
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CCM Mode, VIN = 12V, VOUT = 1V, EN = VCC
Time (100μs/Div)
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RT8228A
Mode Transition CCM to DEM
Mode Transition DEM to CCM
UGATE
(20V/Div)
UGATE
(20V/Div)
MODE
(5V/Div)
VOUT
(200mV/Div)
MODE
(5V/Div)
VOUT
(200mV/Div)
LGATE
(5V/Div)
VIN = 12V, VOUT = 1V, No Load
Time (1ms/Div)
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LGATE
(5V/Div)
VIN = 12V, VOUT = 1V, No Load
Time (1ms/Div)
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RT8228A
Application Information
The RT8228A PWM controller provides high efficiency,
excellent transient response, and high DC output accuracy
needed for stepping down high voltage batteries to
generate low voltage CPU core, I/O, and chipset RAM
supplies in notebook computers. Richtek Mach
ResponseTM technology is specifically designed for
providing 100ns “instant-on” response to load steps while
maintaining a relatively constant operating frequency and
inductor operating point over a wide range of input voltages.
The topology circumvents the poor load transient timing
problems of fixed frequency current mode PWMs while
avoiding the problems caused by widely varying switching
frequencies in conventional constant on-time and constant
off-time PWM schemes. The PSR PWM modulator is
specifically designed to have better noise immunity for
such a single output application.
frequency without the need of a clock generator.
7.06p RTON VOUT
t ON =
33ns
(VIN 0.9)
where RTON is the resistor connected from the input supply
(VIN) to the TON pin.
And then the switching frequency is :
VOUT
Frequency =
VIN tON
Mode Selection Operation
DEM (Diode Emulation Mode) and ASM (Audio Skipping
Mode) operation can be enabled by driving the tri-state
MODE pin to a logic high level. The RT8228A can switch
operation into DEM when the MODE pin is pulled up to
5V. If MODE is pulled to 2.5V, the controller will switch
operation into ASM. Finally, if the pin is pulled to GND,
the RT8228A will operate in CCM mode.
PWM Operation
Diode Emulation Mode
The Mach ResponseTM, PSR (Pulse Shaping Regulator)
mode controller is suitable for low external component
count configuration with appropriate amount of Equivalent
Series Resistance (ESR) capacitor(s) at the output. The
output ripple valley voltage is monitored at a feedback
point voltage. Refer to the function diagrams of the
RT8228A, the synchronous high side MOSFET is turned
on at the beginning of each cycle. After the internal oneshot timer expires, the MOSFET is turned off. The pulse
width of this one shot is determined by the converter's
input and output voltages to keep the frequency fairly
constant over the entire input voltage range. Another oneshot sets a minimum off-time (400ns typ.).
In diode emulation mode, the RT8228A automatically
reduces switching frequency at light load conditions to
maintain high efficiency. This reduction of frequency is
achieved smoothly and without increasing VOUT ripple or
load regulation. As the output current decreases from heavy
load condition, the inductor current is also reduced, and
eventually comes to the point that its valley touches zero
current, which is the boundary between continuous
conduction and discontinuous conduction modes. By
emulating the behavior of diodes, the low side MOSFET
allows only partial of negative current when the inductor
freewheeling current reach negative. As the load current
is further decreased, it takes longer and longer to discharge
the output capacitor to the level than requires the next
“ON” cycle. The on-time is kept the same as that in the
heavy load condition. In reverse, when the output current
increases from light load to heavy load, the switching
frequency increases to the preset value as the inductor
current reaches the continuous condition. The transition
load point to the light load operation can be calculated as
follows (Figure 1) :
On-Time Control
The on-time one-shot comparator has two inputs. One
input looks at the output voltage, while the other input
samples the input voltage and converts it to a current.
This input voltage proportional current is used to charge
an internal on-time capacitor. The on-time is the time
required for the voltage on this capacitor to charge from
zero volts to VOUT, thereby making the on-time of the
high side switch directly proportional to the output voltage
and inversely proportional to the input voltage. The
implementation results in a nearly constant switching
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10
ILOAD
VIN VOUT t
2L
where tON is On-time.
ON
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RT8228A-08 October 2021
RT8228A
IL
Slope = (VIN -VOUT) / L
IPEAK
ILOAD = IPEAK / 2
0
tON
t
Figure 1. Boundary Condition of CCM/DEM
waveform to become the complement of the high side
gate drive waveform. This in turn causes the inductor
current to reverse at light loads as the PWM loop to
maintain a duty ratio VOUT/VIN. The benefit of forced-CCM
mode is to keep the switching frequency fairly constant,
but it comes at a cost. The no load battery current can be
up to 10mA to 40mA, depending on the external
MOSFETs.
Current Limit Setting (OCP)
The switching waveforms may appear noisy and
asynchronous when light loading causes diode emulation
operation, but this is a normal operating condition that
results in high light load efficiency. Trade offs in DEM
noise vs. light load efficiency is made by varying the
inductor value. Generally, low inductor values produce a
broader efficiency vs. load curve, while higher values result
in higher full load efficiency (assuming that the coil
resistance remains fixed) and less output voltage ripple.
The disadvantages for using higher inductor values include
larger physical size and degrade load transient response
(especially at low input voltage levels).
supports temperature compensated MOSFET RDS(ON)
sensing. The CS pin should be connected to GND through
the trip voltage setting resistor, RCS. With the 10μA CS
terminal source current, ICS, and the setting resistor, RCS
the CS trip voltage, VCS, can be calculated as shown in
the following equation.
Audio Skipping Mode
VCS (mV) = RCS (kΩ) x 10 (μA) x (1 / 10)
When the MODE pin is pulled to 2.5V, the controller
operates in audio skipping mode with a minimum switching
frequency of 25kHz. This mode eliminates audio frequency
modulation that would otherwise be present when a lightly
loaded controller automatically skips pulses. In audio
skipping mode, the low side switch gate driver signal is
ORed with an internal oscillator (>25kHz). Once the
internal oscillator is triggered, the audio skipping controller
pulls LGATE logic high, turning on the low side MOSFET
to induce a negative inductor current. After the output
voltage rises above VREF, the controller turns off the low
Inductor current is monitored by the voltage between the
PGND pin and the PHASE pin, so the PHASE pin should
be connected to the drain terminal of the low side
MOSFET. ICS has positive temperature coefficient to
compensate the temperature dependency of the RDS(ON).
PGND is used as the positive current sensing node so
PGND should be connected to the source terminal of the
bottom MOSFET.
side MOSFET (LGATE pulled logic low) and triggers a
constant on-time operation (UGATE driven logic high).
When the on-time operation expires, the controller reenables the low side MOSFET until the inductor current
drops below the zero crossing threshold.
The RT8228A has cycle-by-cycle current limiting control.
The current limit circuit employs a unique “valley” current
sensing algorithm. If PHASE voltage plus the current limit
threshold is below zero, the PWM is not allowed to initiate
a new cycle (Figure 2). In order to provide both good
accuracy and a cost effective solution, the RT8228A
As the comparison is done during the OFF state, VCS
sets the valley level of the inductor current. Thus, the
load current at over current threshold, ILOAD_OC, can be
calculated as follows.
ILOAD_OC =
=
VCS
RDS(ON)
VCS
IRipple
+
RDS(ON)
2
+
V VOUT VOUT
1
IN
2 x Lf
VIN
Forced-CCM Mode
The low noise, forced-CCM mode (MODE = GND) disables
the zero-crossing comparator, which controls the low side
switch on-time. This causes the low side gate drive
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11
RT8228A
Power Good Output (PGOOD)
IL
IPEAK
ILOAD
ILIM
t
0
Figure 2. Valley Current Limit
The power good output is an open drain output and requires
a pull-up resistor. When the output voltage is 25% above
or 10% below its set voltage, PGOOD gets pulled low. It
is held low until the output voltage returns to within these
tolerances once more. In soft-start, PGOOD is actively
held low and is allowed to transition high until soft-start is
over and the output reaches 93% of its set voltage. There
is a 2.5μs delay built into PGOOD circuitry to prevent
false transitions.
MOSFET Gate Driver (UGATE, LGATE)
The high side driver is designed to drive high current, low
RDS(ON) N-MOSFET (s). When configured as a floating
driver, 5V bias voltage is delivered from the VDDP supply.
The average drive current is proportional to the gate charge
at VGS = 5V times switching frequency. The instantaneous
drive current is supplied by the flying capacitor between
BOOT and PHASE pins. A dead time to prevent shoot
through is internally generated between high side
MOSFET off to low side MOSFET on and low side
MOSFET off to high side MOSFET on. The low side driver
is designed to drive high current, low RDS(ON) N-MOSFET (s).
The internal pull down transistor that drives LGATE low is
robust, with a 0.8Ω typical on resistance. A 5V bias voltage
is delivered from the VDDP supply. The instantaneous drive
current is supplied by the flying capacitor between VDDP
and GND.
For high current applications, some combinations of high
and low side MOSFETs might be encountered that will
cause excessive gate drain coupling, which can lead to
efficiency killing, EMI-producing shoot through currents.
This is often remedied by adding a resistor in series with
BOOT, which increases the turn-on time of the high side
MOSFET without degrading the turn-off time (Figure 3).
VIN
BOOT
UGATE
PHASE
Figure 3. Reducing the UGATE Rise Time
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POR, UVLO and Soft-Start
Power On Reset (POR) occurs when VCC rises above to
approximately 3.9V, the RT8228A will reset the fault latch
and preparing the PWM for operation. Below 3.7V, the
VCC Under Voltage Lockout (UVLO) circuitry inhibits
switching by keeping UGATE and LGATE low. A built-in
soft-start is used to prevent surge current from power supply
input after EN is enabled. A current ramping up limit
threshold can eliminate the VOUT folded-back in the softstart duration. The typical soft-start duration is 900μs.
Output Over Voltage Protection (OVP)
The output voltage can be continuously monitored for over
voltage protection. When the output voltage exceeds 25%
of the set voltage threshold, over voltage protection is
triggered and the low side MOSFET is latched on. This
activates the low side MOSFET to discharge the output
capacitor. The RT8228A is latched once OVP is triggered
and can only be released by VCC or EN power on reset.
There is a 5μs delay built into the over voltage protection
circuit to prevent false transitions.
Output Under Voltage Protection (UVP)
The output voltage can be continuously monitored for under
voltage protection. When the output voltage is less than
70% of the set voltage threshold, under voltage protection
is triggered and then both UGATE and LGATE gate drivers
are forced low. During soft-start, the UVP blanking time is
4.5ms.
Output Voltage Setting (FB)
The output voltage can be adjusted from 0.5V to 3.3V by
setting the feedback resistor R1 and R2 (Figure 4). Choose
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RT8228A-08 October 2021
RT8228A
R2 to be approximately 10kΩ, and solve for R1 using the
equation :
R1
VOUT = VREF 1+
R2
where VREF is 0.5V.(typ.)
by an external circuit to reduce the jitter level. The required
signal level is approximately 15 mV at the comparing point.
This generates VRIPPLE = (VOUT / 0.5) x 15mV at the output
node. The output capacitor ESR should meet this
requirement.
VIN
Thermal Considerations
VOUT
UGATE
PHASE
LGATE
R1
FB
R2
GND
Figure 4. Setting VOUT with a Resistor Divider
Output Inductor Selection
The switching frequency (on-time) and operating point (%
ripple or LIR) determine the inductor value as follows :
T VIN VOUT
L = ON
LIR ILOAD(MAX)
where LIR is the ratio of peak-of-peak ripple current to the
maximum average inductor current. Find a low pass
inductor having the lowest possible DC resistance that
fits in the allowed dimensions. Ferrite cores are often the
best choice, although powdered iron is inexpensive and
can work well at 200kHz. The core must be large enough
and not to saturate at the peak inductor current (IPEAK) :
L
IPEAK = ILOAD(MAX) + IR ILOAD(MAX)
2
Output Capacitor Selection
The output filter capacitor must have low enough Equivalent
Series Resistance (ESR) to meet output ripple and loadtransient requirements, yet have high enough ESR to
satisfy stability requirements. The output capacitance
must also be high enough to absorb the inductor energy
while transitioning from full-load to no-load conditions
without tripping the overvoltage fault latch.
For continuous operation, do not exceed absolute
maximum operation junction temperature. The maximum
power dissipation depends on the thermal resistance of
IC package, PCB layout, the rate of surroundings airflow
and temperature difference between junction to ambient.
The maximum power dissipation can be calculated by
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where T J(MAX) is the maximum operation junction
temperature 125°C, TA is the ambient temperature and
the θJA is the junction to ambient thermal resistance.
For recommended operating conditions specification of
RT8228A, the maximum junction temperature is 125°C
and TA is the ambient temperature. The junction to ambient
thermal resistance, θJA, is layout dependent. For WQFN12L 2x2 package, the thermal resistance, θJA, is 165°C/
W on a standard JEDEC 51-3 single-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) / (165°CW) = 0.606W for
WQFN-12L 2x2 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. For the RT8228A package, the derating
curve in Figure 5 allows the designer to see the effect of
rising ambient temperature on the maximum power
dissipation.
Although Mach ResponseTM DRVTM dual ramp valley mode
provides many advantages such as ease-of-use, minimum
external component configuration, and extremely short
response time, due to not employing an error amplifier in
the loop, a sufficient feedback signal needs to be provided
Copyright © 2021 Richtek Technology Corporation. All rights reserved.
RT8228A-08 October 2021
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
13
Maximum Power Dissipation (W)1
RT8228A
0.65
0.60
0.55
0.50
Keep current limit setting network as close as possible
to the IC. Routing of the network should avoid coupling
to high voltage switching node.
Connections from the drivers to the respective gate of
the high side or the low side MOSFET should be as
short as possible to reduce stray inductance.
All sensitive analog traces and components such as
MODE, FB, GND, EN, PGOOD, CS, VCC, and TON
should be placed away from high voltage switching
nodes such as PHASE, LGATE, UGATE, or BOOT
nodes to avoid coupling. Use internal layer (s) as ground
plane (s) and shield the feedback trace from power traces
and components.
Current sense connections must always be made using
Kelvin connections to ensure an accurate signal, with
the current limit resistor located at the device.
Power sections should connect directly to ground plane
(s) using multiple vias as required for current handling
(including the chip power ground connections). Power
components should be placed to minimize loops and
reduce losses.
Single-Layer PCB
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 5. Derating Curves for RT8228A Packages
Layout Considerations
Layout is very important in high frequency switching
converter design. If the layout is designed improperly, the
PCB could radiate excessive noise and contribute to the
converter instability. The following points must be followed
for a proper layout of RT8228A.
Connect a filter capacitor to VCC, 1μF to 4.7μF range is
recommended. Place the filter capacitor close to the
IC.
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14
is a registered trademark of Richtek Technology Corporation.
RT8228A-08 October 2021
RT8228A
Outline Dimension
1
1
2
2
DETAIL A
Pin #1 ID and Tie Bar Mark Options
Note : The configuration of the Pin #1 identifier is optional,
but must be located within the zone indicated.
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
0.150
0.250
0.006
0.010
D
1.900
2.100
0.075
0.083
E
1.900
2.100
0.075
0.083
e
0.400
0.016
D2
0.850
0.950
0.033
0.037
E2
0.850
0.950
0.033
0.037
L
0.250
0.350
0.010
0.014
W-Type 12L QFN 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.
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