RT8270
1
DS8270-01 March 2011 www.richtek.com
Ordering Information
Pin Configurations
(TOP VIEW)
SOP-8
Note :
Richtek products are :
` RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
` Suitable for use in SnPb or Pb-free soldering processes.
2A, 22V, 1.2MHz Step-Down Converter
Applications
zDistributive Power Systems
zBattery Charger
zDSL Modems
zPre-regulator for Linear Regulators
Marking Information
For marking information, contact our sales representative
directly or through a Richtek distributor located in your
area, otherwise visit our website for detail.
General Description
The RT8270 is an asynchronous high voltage buck
converter that can support the input voltage range from
4.75V to 22V and the output current can be up to 2A.
Current Mode operation provides fa st tra nsient response
a nd ea ses loop stabilization.
The chip provides protection functions such a s cycle-by-
cycle current limiting and thermal shutdown protection.
In shutdown mode, the regulator draws 25μA of supply
current. The RT8270 is available in a SOP-8 surfa ce mount
package.
Features
zz
zz
zWide Operating Input Range : 4.75V to 22V
zz
zz
zAdjustable Output Voltage Range : 1.222V to 16V
zz
zz
zOutput Current up to 2A
zz
zz
z25μμ
μμ
μA Low Shutdown Current
zz
zz
zPower MOSFET : 0.18ΩΩ
ΩΩ
Ω
zz
zz
zHigh Efficiency up to 95%
zz
zz
z1.2MHz Fixed Switching Frequency
zz
zz
zSta ble with Low ESR Output Ceramic Capacitors
zz
zz
zThermal Shutdown Protection
zz
zz
zCycle-By-Cycle Over Current Protection
zz
zz
zRoHS Compliant and Halogen Free
Typical Application Circuit
BOOT
VIN
SW
GND
NC
EN
FB
COMP
2
3
45
6
7
8
Package Type
S : SOP-8
RT8270
Lead Plating System
G : Green (Halogen Free and Pb Free)
VIN
EN
GND
BOOT
FB
SW
7
5
2
3
1
L1
4.7µH
CB
10nF
COUT
22µF
R1
17k
R2
10k
VOUT
3.3V/2A
CIN
10µF
Chip Enable
VIN
4.75V to 22V RT8270
D1
B330
COMP
CC
1.8nF
RC
18k
CP
NC
6
4
RT8270
2DS8270-01 March 2011www.richtek.com
Functional Pin Description
Function Block Diagram
Pin No. Pin Name Pin Function
1 BOOT High Side Gate Drive Boost Input. BOOT supplies the drive for the high side N-MOSFET
switch. Connect a 10nF or greater capac itor from SW to BOOT to power the high side
switch.
2 VIN Power Input. VIN supplies the power to the IC, as well as the step-down converter
switches. Bypass VIN to GN D with a suitable large capacitor to elimi nate nois e on the
input to the IC.
3 SW Power Sw itching Output. SW is the switching node that supplies power to the output.
Connect the output LC filter fr om SW to the output load. Note that a capacitor is required
from SW to BOOT to power the high side switch.
4 GND Ground.
5 FB Feedback Input. FB s enses the output voltage to regulate said voltage. The feedback
refer ence voltage is 1.222V typically.
6 COMP Compensation Node. COMP is used to compensate the regulation control loop.
Connect a series RC network from COMP to GND to compensate the re gulation contr ol
loop. In some cases, an additional capacitor fr om COMP to GND is r equ ired.
7 EN Enable Input. EN is a digital input that turns the regulator on or off. Drive EN higher than
1.4V t o turn o n the r egulat o r, lower than 0.4V to turn it off. If t he EN pin is open , it will be
pulled to high by internal cir cuit.
8 NC No Intern al Connection.
VOUT (V) R1 (kΩ) R2 (kΩ) RC (kΩ) CC (nF) L1 (μH) COUT (μF)
12 88.7 10 51 0.86 10 22
5 30 10 23.1 1.2 6.8 22
3.3 17 10 18 1.8 4.7 22
2.5 10.45 10 12 2.2 4.7 22
1.8 4.75 10 10 2.2 2.2 22
1.222 0 10 9.1 2.2 2.2 22
Table 1. Recommended Component Selection
Logic
VA
+
-
+
-
+
-
+
-
EA
UV
Comparator
Oscillator
1.2MHz/440kHz
Foldback
Control
0.6V
Internal
Regulator
+
-
1V
1µA
Shutdown
Comparator
Current Sense
Amplifier
BOOT
VIN
GND
SW
FB
EN
COMP
1.222V
3V
10k VA VCC
VCC Slope Comp
Current
Comparator
Gm = 780µA/V
RT8270
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DS8270-01 March 2011 www.richtek.com
Electrical Characteristics
Parameter Symbol Test Conditions Min Typ Max Unit
F eed back Reference Voltage V FB 4.75V ≤ VIN ≤ 22V 1.184 1.222 1.258 V
High Side Switch-On R esistance RDS(ON)1 -- 0.18 -- Ω
Low Side S witch-On Resistance RDS(ON)2 -- 10 -- Ω
Switch Leakage VEN = 0V, VSW = 0V - - - - 10 μA
Cur ren t L imit ILIM Duty = 90%; VBOOT−SW = 4.8V -- 3 -- A
Curren t Sense Tr anscondu ctance GCS Output Current to VCOMP -- 2.5 -- A/V
Error A mplifier T a nsc onduct ance Gm ΔIC = ±10μA -- 780 -- μA/V
Oscillator F requ ency fSW -- 1.2 -- MHz
Short Cir cuit Oscillation Fr eq uen cy VFB = 0V -- 440 - - kHz
Maximum Du ty Cycle DMAX V
FB = 0.8V -- 80 -- %
Minimum On-Time tON -- 100 -- ns
Under Voltage Lockout Threshold
Rising 4 4.2 4.5 V
Under Voltage Lockout Threshold
Hysteresis -- 300 -- mV
En in put Low Voltage -- -- 0.4 V
En in put Hig h Voltage 1.4 -- -- V
Enable Pull Up Current -- 1 -- μA
(VIN = 12V, TA = 25°C unless otherwise specified)
Absolute Maximum Ratings (Note 1)
zSupply Voltage, VIN -----------------------------------------------------------------------------------------23V
zSwitching Voltage, SW -------------------------------------------------------------------------------------−0.3V to (VIN + 0.3V)
zBOOT Voltage ------------------------------------------------------------------------------------------------(VSW − 0.3V) to (VSW + 6V)
zAll Other Voltage---------------------------------------------------------------------------------------------−0.3V to 6V
zPower Dissipation, PD @ TA = 25°C
SOP-8 ----------------------------------------------------------------------------------------------------------0.833W
zPackage Thermal Resistance (Note 2)
SOP-8, θJA ----------------------------------------------------------------------------------------------------120°C/W
zJunction T emperature ---------------------------------------------------------------------------------------150°C
zLead T emperature (Soldering, 10 sec.) -----------------------------------------------------------------260°C
zStorage T emperature Range -------------------------------------------------------------------------------−65°C to 150°C
zESD Susceptibility (Note 3)
HBM (Human Body Mode) ---------------------------------------------------------------------------------2kV
MM (Ma chine Mode) ----------------------------------------------------------------------------------------200V
Recommended Operating Conditions (Note 4)
zSupply Voltage, VIN -----------------------------------------------------------------------------------------4.75V to 22V
zEnable V oltage, VEN -----------------------------------------------------------------------------------------0V to 5.5V
zJunction T emperature Range ------------------------------------------------------------------------------−40°C to 125°C
zAmbient T emperature Range ------------------------------------------------------------------------------−40°C to 85°C
To be continued
RT8270
4DS8270-01 March 2011www.richtek.com
Note 1. Stresses listed as the above "Absolute Maximum Ratings" may cause permanent damage to the device. These are for
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 high effective four layers thermal conductivity test board of
JEDEC 51-7 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.
Parameter Symbol Test Conditions Min Typ Max Unit
Shutdown Current ISHDN V
EN = 0V -- 25 50 μA
Quiesce nt Current IQ V
EN = 2V, VFB = 1.5V -- 0.7 1 mA
Thermal Shutdown TSD -- 150 -- °C
RT8270
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DS8270-01 March 2011 www.richtek.com
Typical Operating Characteristics
Freque ncy vs. Temperature
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
1.40
-50 -25 0 25 50 75 100 125
Temperature
Frequency (MHz)
VIN = 12V, VOUT = 3.3V
(°C)
Freque ncy vs. Input Voltage
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
1.40
4 6 8 10121416182022
Input V olt age (V)
Frequency (MHz)
VIN = 4.75V to 22V, VOUT = 3.3V
Output Voltage vs. Temperature
3.200
3.225
3.250
3.275
3.300
3.325
3.350
3.375
3.400
-50 -25 0 25 50 75 100 125
Temperature
Ou t pu t Vol tage ( V)
VIN = 12V, VOUT = 3.3V, IOUT = 0A
(°C)
Output Voltage vs. Output Current
3.279
3.282
3.285
3.288
3.291
3.294
3.297
3.300
00.40.81.21.62
Output Cu rren t (A)
Output Voltage (V)
VIN = 9V
VIN = 12V
VIN = 22V
VOUT = 3.3V
Reference Voltage vs. Input Voltage
1.214
1.216
1.218
1.220
1.222
1.224
1.226
4 7 10 13 16 19 22
Input Vol tage (V)
Refer ence Vo lt ag e (V)
VIN = 4.75V to 22V, VOUT = 3.3V
Efficiency vs. Output Current
0
10
20
30
40
50
60
70
80
90
100
0 0.4 0.8 1.2 1.6 2
Output Current (A)
Eff icienc y (%)
VIN = 4.75V
VIN = 12V
VIN = 22V
VOUT = 3.3V
RT8270
6DS8270-01 March 2011www.richtek.com
Current Limit vs. Temperature
2.50
2.75
3.00
3.25
3.50
3.75
4.00
4.25
4.50
-50-250 255075100125
Temperature
Current Li mit (A)
VIN = 12V, VOUT = 3.3V
(°C)
Current Limit vs. Input Voltage
2.50
2.75
3.00
3.25
3.50
3.75
4.00
4.25
4.50
4 6 8 10 12 14 16 18 20 22
In put Vol ta g e (V)
Current Li mit (A)
VIN = 4.75 to 22V, VOUT = 3.3V
VOUT = 3.3V, IOUT = 0.3A
Load Transient Response
Time (100μs/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 1A to 2A
IOUT
(1A/Div)
VOUT
(100mV/Div)
Load Transient Response
Time (100μs/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 0A to 2A
VOUT
(100mV/Div)
IOUT
(1A/Div)
Power On from EN
Time (5ms/Div)
VOUT
(2V/Div)
VEN
(2V/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 2A
Power Off from EN
Time (5ms/Div)
VOUT
(2V/Div)
VEN
(2V/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 2A
RT8270
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DS8270-01 March 2011 www.richtek.com
Switching
Time (500ns/Div)
VOUT
(10mV/Div)
VSW
(10V/Div)
ISW
(1A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 2A
Power On from VIN
Time (5ms/Div)
VOUT
(2V/Div)
VIN
(5V/Div)
IIN
(1A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 2A
RT8270
8DS8270-01 March 2011www.richtek.com
Application Information
The RT8270 is an asynchronous high voltage buck
converter that can support the input voltage range from
4.75V to 22V a nd the output current ca n be up to 2A.
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage a s shown in Figure 1.
Figure 1. Output Voltage Setting
The output voltage is set by a n external resistive divider
according to the following equation :
⎛⎞
+
⎜⎟
⎝⎠
OUT FB R1
V = V1
R2
Where VFB is the feedback reference voltage (1.222V typ.).
External Bootstrap Diode
Connect a 10nF low ESR cera mic ca pacitor between the
BOOT pin a nd SW pin. This capa citor provides the gate
driver voltage for the high side MOSFET.
It is recommended to add an external bootstrap diode
between a n external 5V and the BOOT pin for ef ficiency
improvement when input voltage is lower than 5.5V or duty
ratio is higher than 65%. The bootstrap diode can be a
low cost one such a s 1N4148 or BAT54.
The external 5V can be a 5V fixed input from system or a
5V output of the RT8270.
Figure 2
Soft-Start
The RT8270 contains an internal soft-start clamp that
gradually raises the output voltage. The soft-start time is
designed by the internal capacitor . The typical soft-start
time is 2ms.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ΔIL incre ase s with higher VIN
and decrea ses with higher inducta nce.
OUT OUT
LIN
VV
I = 1
fL V
⎡⎤⎡ ⎤
Δ×−
⎢⎥⎢ ⎥
×
⎣⎦⎣ ⎦
Having a lower ripple current reduces not only the ESR
losses in the output ca pa citors but also the output voltage
ripple. High frequency with small ripple current can achieve
highest efficiency operation. However , it requires a large
inductor to a chieve this goal.
For the ripple current selection, the value of ΔIL = 0.4(IMAX)
will be a reasonable starting point. The largest ripple
current occurs at the highest VIN. To guarantee that the
ripple current stays below the specified maximum, the
inductor value should be chosen according to the following
equation :
OUT OUT
L(MAX) IN(MAX)
VV
L = 1
fI V
⎡⎤⎡ ⎤
×−
⎢⎥⎢ ⎥
×Δ
⎣⎦⎣ ⎦
Inductor Core Selection
The inductor type must be selected once the value for L
is known. Generally speaking, high efficiency converters
ca n not af f ord the core loss f ound in low cost powdered
iron cores. So, the more expensive ferrite or
mollypermalloy cores will be a better choice.
The selected inductance rather than the core size for a
fixed inductor value is the key for actual core loss. As the
inductance increa ses, core losses decrease. Unfortunately ,
increase of the inductance requires more turns of wire
and therefore the copper losses will increa se.
Ferrite designs are preferred at high switching frequency
due to the characteristics of very low core losses. So,
design goals can focus on the reduction of copper loss
and the saturation prevention.
RT8270
GND
FB
R1
R2
VOUT
SW
BOOT
5V
RT8270 10nF
RT8270
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DS8270-01 March 2011 www.richtek.com
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design
current is exceeded. The previous situation results in a n
abrupt increa se in inductor ripple current and consequent
output voltage ripple.
Do not allow the core to saturate!
Different core materi als and sha pes will change the size/
current and price/current relationship of a n inductor .
T oroid or shielded pot cores in ferrite or permalloy materials
are small and do not radi ate energy. However, they are
usually more expensive than the similar powdered iron
inductors. The rule for inductor choice mainly depends
on the price vs. size requirement a nd a ny ra diated f ield/
EMI requirements.
Diode Selection
When the power switch turns off, the path for the current
is through the diode connected between the switch output
and ground. This forward biased diode must have a
minimum voltage drop a nd recovery times. Schottky diode
is recommended and it should be able to handle those
current. The reverse voltage rating of the diode should be
greater than the maximum input voltage, a nd current rating
should be greater than the maximum load current. For
more detail, plea se refer to Table 4.
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the
tra pezoidal current at the source of the high side MOSFET .
To prevent large ripple current, a low ESR input ca pacitor
sized for the maximum RMS current should be used. The
RMS current is given by :
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is
commonly used for design because even significant
deviations do not offer much relief.
Choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design.
For the input ca pacitor , a 10μF low ESR cera mic capacitor
is recommended. For the recommended ca pacitor, plea se
refer to table 3 for more detail.
The selection of COUT is determined by the required ESR
to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key
for COUT selection to ensure that the control loop is stable.
Loop stability can be checked by viewing the load transient
respon se as described in a later section.
The output ripple, ΔVOUT , is determined by :
The output ripple will be highest at the maximum input
voltage since ΔIL increases with input voltage. Multiple
ca pa citors pla ced in parallel may be needed to meet the
ESR and RMS current handling requirement. Dry tantalum,
special polymer, aluminum electrolytic and ceramic
capacitors are all available in surface mount packages.
Special polymer capacitors offer very low ESR value.
However, it provides lower capacitance density than other
types. Although Tantalum capacitors have the highest
ca pa cita nce density , it is important to only use type s that
pass the surge test for use in switching power supplie s.
Aluminum electrolytic ca pacitors have significantly higher
ESR. However, it can be used in cost-sensitive a pplications
for ripple current rating and long term reliability
considerations. Ceramic capacitors have excellent low
ESR characteristics but can have a high voltage coefficient
a nd audible piezoelectric effe cts. The high Q of ceramic
ca pacitors with trace inductance can also lead to significant
ringing.
Higher values, lower cost ceramic capacitors are now
becoming available in smaller ca se sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However , care must
be taken when these capacitors are used at input and
output. When a ceramic capacitor is used at the input
a nd the power is supplied by a wall ada pter through long
wires, a loa d step at the output ca n induce ringing at the
input, VIN. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at VIN large enough to da mage the
part.
OUT IN
RMS OUT(MAX) IN OUT
VV
I = I 1
VV
−
OUT L OUT
1
VIESR
8fC
⎡⎤
Δ≤Δ +
⎢⎥
⎣⎦
RT8270
10 DS8270-01 March 2011www.richtek.com
Checking T ransient Re sponse
The regulator loop response ca n be checked by looking
at the load tra nsient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an a mount
equal to ΔILOAD (ESR) and also begins to charge or
discharge COUT generating a feedback error signal for the
regulator to return VOUT to its steady-state value. During
this recovery time, VOUT can be monitored for overshoot or
ringing that would indicate a stability problem.
Thermal Considerations
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 a mbient. The maximum power dissipation can
be calculated by following formula :
PD(MAX) = ( TJ(MAX) − TA ) / θJA
Where TJ(MAX) is the maximum operation junction
temperature, TA is the a mbient temperature and the θJA is
the junction to a mbient thermal resista nce.
For recommended operating conditions specification of
RT8270, the maximum junction temperature is 125°C. The
junction to ambient thermal resistance θJA for SOP-8
pa ck age is 120 °C/W on the standard JEDEC 51-7 f our-
layers thermal test board. The maximum power dissipation
at TA = 25°C ca n be calculated by following f ormula :
PD(MAX) = (125°C − 25°C) / (120°C/W) = 0.833W for
SOP-8 pa ckages
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal
resistance θJA. For RT8270 packages, the Figure 3 of
derating curves allows the designer to see the effect of
rising ambient temperature on the maximum power
allowed.
Layout Consideration
Follow the PCB layout guidelines for optimal performa nce
of the RT8270.
`Keep the tra ces of the main current paths a s short a nd
wide a s possible.
`Put the input ca pacitor as close as possible to the device
pins (VIN and GND).
`LX node is with high frequency voltage swing and should
be kept at small area. Keep sensitive components away
from the LX node to prevent stray ca pacitive noise pick-
up.
`Pla ce the feedback components to the FB pin a s close
as possible.
`The GND a nd Exposed Pa d should be connected to a
strong ground plane for heat sinking and noise protection.
Figure 3. Derating Curves for RT8270 Packages
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 25 50 75 100 125
Am bien t Tempera tu re ( °C )
Pow er Dissipati on (W)
Four Layer PCB
SOP-8
RT8270
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DS8270-01 March 2011 www.richtek.com
Figure 4. PCB Layout Guide
Table 3. Suggested Capacitors for CIN and COUT
Component Supplier Series D ime nsions (mm)
TDK SLF12555T 12.5 x 12.5 x 5.5
TAIYO YUDEN NR8040 8 x 8 x 4
TDK SLF12565T 12.5 x 12.5 x 6.5
Table 2. Sugge sted Inductors for Typical Application Circuit
Component Supplier Series VRRM (V) IOUT (A) Package
DIODES B330A 30 3 SMA
PANJIT SK23 30 2 DO-214AA
Table 4. Suggested Diode
Location Component Supplier Part No. Capacitance (μF) Case Size
CIN MU RATA GRM31CR61E106K 10 1206
CIN TDK C3225X5R1E106K 10 1206
CIN TAI YO YUDEN TMK316BJ106ML 10 1206
COUT MU RATA GRM32ER61E226M 22 1210
COUT TDK C3225X5R0J226M 22 1210
COUT TAIYO YUDEN EMK325BJ226MM 22 1210
VIN
VOUT
GND
CIN CB
2
3
45
8
7
6
NC
BOOT
VIN
GND
SW FB
EN
COMP
GND
CP
CC
RC
SW
D1
VOUT
COUT
L1 R1
R2
Input capacitor must
be placed as close
to the IC as possible.
SW should be connected to inductor by
wide and short trace. Keep sensitive
components away from this trace.
The feedback and
compensation components
must be connected as close
to the device as possible.
RT8270
12 DS8270-01 March 2011www.richtek.com
Information that is provided by Richtek Technology Corporation is believed to be accurate and reliable. Richtek reserves the right to make any change in circuit
design, specification or other related things if necessary without notice at any time. No third party intellectual property infringement of the applications should be
guaranteed by users when integrating Richtek products into any application. No legal responsibility for any said applications is assumed by Richtek.
Richtek Technology Corporation
Headquarter
5F, No. 20, Taiyuen Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789 Fax: (8863)5526611
Richtek Technology Corporation
Taipei Office (Marketing)
5F, No. 95, Minchiuan Road, Hsintien City
Taipei County, Taiwan, R.O.C.
Tel: (8862)86672399 Fax: (8862)86672377
Email: marketing@richtek.com
Outline Dimension
A
B
J
F
H
M
C
D
I
8-Lead SOP Plastic Package
Dimension s In Millimet ers Dimen sions In Inch es
Symbol Min Max Min Max
A 4.801 5.004 0.189 0.197
B 3.810 3.988 0.150 0.157
C 1.346 1.753 0.053 0.069
D 0.330 0.508 0.013 0.020
F 1.194 1.346 0.047 0.053
H 0.170 0.254 0.007 0.010
I 0.050 0.254 0.002 0.010
J 5.791 6.200 0.228 0.244
M 0.400 1.270 0.016 0.050
