DS8258 02

RT8258
1.2A, 24V, 700kHz Step-Down Converter
General Description
Features
The RT8258 is a high voltage buck converter that can support
the input voltage range from 4.5V to 24V and the output
current can be up to 1.2A. Current Mode operation provides
fast transient response and eases loop stabilization.
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Wide Operating Input Voltage Range : 4.5V to 24V
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Adjustable Output Voltage Range : 0.8V to 15V
1.2A Output Current
0.3Ω
Ω Internal Power MOSFET Switch
High Efficiency up to 92%
700kHz Fixed Switching Frequency
Stable with Low ESR Output Ceramic Capacitors
Thermal Shutdown
Cycle-By-Cycle Over Current Protection
RoHS Compliant and Halogen Free
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The chip also provides protection functions such as cycleby-cycle current limiting and thermal shutdown protection.
The RT8258 is available in a SOT-23-6 and TSOT-23-6
packages.
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Ordering Information
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RT8258
Applications
Package Type
E : SOT-23-6
J6 : TSOT-23-6
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Lead Plating System
G : Green (Halogen Free and Pb Free)
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Note :
Richtek products are :
`
Distributed Power Systems
Battery Charger
Pre-Regulator for Linear Regulators
WLED Drivers
Pin Configurations
RoHS compliant and compatible with the current require-
(TOP VIEW)
ments of IPC/JEDEC J-STD-020.
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Suitable for use in SnPb or Pb-free soldering processes.
PHASE VIN EN
6
Marking Information
For marking information, contact our sales representative
directly or through a Richtek distributor located in your
area.
5
4
2
3
BOOT GND FB
SOT-23-6/TSOT-23-6
Typical Application Circuit
VIN
4.5V to 24V
CIN
10µF
Chip Enable
5 VIN
BOOT
RT8258
PHASE 6
4 EN
GND
2
DS8258-02 March 2011
1
FB 3
CBOOT
L
10nF 10µH
D1
B230A
VOUT
3.3V
R1
100k
R2
32.4k
COUT
22µF
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RT8258
Table 1. Recommended Component Selection
VOUT
1.2V
1.5V
1.8V
2.5V
3.3V
5V
8V
10V
15V
R1 (kΩ)
100
91
91
100
100
91
91
91
120
R2 (kΩ)
200
100
75
47
32.4
17.4
10
7.87
6.8
L (μH)
3.6
3.6
4.7
6.8
10
15
22
22
33
COUT (μF)
22
22
22
22
22
22
22
22
22
Note : The value of R1 is related to the loop bandwidth of the RT8258. It is strongly recommended to follow the
parameters in above table for the specific output voltage.
Function Block Diagram
VIN
-
X20
1µA
Current Sense Amp
EN
1.1V
3V
-
BOOT
Oscillator
700kHz
+
Shutdown Reference
Comparator
S
+
EA
-
FB
25mΩ
Ramp
Generator
Regulator
10k
+
400k
30pF
+
-
Driver
Q
R
PHASE
PWM
Comparator
Bootstrap
Control
OC Limit Clamp
GND
1pF
Functional Pin Description
Pin No.
Pin Name
1
BOOT
2
GND
3
FB
4
EN
5
VIN
6
PHASE
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2
Pin Function
Gate Driver Bootstrap Input Pin. Connect a 10nF or greater capacitor between PHASE
and BOOT pins to supply the MOSFET driver.
Ground Pin. This pin should be connected to the (-) terminal of the output capacitor and
it should be kept away from the D1 and input capacitor for noise prevention.
Output Voltage Feedback Input Pin. An external resistor divider from the output to GND
tapped to the FB pin sets the output voltage. The value of the divider resistors also set
loop bandwidth.
Chip Enable (Active High). If the EN pin is open, it will be pulled to high by internal
circuit.
Power Supply Input Pin. Bypass VIN to GND with a suitable large capacitor to prevent
large voltage spikes from appearing at the input.
Power Switching Output Pin. Connect this pin to the output inductor.
DS8258-02 March 2011
RT8258
Absolute Maximum Ratings
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(Note 1)
Supply Voltage, VIN -----------------------------------------------------------------------------------------------PHASE Voltage ----------------------------------------------------------------------------------------------------BOOT Voltage ------------------------------------------------------------------------------------------------------All Other Pins -------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
T/SOT-23-6 ----------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
T/SOT-23-6, θJA -----------------------------------------------------------------------------------------------------Junction Temperature ---------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -----------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Mode) ---------------------------------------------------------------------------------------MM (Machine Mode) -----------------------------------------------------------------------------------------------
Recommended Operating Conditions
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26V
−0.3V to (VIN + 0.3V)
VPHASE + 6V
0.3V to 6V
0.4W
250°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Voltage, VIN ------------------------------------------------------------------------------------------------ 4.5V to 24V
Output Voltage, VOUT ---------------------------------------------------------------------------------------------- 0.8V to 15V
EN Voltage, VEN ----------------------------------------------------------------------------------------------------- 0V to 5.5V
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
Feedback Reference Voltage
V FB
4.5V ≤ VIN ≤ 24V
0.784
0.8
0.816
V
Feedback Current
IFB
VFB = 0.8V
--
0.1
0.3
μA
Switch On Resistance
Switch Leakage
RDS(ON)
VEN = 0V, VPHASE = 0V
---
0.3
--
-10
Ω
μA
Current Limit
ILIM
VBOOT − VPHASE = 4.8V
1.6
2.1
--
A
Oscillator Frequency
fSW
600
700
800
kHz
--
90
--
%
--
100
--
ns
3.9
4.2
4.5
V
--
270
--
mV
1.4
--
--
--
--
0.4
Maximum Duty Cycle
Minimum On-Time
tON
Under Voltage Lockout
Rising
Threshold Voltage
Under Voltage Lockout
Threshold Hysteresis
Logic High
EN Input Voltage
Logic Low
V
VEN = 0V
--
1
--
μA
Shutdown Current
ISHDN
VEN = 0V
--
25
--
μA
Quiescent Current
IQ
VEN = 2V, VFB = 1V (No Switching)
--
0.55
1
mA
Thermal Shutdown
T SD
--
150
--
°C
EN Pull Up Current
DS8258-02 March 2011
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RT8258
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 low effective single layer thermal conductivity test board of
JEDEC 51-3 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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DS8258-02 March 2011
RT8258
Typical Operating Characteristics
Efficiency vs. Output Current
100
90
90
VIN = 24V
80
70
VIN = 12V
80
VIN = 12V
Efficiency (%)
Efficiency (%)
Efficiency vs. Output Current
100
60
50
40
30
20
VIN = 24V
70
60
50
40
30
20
10
10
VOUT = 5V
0
0
0.2
0.4
0.6
0.8
1
VOUT = 3.3V
0
1.2
0
0.2
0.4
Output Current (A)
0.8
1
1.2
Output Voltage vs. Temperature
Output Voltage vs. Output Current
3.366
3.366
3.344
3.344
VIN = 24V
3.322
Output Voltage (V)
Output Voltage (V)
0.6
Load Current (A)
VIN = 12V
3.300
3.278
3.278
3.256
3.234
3.234
0.2
0.4
0.6
0.8
1
VIN = 24V
3.300
3.256
0
VIN = 12V
3.322
1.2
IOUT = 0A
-50
-25
0
25
50
75
100
125
Temperature (°C)
Output Current (A)
Frequency vs. Temperature
Quiescent Current vs. Temperature
600
750
Quiescent Current (μA)
Frequency (kHz)1
575
700
650
600
550
VIN = 24V
525
500
VIN = 12V
475
450
425
VIN = 12V, VOUT = 3.3V, IOUT = 0A
550
VEN = 2V, VFB = 1V
400
-50
-25
0
25
50
Temperature (°C)
DS8258-02 March 2011
75
100
125
-50
-25
0
25
50
75
100
125
Temperature (°C)
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RT8258
Power Off from EN
Power On from EN
VIN = 12V, VOUT = 3.3V, IOUT = 2A
VEN
(2V/Div)
VEN
(2V/Div)
VOUT
(1V/Div)
VOUT
(1V/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 1.2A
Time (100μs/Div)
Time (50μs/Div)
Output Ripple Voltage
Output Ripple Voltage
VOUT
(10mV/Div)
VOUT
(10mV/Div)
VPHASE
(10V/Div)
VPHASE
(10V/Div)
IL
(1A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 1.2A
Time (500ns/Div)
Load Transient Response
Load Transient Response
VOUT
(0.1V/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 0.6A to 1.2A
Time (50μs/Div)
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VIN = 24V, VOUT = 3.3V, IOUT = 1.2A
Time (500ns/Div)
VOUT
(0.1V/Div)
IOUT
(0.5A/Div)
IL
(1A/Div)
IOUT
(0.5A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 0A to 1.2A
Time (250μs/Div)
DS8258-02 March 2011
RT8258
Application Information
The RT8258 is an asynchronous high voltage buck converter
that can support the input voltage range from 4.5V to 24V
and the output current can be up to 1.2A.
Inductor Selection
Output Voltage Setting
and decreases with higher inductance.
The resistive voltage divider allows the FB pin to sense a
fraction of the output voltage as shown in Figure 1.
VOUT
R1
FB
RT8258
R2
GND
Figure 1. Output Voltage Setting
For adjustable voltage mode, the output voltage is set by
an external resistive voltage divider according to the
following equation :
VOUT = VFB ⎛⎜ 1+ R1 ⎞⎟
⎝ R2 ⎠
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ΔIL increases with higher VIN
V
V
ΔIL = ⎡⎢ OUT ⎤⎥ × ⎡⎢1− OUT ⎤⎥
f
×
L
VIN ⎦
⎣
⎦ ⎣
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
highest efficiency operation. However, it requires a large
inductor to achieve this goal.
For the ripple current selection, the value of ΔIL = 0.34(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 :
Where VFB is the feedback reference voltage (0.8V typ.).
⎡ VOUT ⎤ ⎡
VOUT ⎤
L =⎢
⎥ × ⎢1− VIN(MAX) ⎥
f
×
Δ
I
L(MAX)
⎣
⎦ ⎣
⎦
External Bootstrap Diode
Inductor Core Selection
Connect a 10nF low ESR ceramic capacitor between the
BOOT pin and PHASE pin. This capacitor provides the
gate driver voltage for the high side MOSFET.
The inductor type must be selected once the value for L is
known. Generally speaking, high efficiency converters can
not afford the core loss found in low cost powdered iron
cores. So, the more expensive ferrite or mollypermalloy
cores will be a better choice.
It is recommended to add an external bootstrap diode
between an external 5V and the BOOT pin for efficiency
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 as 1N4148 or BAT54.
The external 5V can be a 5V fixed input from system or a
5V output of the RT8268.
5V
BOOT
RT8258
10nF
PHASE
Figure 2. External Bootstrap Diode
DS8258-02 March 2011
The selected inductance rather than the core size for a
fixed inductor value is the key for actual core loss. As the
inductance increases, core losses decrease. Unfortunately,
increase of the inductance requires more turns of wire and
therefore the copper losses will increase.
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.
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design current
is exceeded. The previous situation results in an abrupt
increase in inductor ripple current and consequent output
voltage ripple.
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RT8258
Do not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy materials
are small and do not radiate 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 and any radiated field/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 and 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, and current rating
should be greater than the maximum load current. For
more detail, please refer to Table 3.
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the trapezoidal
current at the source of the top MOSFET. To prevent large
ripple current, a low ESR input capacitor sized for the
maximum RMS current should be used. The RMS current
is given by :
V
VIN
IRMS = IOUT(MAX) OUT
−1
VIN
VOUT
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.
The selection of COUT is determined by the required Effective
Series Resistance (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
response as described in a later section.
The output ripple, ΔVOUT , is determined by :
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8
1
⎤
ΔVOUT ≤ ΔIL ⎡⎢ESR +
8fCOUT ⎥⎦
⎣
The output ripple will be highest at the maximum input
voltage since ΔIL increases with input voltage. Multiple
capacitors placed 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
capacitance density, it is important to only use types that
pass the surge test for use in switching power supplies.
Aluminum electrolytic capacitors have significantly higher
ESR. However, it can be used in cost-sensitive applications
for ripple current rating and long term reliability
considerations. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient and
audible piezoelectric effects. The high Q of ceramic
capacitors with trace inductance can also lead to significant
ringing.
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case 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 and
the power is supplied by a wall adapter through long wires,
a load step at the output can 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 damage the part.
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
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.
DS8258-02 March 2011
RT8258
Thermal Considerations
Layout Consideration
For continuous operation, do not exceed the maximum
operation junction temperature 125°C. 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 :
Follow the PCB layout guidelines for optimal performance
of RT8258.
PD(MAX) = (TJ(MAX) − TA ) / θJA
`
Keep the traces of the main current paths as short and
wide as possible.
`
Put the input capacitor as close as possible to the device
pins (VIN and GND).
`
PHASE node is with high frequency voltage swing and
should be kept at small area. Keep sensitive components
away from the PHASE node to prevent stray capacitive
noise pick-up.
`
Place the feedback components to the FB pin as close
as possible.
`
Connect the GND to a ground plane for noise reduction
and thermal dissipation.
where T J(MAX) is the maximum operation junction
temperature, TA is the ambient temperature and the θJA is
the junction to ambient thermal resistance.
For recommended operating conditions specification of the
RT8258, the maximum junction temperature of the die is
125°C. The junction to ambient thermal resistance θJA is
layout dependent. For T/SOT-23-6 package, the thermal
resistance θJA is 250°C/W on standard JEDEC 51-3 single
COUT
layer thermal test board. The maximum power dissipation
at TA = 25°C can be calculated by following formula :
VOUT
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal resistance
θJA . For RT8258 package, the Figure 3 of derating curve
allows the designer to see the effect of rising ambient
temperature on the maximum power dissipation allowed.
Maximum Power Dissipation (W)
0.50
L
CB
PD(MAX) = (125°C − 25°C) / (250°C/W) = 0.4W for
T/SOT-23-6 package
GND
D1
BOOT
1
6
PHASE
GND
2
5
VIN
FB
3
4
EN
CIN
R2
VOUT
R1
Figure 4. PCB Layout Guide
Single Layer PCB
0.45
0.40
0.35
0.30
T/SOT-23-6
0.25
0.20
0.15
0.10
0.05
0.00
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 3. Derating Curve for RT8258 Package
DS8258-02 March 2011
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RT8258
Table 2. Suggested Inductors for L
Component Supplier
Series
Dimensions (mm)
TDK
SLF12555T
12.5x12.5x5.5
TAIYO YUDEN
NR8040
8x8x4
TDK
SLF12565T
12.5x12.5x6.5
Table 3. Suggested Capacitors for CIN and COUT
Capacitance
Case Size
(μF)
GRM31CR61E106K
10
1206
Location
Component Supplier
Part No.
CIN
MURATA
CIN
TDK
C3225X5R1E106K
10
1206
CIN
TAIYO YUDEN
TMK316BJ106ML
10
1206
COUT
MURATA
GRM31CR61C226M
22
1206
COUT
TDK
C3225X5R1C226M
22
1206
COUT
TAIYO YUDEN
EMK316BJ226ML
22
1206
Table 4. Suggested Diode for D1
Component Supplier
Series
DIODES
DIODES
PANJIT
PANJIT
B230A
B330A
SK23
SK33
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10
VRRM
(V)
30
30
30
30
IOUT
(A)
2
3
2
3
Package
DO-214AC
DO-214AC
DO-214AC
DO-214AB
DS8258-02 March 2011
RT8258
Outline Dimension
H
D
L
C
B
b
A
A1
e
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
0.889
1.295
0.031
0.051
A1
0.000
0.152
0.000
0.006
B
1.397
1.803
0.055
0.071
b
0.250
0.560
0.010
0.022
C
2.591
2.997
0.102
0.118
D
2.692
3.099
0.106
0.122
e
0.838
1.041
0.033
0.041
H
0.080
0.254
0.003
0.010
L
0.300
0.610
0.012
0.024
SOT-23-6 Surface Mount Package
DS8258-02 March 2011
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RT8258
H
D
L
C
B
b
A
A1
e
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
0.700
1.000
0.028
0.039
A1
0.000
0.100
0.000
0.004
B
1.397
1.803
0.055
0.071
b
0.300
0.559
0.012
0.022
C
2.591
3.000
0.102
0.118
D
2.692
3.099
0.106
0.122
e
0.838
1.041
0.033
0.041
H
0.080
0.254
0.003
0.010
L
0.300
0.610
0.012
0.024
TSOT-23-6 Surface Mount Package
Richtek Technology Corporation
Richtek Technology Corporation
Headquarter
Taipei Office (Marketing)
5F, No. 20, Taiyuen Street, Chupei City
5F, No. 95, Minchiuan Road, Hsintien City
Hsinchu, Taiwan, R.O.C.
Taipei County, Taiwan, R.O.C.
Tel: (8863)5526789 Fax: (8863)5526611
Tel: (8862)86672399 Fax: (8862)86672377
Email: marketing@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.
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DS8258-02 March 2011