RT8258
1.2A, 24V, 700kHz Step-Down Converter
General Description
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.
The chip also provides protection functions such as cycle-
by-cycle current limiting and thermal shutdown protection.
The RT8258 is available in a SOT-23-6 and TSOT-23-6
packages.
Features
Wide Operating Input Voltage Range : 4.5V to 24V
Adjustable Output Voltage Range : 0.8V to 15V
1.2A Output Current
0.3Ω Internal Power MOSFET Switch
Ω
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
Ordering Information
RT8258
Package Type
E : SOT-23-6
J6 : TSOT-23-6
Lead Plating System
G : Green (Halogen Free and Pb Free)
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.
Applications
Distributed Power Systems
Battery Charger
Pre-Regulator for Linear Regulators
WLED Drivers
Pin Configurations
(TOP VIEW)
PHASE VIN EN
6
5
2
4
3
Marking Information
For marking information, contact our sales representative
directly or through a Richtek distributor located in your
area.
BOOT GND FB
SOT-23-6/TSOT-23-6
Typical Application Circuit
V
IN
4.5V to 24V
C
IN
10µF
Chip Enable
5 VIN
BOOT
1
C
BOOT
L
10nF 10µH
D1
B230A
V
OUT
3.3V
RT8258
PHASE 6
4 EN
GND
2
FB 3
R1
100k
R2
32.4k
C
OUT
22µF
DS8258-02 March 2011
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1
RT8258
Table 1. Recommended Component Selection
V
OUT
R1 (kΩ)
R2 (kΩ)
L (μH)
C
OUT
(μF)
1.2V
100
200
3.6
22
1.5V
91
100
3.6
22
1.8V
91
75
4.7
22
2.5V
100
47
6.8
22
3.3V
100
32.4
10
22
5V
91
17.4
15
22
8V
91
10
22
22
10V
91
7.87
22
22
15V
120
6.8
33
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
1µA
X20
Current Sense Amp
-
+
25mΩ
Regulator
10k
3V
1.1V
-
Ramp
Generator
Oscillator
700kHz
S
+
EA
-
+
-
Q
R
PHASE
Bootstrap
Control
OC Limit Clamp
1pF
Driver
BOOT
EN
+
Shutdown Reference
Comparator
FB
400k
GND
30pF
PWM
Comparator
Functional Pin Description
Pin No.
1
2
3
4
5
6
Pin Name
BOOT
GND
FB
EN
VIN
PHASE
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.
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2
DS8258-02 March 2011
RT8258
Absolute Maximum Ratings
(Note 1)
26V
−0.3V
to (V
IN
+ 0.3V)
V
PHASE
+ 6V
0.3V to 6V
0.4W
Supply Voltage, V
IN
------------------------------------------------------------------------------------------------
PHASE Voltage -----------------------------------------------------------------------------------------------------
BOOT Voltage -------------------------------------------------------------------------------------------------------
All Other Pins --------------------------------------------------------------------------------------------------------
Power Dissipation, P
D
@ T
A
= 25°C
T/SOT-23-6 -----------------------------------------------------------------------------------------------------------
Package Thermal Resistance (Note 2)
T/SOT-23-6,
θ
JA
------------------------------------------------------------------------------------------------------ 250°C/W
Junction Temperature ---------------------------------------------------------------------------------------------- 150°C
Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------ 260°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)
4.5V to 24V
0.8V to 15V
0V to 5.5V
−
40°C to 125°C
−
40°C to 85°C
Supply Voltage, V
IN
------------------------------------------------------------------------------------------------
Output Voltage, V
OUT
----------------------------------------------------------------------------------------------
EN Voltage, V
EN
-----------------------------------------------------------------------------------------------------
Junction Temperature Range -------------------------------------------------------------------------------------
Ambient Temperature Range -------------------------------------------------------------------------------------
Electrical Characteristics
Parameter
Feedback Reference Voltage
Feedback Current
Switch On Resistance
Switch Leakage
Current Limit
Oscillator Frequency
Maximum Duty Cycle
Minimum On-Time
Under Voltage Lockout
Threshold Voltage
Under Voltage Lockout
Threshold Hysteresis
Logic High
EN Input Voltage
Logic Low
EN Pull Up Current
Shutdown Current
Quiescent Current
Thermal Shutdown
DS8258-02 March 2011
(V
IN
= 12V, T
A
= 25° C unless otherwise specified)
Symbol
V
FB
I
FB
R
DS(ON)
Test Conditions
4.5V
≤
V
IN
≤
24V
V
FB
= 0.8V
V
EN
= 0V, V
PHASE
= 0V
Min
0.784
--
--
--
1.6
600
--
--
Typ
0.8
0.1
0.3
--
2.1
700
90
100
4.2
270
--
--
1
25
0.55
150
Max
0.816
0.3
--
10
--
800
--
--
4.5
--
--
0.4
--
--
1
--
Unit
V
μA
Ω
μA
A
kHz
%
ns
V
mV
V
μA
μA
mA
°C
I
LIM
f
SW
t
ON
V
BOOT
−
V
PHASE
= 4.8V
Rising
3.9
--
1.4
--
V
EN
= 0V
I
SHDN
I
Q
T
SD
V
EN
= 0V
V
EN
= 2V, V
FB
= 1V (No Switching)
--
--
--
--
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3
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.
θ
JA
is measured in the natural convection at T
A
= 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
80
Efficiency vs. Output Current
100
90
80
V
IN
= 12V
V
IN
= 24V
V
IN
= 24V
V
IN
= 12V
Efficiency (%)
60
50
40
30
20
10
0
0
0.2
0.4
0.6
0.8
Efficiency (%)
70
70
60
50
40
30
20
V
OUT
= 5V
1
1.2
10
0
0
0.2
0.4
0.6
0.8
V
OUT
= 3.3V
1
1.2
Output Current (A)
Load Current (A)
Output Voltage vs. Output Current
3.366
3.344
Output Voltage vs. Temperature
3.366
3.344
Output Voltage (V)
Output Voltage (V)
V
IN
= 24V
3.322
3.300
3.278
3.256
3.234
0
0.2
0.4
0.6
0.8
1
1.2
V
IN
= 12V
3.322
3.300
3.278
3.256
3.234
V
IN
= 12V
V
IN
= 24V
I
OUT
= 0A
-50
-25
0
25
50
75
100
125
Output Current (A)
Temperature (°C)
Frequency vs. Temperature
750
600
575
Quiescent Current vs. Temperature
Quiescent Current (μA)
Frequency (kHz)1
700
550
525
500
475
450
425
V
IN
= 24V
650
V
IN
= 12V
600
V
IN
= 12V, V
OUT
= 3.3V, I
OUT
= 0A
550
-50
-25
0
25
50
75
100
125
400
-50
-25
0
25
50
V
EN
= 2V, V
FB
= 1V
75
100
125
Temperature (°C)
Temperature (°C)
DS8258-02 March 2011
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