RT8259
1.2A, 24V, 1.4MHz Step-Down Converter
General Description
The RT8259 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 RT8259 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%
1.4MHz 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
RT8259
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
6
5
2
BOOT GND
EN
4
3
FB
Marking Information
For marking information, contact our sales representative
directly or through a Richtek distributor located in your
area.
SOT-23-6/TSOT-23-6
Typical Application Circuit
V
IN
4.5V to 24V
5 VIN
C1
10µF
Chip Enable
Open =
Automatic Startup
BOOT
1
CB
10nF
D1
B230A
L1
4.7µH
R1
62k
C2
22µF
R2
19.6k
V
OUT
3.3V
RT8259
PHASE 6
4 EN
GND
2
FB 3
DS8259-03 March 2011
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1
RT8259
μ
Table 1. Recommended Component Selection, C2 = 22μF
V
OUT
(V)
L1 (μH)
R1 (kΩ)
R2 (kΩ)
1.2
2
62
124
1.8
2
62
49.9
2.5
3.6
62
29.4
3.3
4.7
62
19.6
5
6.8
62
12
8
10
68
7.5
10
10
68
5.9
15
15
68
3.9
Functional Pin Description
Pin No.
1
Pin Name
BOOT
Pin Function
Bootstrap. A capacitor is connected between PHASE and BOOT pins to form a floating
supply across the power switch driver. This capacitor is needed to drive the power switch‘s
gate above the supply voltage.
Ground. This pin is the voltage reference for the regulated output voltage. For this reason,
care must be taken in its layout. This node should be placed outside of the D1 to C1 ground
path to prevent switching current spikes from inducing voltage noise into the part.
Feedback. 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.
Supply Voltage. Bypass VIN to GND with a suitable large capacitor to prevent large voltage
spikes from appearing at the input.
Switch Output.
2
3
4
5
6
GND
FB
EN
VIN
PHASE
Function Block Diagram
VIN
1µA
X20
Current Sense Amp
-
+
Ω
25mΩ
Regulator
10k
EN
3V
1.1V
-
Ramp
Generator
BOOT
Oscillator
1.4MHz
S
Q
+
EA
-
+
-
R
PHASE
Bootstrap
Control
OC Limit Clamp
1pF
Driver
+
Shutdown Reference
Comparator
FB
400k
GND
30pF
PWM
Comparator
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2
DS8259-03 March 2011
RT8259
Absolute Maximum Ratings
(Note 1)
26V
−0.3V
to (V
IN
+ 0.3V)
V
PHASE
+ 6V
0.3V to 6V
−0.3V
to 15V
0.4W
Supply Voltage, V
IN
------------------------------------------------------------------------------------------------
PHASE Voltage -----------------------------------------------------------------------------------------------------
BOOT Voltage -------------------------------------------------------------------------------------------------------
All Other Pins --------------------------------------------------------------------------------------------------------
Output Voltage ------------------------------------------------------------------------------------------------------
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
(V
IN
= 12V, T
A
= 25° C unless otherwise specified)
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
Under Voltage Lockout
Threshold Hysteresis
Logic-High
EN Input Voltage
Logic-Low
EN Pull Up Current
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
1.2
--
--
Typ
0.8
0.1
0.3
--
2.1
1.4
80
100
4.2
200
--
--
1
Max
0.816
0.3
--
10
--
1.6
--
--
4.5
--
--
0.4
--
Unit
V
μA
Ω
μA
A
MHz
%
ns
V
mV
V
μA
I
LIM
f
SW
t
ON
V
BOOT
−
V
PHASE
= 4.8V
Rising
3.9
--
V
IH
V
IL
V
EN
= 0V
1.4
--
--
To be continued
DS8259-03 March 2011
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3
RT8259
Parameter
Shutdown Current
Quiescent Current
Thermal Shutdown
Symbol
I
SHDN
I
Q
T
SD
V
EN
= 0V
V
EN
= 2V, V
FB
= 1V (Not Switching)
Test Conditions
Min
--
--
--
Typ
25
0.55
150
Max
--
1
--
Unit
μA
mA
°C
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 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.
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DS8259-03 March 2011
RT8259
Typical Operating Characteristics
Efficiency vs. Load Current
100
90
80
Efficiency vs. Load Current
100
90
V
IN
= 12V
V
IN
= 24V
V
IN
= 12V
V
IN
= 24V
80
Efficiency (%)
Efficiency (%)
70
60
50
40
30
20
10
70
60
50
40
30
20
10
V
OUT
= 5V
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1.1 1.2
V
OUT
= 3.3V
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1.1 1.2
0
Load Current (A)
Load Current (A)
Output Voltage vs. Load Current
3.333
3.328
3.38
3.36
Output Voltage vs. Temperature
Output Voltage (V)
3.323
3.318
3.313
3.308
3.303
3.298
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1.1 1.2
Output Voltage (V)
3.34
3.32
3.30
3.28
3.26
3.24
V
IN
= 24V
V
IN
= 24V
V
IN
= 12V
V
IN
= 12V
I
OUT
= 0A
3.22
-50
-25
0
25
50
75
100
125
Load Current (A)
Temperature (°C)
Frequency vs. Input Voltage
1.55
1.50
Frequency vs. Temperature
1.55
1.50
1.40
1.35
1.30
1.25
1.20
Frequency (MHz)1
V
OUT
= 3.3V, I
OUT
= 0.3A
Frequency (MHz)
1.45
1.45
1.40
1.35
1.30
1.25
1.20
V
IN
= 12V, V
OUT
= 3.3V, I
OUT
= 0.3A
1.15
-50
-25
0
25
50
75
100
125
1.15
4
6.5
9
11.5
14
16.5
19
21.5
24
Input Voltage (V)
Temperature (°C)
DS8259-03 March 2011
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