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RS6508A
2A, 20V, 1.2MHz DC/DC Asynchronous Step‐Down Converter
General Description
The RS6508A is a high‐efficiency asynchronous step‐down DC/DC converter that can deliver up to 2A output current from
4.75V to 20V input supply. The RS6508A's current mode architecture and external compensation allow the transient
response to be optimized over a wide range of loads and output capacitors. Cycle‐by‐cycle current limit provides protection
against shorted outputs and soft‐start eliminates input current surge during start‐up.
The RS6508A also provides output under voltage protection and thermal shutdown protection. The low current (<30μA)
shutdown mode provides output disconnection, enabling easy power management in battery‐powered systems.
Features
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2A Output Current
Up to 93% Efficiency
Integrated 100mΩ Power MOSFET Switches
Fixed 1.2MHz Frequency
Cycle‐by‐Cycle Over Current Protection
Thermal Shutdown function
Wide 4.75V to 20V Operating Input Range
Output Adjustable from 0.925V to 18V
Programmable Under Voltage Lockout
Available in an SOP‐8 Package
RoHS Compliant and 100% Lead (Pb)‐Free and Green
(Halogen Free with Commercial Standard)
Applications
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PC Motherboard, Graphic Card
LCD Monitor
Set‐Top Boxes
DVD‐Video Player
Telecom Equipment
ADSL Modem
Printer and other Peripheral Equipment
Microprocessor core supply
Networking power supply
Pre‐Regulator for Linear Regulators
Green Electronics/Appliances
Application Circuits
C5
1
INPUT
4.75V to 21V
C1
10uF/35V
CERAMIC x2
OFF ON
BS
2
7
8
C4
0.1uF
4
U1
IN
EN
SS
GND
10nF
SW
FB
COMP
3
5
6
C6
R3
5.6KΩ
1nF
C3
8.2nF
R2
10KΩ 1%
D1
B340A
R1
25.8KΩ 1%
C2
22uF/6.3V
CERAMIC x2
L1
4.7uH
OUTPUT
3.3V/2A
RS6508A‐ADS
This integrated circuit can be damaged by ESD. Orister Corporation recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more
susceptible to damage because very small parametric changes could cause the device not to meet its published specifications.
DS‐RS6508A‐05
February, 2010
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Pin Assignments
SOP‐8
PACKAGE
PIN
1
SYMBOL
BS
DESCRIPTION
Bootstrap. This capacitor (C5) is needed to drive the power switch’s gate above
the supply voltage. It is connected between the SW and BS pins to form a floating
supply across the power switch driver. The voltage across C5 is about 5V and is
supplied by the internal +5V supply when the SW pin voltage is low.
Supply Voltage. The RS6508A operates from a 4.75V to 20V unregulated input.
C1 is needed to prevent large voltage spikes from appearing at the input.
Power Switching Output. SW is the switching node that supplies power to the
output. Connect the output LC filter from SW to the output load. Note that a
capacitor is required from SW to BS to power the high‐side switch.
Ground.
Feedback Input. FB senses the output voltage and regulates it. Drive FB with a
resistive voltage divider from the output voltage to ground. The feedback
threshold is 0.925V. See Setting the Output Voltage.
Compensation Node. COMP is used to compensate the regulation control loop.
Connect a series RC network from COMP to GND. In some cases, an additional
capacitor from COMP to GND is required. See Compensation.
Enable Input. EN is a digital input that turns the regulator on or off. Drive EN high
to turn on the regulator, drive it low to turn it off. For automatic startup, leave
EN unconnected.
Soft‐Start. Connect SS to an external capacitor to program the soft‐start. If
unused, leave it open.
2
3
4
SOP‐8
5
IN
SW
GND
FB
6
COMP
7
8
EN
SS
Ordering Information
DEVICE
DEVICE CODE
XX is nominal output voltage :
AD : ADJ
Y is package & Pin Assignments designator :
S : SOP‐8
Z is Lead Free designator :
P: Commercial Standard, Lead (Pb) Free and Phosphorous (P) Free Package
G: Green (Halogen Free with Commercial Standard)
RS6508A‐XX Y Z
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February, 2010
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Block Diagram
Absolute Maximum Ratings
Symbol
V
IN
V
SW
V
BS
V
FB
V
EN
V
COMP
V
SS
T
J
T
OPR
T
STG
T
LEAD
Parameter
Supply Voltage
SW Pin Voltage
Boot Strap Voltage
Feedback Voltage
Enable/UVLO Voltage
Comp Voltage
SS Voltage
Junction Temperature
Operating Temperature Range
Storage Temperature Range
Lead Temperature
Range
‐0.3 to +21
‐0.3 to V
IN
+0.3
V
SW
‐0.3 to V
SW
+6
‐0.3 to +6
‐0.3 to +6
‐0.3 to +6
‐0.3 to +6
150
‐20 to +85
‐65 to +150
260
Units
V
V
V
V
V
V
V
o
C
o
C
o
C
o
C
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Electrical Characteristics
(V
IN
=12V, T
A
=25°C, unless otherwise specified)
Symbol
V
IN
V
FB
R
DS(ON)1
R
DS(ON)2
I
Sw
I
LIM
G
CS
A
VEA
G
EA
F
S
F
OSC1
‐
D
MAX
t
ON
‐
‐
‐
‐
I
SD
I
Q
T
SD
Notes:
1.
2.
Slope compensation changes current limit above 40% duty cycle.
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.
Devices are ESD sensitive. Handling precaution is recommended.
The device is not guaranteed to function outside its operating conditions.
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.
Parameter
Input Voltage
Feedback Voltage
Upper Switch On Resistance
Lower Switch On Resistance
Upper Switch Leakage
Current Limit
(NOTE 1)
Current Sense Transconductance Output
Current to Comp Pin Voltage
Error Amplifier Voltage Gain
Error Amplifier Transconductance
Oscillator Frequency
Short Circuit Frequency
Soft‐Start Pin Equivalent Output Resistance
Maximum Duty Cycle
Minimum On Time
EN Shutdown Threshold
Enable Pull Up Current
EN UVLO Threshold Rising
EN UVLO Threshold Hysteresis
Supply Current (Shutdown)
Supply Current (Quiescent)
Thermal Shutdown
Conditions
‐
4.75V ≤ V
IN
≤ 20V
‐
‐
V
EN
= 0V, V
SW
= 0V
‐
‐
‐
‐
‐
V
FB
= 0V
‐
V
FB
= 1.0V
‐
I
CC
＞100uA
V
EN
= 0V
V
IN
Rising
‐
V
IN
≤0.4V
V
EN
≥3V
‐
Min. Typ. Max.
4.75
‐
20
0.898 0.925 0.952
‐
0.22
‐
‐
10
‐
‐
‐
10
‐
3.8
‐
‐
‐
550
‐
‐
‐
‐
‐
0.7
‐
2.35
‐
‐
‐
‐
1.95
400
830
1.2
240
9
90
100
1.0
1.0
2.50
200
23
1.1
160
‐
‐
1150
‐
‐
‐
‐
‐
1.3
‐
2.65
‐
36
1.3
‐
Unit
V
V
Ω
Ω
uA
A
A/V
V/V
uA/V
MHz
KHz
KΩ
%
ns
V
uA
V
mV
uA
mA
o
C
3.
4.
5.
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Detail Description
The RS6508A is a synchronous high voltage buck converter that can support the input voltage range from 4.75V to 20V and
the output current can be up to 2A.
Output Voltage Setting
The resistive divider allows the FB pin to sense the output voltage as shown in Figure 1.
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive divider according to the following equation：
V
OUT
=
V
FB
⎜
1
+
⎝
⎛
R1
⎞
⎟
R2
⎠
Where VFB is the feedback reference voltage (0.925V typ.).
External Bootstrap Diode
Connect a 10nF low ESR ceramic capacitor between the BOOT pin and SW pin. This capacitor provides the gate driver voltage
for the high side MOSFET.
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.
Soft‐Start
The RS6508A contains an external soft‐start clamp that gradually raises the output voltage. The soft‐start timing can be
programmed by the external capacitor between SS pin and GND. The chip provides a 7μA charge current for the external
capacitor. If a 0.1μF capacitor is used to set the soft‐start and its period will be 13ms(typ.).
Inductor Selection
The inductor value and operating frequency determine the ripple current according to a specific input and output voltage.
The ripple current ΔI
L
increases with higher V
IN
and decreases with higher inductance.
V
OUT
⎡
V
OUT
⎤ ⎡
Δ
I
L
= ⎢
× ⎢
1
−
⎥
V
IN
⎣
f
×
L
⎦ ⎣
⎤
⎥
⎦
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 ΔI
L
= 0.2375(I
MAX
) will be a reasonable starting point. The largest ripple current
occurs at the highest V
IN
. To guarantee that the ripple current stays below the specified maximum, the inductor value should
be chosen according to the following equation：
V
OUT
V
OUT
⎤
⎡
⎤ ⎡
L
=⎢
× ⎢
1
−
f
× Δ
I
L
(
MAX
)
⎥ ⎣
V
IN
(
MAX
)
⎥
⎣
⎦
⎦
DS‐RS6508A‐05
February, 2010
www.Orister.com
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