DS8481A 00

®
RT8481A
SPS Secondary-Side CC/CV Controller
General Description
Features
The RT8481A is a secondary-side CC/CV controller for
SPS applications. It integrates a Constant Current (CC)
regulating amplifier, a Constant Voltage (CV) regulating
amplifier, and 2 precision reference voltages.

Secondary-Side Constant Voltage (CV) and Constant
Current (CC) Control

4.75V to 50V Operation Voltage Range
±1% Output Voltage Accuracy at Full Temperature
Range
0.6mA Quiescent Current
Smooth Transition Between CC and CV Control
Loops
−5V Negative Voltage Tolerance at CP pin
The CC regulating amplifier is featured with an extended
input common mode voltage below GND level to insure
the performance of low-side current sense, and a very low
input offset voltage to guarantee the sensing accuracy. A
60mV reference voltage is internally connected between
the inverting input of the CC regulating amplifier and the
CP pin. The non-inverting input is the CN pin, at which the
voltage will be regulated 60mV higher than that at the CP
pin. The inverting input pin CP is equiped with −5V antireverse immunity.




Applications



Battery Chargers
AC/DC Adaptors
LED Drivers
The CV regulating amplifier with low input offset voltage is
biased with a 2.5V reference voltage at the inverting input.
The non-inverting input is the FB pin, at which the voltage
will be regulated with 2.5V from GND. The CC and CV
amplifiers share an open-collector output pin to minimize
application circuit.
The RT8481A is available in the SOT-23-6 package.
Simplified Application Circuit
VOUT
Opto
VCC
R1
RVC1 CVC1
RT8481A
OUT
FB
CIC1
CP
CN
GND
R2
RIC1
RIC2
RS
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
DS8481A-00 January 2016
GND
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RT8481A
Ordering Information
Pin Configurations
RT8481A
(TOP VIEW)
Package Type
E : SOT-23-6
Lead Plating System
G : Green (Halogen Free and Pb Free)
VCC OUT CP
6
Note :
4
2
3
CN GND FB
Richtek products are :

5
SOT-23-6
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.

Suitable for use in SnPb or Pb-free soldering processes.
Marking Information
3Y= : Product Code
3Y=DNN
DNN : Date Code
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
CN
Non-inverting Input of the CC Regulating Amp. It has 60mV offset from the CP pin. The
CN pin should be connected to the “current-in” node of the current sensing resistor, Rs.
2
GND
Ground.
3
FB
Non-inverting Input of the CV Regulating Amp. The pin should be connected to the
mid-point of a resistor divider from “Secondary Side VOUT” (usually the VCC) to GND.
4
CP
Inverting Input of the CC Regulating Amp with 60mV offset from CN pin. The CP pin
should be connected to the “current-out” node of the current sensing resistor.
5
OUT
Common Open-collector Output of CC and CV Regulating Amps. The pin sinks a
regulated current and driver the opto-coupler to transmit the error signal to primary-side.
6
VCC
Supply Voltage Input. A 0.1F bypass capacitor should be connected between VCC and
GND.
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DS8481A-00 January 2016
RT8481A
Function Block Diagram & Typical Application Circuit
VOUT
Opto
VCC
R1
RVC1 CVC1
RT8481A
2.5V
OUT
- CV
+
FB
60mV
- CC
CIC1
+
CP
CN
GND
R2
RIC1
RIC2
RS
GND
Operation
The available input voltage range is from 4.75V to 50V for
the RT8481A. An internal 2.5V reference voltage is
generated from VCC input power. The RT8481A can be
used to monitor the transformer secondary-side output
voltage by the CV control loop and regulate the output
current by the CC control loop at the same time.
The transformer secondary-side output voltage can be
monitored by the FB pin voltage. The sensed FB pin
voltage is compared with the 2.5V internal reference. When
the FB pin voltage is higher than 2.5V, the OUT pin will
sink more current at the external opto-coupler and instruct
the controller at primary-side to adjust the output voltage.
The output current can be regulated by the voltage across
the CN and CP pins through the current sense resistor
connected between the CN and CP pins. The voltage
difference between CN and CP pins is compared with the
60mV internal reference. When the voltage difference
between CN and CP pins is greater than 60mV, the OUT
pin will sink more current at the external opto-coupler and
instruct the controller at primary-side to adjust the output
current.
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
DS8481A-00 January 2016
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3
RT8481A
Absolute Maximum Ratings
(Note 1)
Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------CP --------------------------------------------------------------------------------------------------------------------------- CN --------------------------------------------------------------------------------------------------------------------------- FB --------------------------------------------------------------------------------------------------------------------------- OUT ------------------------------------------------------------------------------------------------------------------------- OUT Current -------------------------------------------------------------------------------------------------------------- Power Dissipation, PD @ TA = 25°C
SOT-23-6 ------------------------------------------------------------------------------------------------------------------ Package Thermal Resistance (Note 2)
SOT-23-6, θJA ------------------------------------------------------------------------------------------------------------- Lead Temperature (Soldering, 10 sec.) ----------------------------------------------------------------------------- Junction Temperature --------------------------------------------------------------------------------------------------- Storage Temperature Range ------------------------------------------------------------------------------------------ ESD Susceptibility (Note 3)
HBM (Human Body Model) --------------------------------------------------------------------------------------------MM (Machine Model) ---------------------------------------------------------------------------------------------------

Recommended Operating Conditions



−0.3V to 60V
−5V to 1V
−0.3V to 1V
−0.3V to VCC
−0.3V to 50V
−20mA to 20mA
0.41W
243.3°C/W
260°C
150°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Input Voltage, VCC (Note 5) -------------------------------------------------------------------------------- 4.75 to 50V
Junction Temperature Range ------------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range ------------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VCC = 12V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Condition
Min
Typ
Max
Unit
Device Supply
Quiescent Current
ICC
CV Close Loop, VCN = VCP = 0V
--
500
600
CV Close Loop, VCN = VCP = 0V,
TA = 25°C to 105°C
--
--
800
A
Voltage Control Loop OP Amp
Transconductance
Power Supply Rejection
Rate
GMv
VCC = 4.75V to 45V
--
1
--
S
PSRR
VCC = 4.75V to 45V
--
60
--
dB
FB Voltage
VFB
VCN = VCP = 0V
2.487
2.5
2.513
VCN = VCP = 0V, TA = 25°C to 105°C
2.475
--
2.525
FB Line Regulation
dVLINE-FB
FB Input Bias Current
IFB
VCN = VCP = 0V, VCC = 4.75V to 45V
--
0.2
--
VFB = 2.4 to 2.6V
--
--
100
VFB = 2.4 to 2.6V, TA = 25°C to 105°C
--
--
200
VCC = 4.75V to 45V
--
6
--
VFB = 2.4V
59
61
63
TA = 25°C to 105°C
58
--
64
V
%
nA
Current Control Loop
Transconductance
CN – CP Voltage
VCN-CP
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S
mV
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DS8481A-00 January 2016
RT8481A
Parameter
Symbol
Test Condition
Min
Typ
Max
Unit
--
0.2
--
%/V
Close Loop
--
--
100
Close Loop, TA = 25°C to 105°C
--
--
200
VOUT = 1.5V
VOUT = 1.5V, TA = 25°C to 105°C
IOUT = 2mA
IOUT = 2mA, TA = 25°C to 105°C
-----
8
8
1
--
--1.2
1.5
CN – CP Line Regulation dVLINE-CN-CP VFB = 2.4V, VCC = 4.75V to 45V
CN Input Bias Current
Output Stage
OUT Maximum Sink
Current
OUT Minimum Voltage
ICN
IOUTH
VOUTL
nA
mA
V
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and 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 may affect
device reliability.
Note 2. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Note 5. RT8481A starts regulation at VCC ≥ 4.5V, and meets all parameter specs at VCC ≥ 4.75V.
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
DS8481A-00 January 2016
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RT8481A
Typical Operating Characteristics
Quiescent Current vs. Temperature
520
2.515
500
480
VCC
VCC
VCC
VCC
VCC
VCC
460
440
420
=
=
=
=
=
=
2.505
VFB (V)
Quiescent Current (μA)
VFB vs. Temperature
2.525
540
4.75V
5V
12V
24V
36V
50V
VCC
VCC
VCC
VCC
VCC
VCC
2.495
2.485
=
=
=
=
=
=
4.75V
5V
12V
24V
36V
50V
2.475
400
-50
-25
0
25
50
75
100
-50
125
-25
0
25
50
75
100
125
Temperature (°C)
Temperature (°C)
IFB vs. Temperature
VOUTL vs. Temperature
120
1.0
0.9
100
0.8
0.7
VOUTL (V)
I FB (nA)
80
60
VCC
VCC
VCC
VCC
VCC
VCC
40
20
=
=
=
=
=
=
4.75V
5V
12V
24V
36V
50V
-25
0
0.6
VCC
VCC
VCC
VCC
VCC
VCC
0.5
0.4
0.3
0.2
=
=
=
=
=
=
0.1
0
0.0
-50
25
50
75
100
125
-50
-25
0
Temperature (°C)
25
50
75
100
125
100
125
Temperature (°C)
VCN-CP vs. Temperature
IOUTH vs. Temperature
16
62.5
14
62.0
12
61.5
VCN-CP (mV)
I OUTH (mA)
4.75V
5V
12V
24V
36V
50V
10
VCC
VCC
VCC
VCC
VCC
VCC
8
6
4
=
=
=
=
=
=
4.75V
5V
12V
24V
36V
50V
61.0
VCC
VCC
VCC
VCC
VCC
VCC
60.5
60.0
59.5
2
=
=
=
=
=
=
4.75V
5V
12V
24V
36V
50V
59.0
0
-50
-25
0
25
50
75
100
Temperature (°C)
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125
-50
-25
0
25
50
75
Temperature (°C)
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DS8481A-00 January 2016
RT8481A
GMi vs. Temperature
GMv vs. Temperature
20
10
18
9
GMi (mA/mV)
14
12
10
=
=
=
=
=
=
4.75V
5V
12V
24V
36V
50V
VCC
VCC
VCC
VCC
VCC
VCC
8
GMv (mA/mV)
VCC
VCC
VCC
VCC
VCC
VCC
16
8
6
7
6
5
4.75V
5V
12V
24V
36V
50V
4
3
4
2
2
1
0
=
=
=
=
=
=
0
-50
-25
0
25
50
75
100
Temperature (°C)
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DS8481A-00 January 2016
125
-50
-25
0
25
50
75
100
125
Temperature (°C)
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RT8481A
Application Information
Output Voltage Setting
The voltage control loop is controlled via the first transconductance operational amplifier. An optocoupler which
is directly connected to the output and an external resistor
bridge is connected between the output positive line and
the ground reference. The middle point is to be connected
to the FB pin of RT8481A, where R2 is the upper resistor
and R1 the lower resistor of the bridge. The relationship
between R2 and R1 is shown below :
VOUT = VFB 
R1 = R2 
R1 + R2 
R2
V
 OUT  VFB 
VFB
where VOUT is the desired maximum output voltage and
VFB is the feedback voltage (2.5V typ). When under
constant voltage control mode, the output voltage is fixed
due to the R1/R2 resistor bridge. To avoid discharge of
the load, the resistor bridge R1, R2, should be highly
resistive. For this type of application a total value of 100kΩ
(or more) would be appropriate for the resistors R1 and
R2.
As an example, with R1 = 80kΩ and R2 = 20kΩ,
VOUT = 12.5V
Output Current Setting
The current control loop is controlled via the second
transconductance operational amplifier. An optocoupler and
the sense resistor, Rs, is placed in series on the output
negative line. VCN−CP threshold is achieved externally by
a resistor bridge tied to the Vref voltage reference. Its
middle point is tied to the positive input of the current
control operational amplifier and its foot is to be connected
to the lower potential point of the sense resistor. The
resistors of the bridge are matched to provide the best
precision. With VCN−CP and Rs, the expected output
current IOUT can be obtained as below equation.
IOUT =
VCN-CP
RS
As an example, with RS = 200mΩ
VCN−CP = 60mV, IOUT = 300mA
where IOUT is the desired maximum output current, and
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VCN−CP the threshold voltage for the current control loop.
Note that RSENSE resistor should be chosen taking into
account its maximum power dissipation (PLIM) during full
load operation.
Compensation
Both the voltage control trans-conductance amplifier and
the current control trans-conductance amplifier can be fully
compensated. The output and negative inputs are directly
accessible for external compensation components, as
shown in the Typical Application Circuit.
The typical component values for the compensation
network of voltage control loop is CVC1 = 2.2nF and RVC1
= 22kΩ. The typical component values for the
compensation network of current control loop is CIC1 =
2.2nF, RIC1 = 22kΩ and RIC2 = 1kΩ. However, in many
application conditions, the current control loop can be
stable even without compensation network (RIC2 = 0, no
CIC1 nor RIC1).
When the voltage control loop is used as the voltage limit
protection or the current control loop is used as the current
limit protection, no compensation network is needed for
the protecting control loop.
A resistor, ROPT, must be connected in series with the
opto-coupler since it is part of the compensation network.
Although the value of ROPT is not critical, it's recommended
to be in the range from 0.33kΩ to (VOUT − 2) / (0.005) Ω.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction to ambient
thermal resistance.
is a registered trademark of Richtek Technology Corporation.
DS8481A-00 January 2016
RT8481A
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
SOT-23-6 packages, the thermal resistance, θJA, is
243.3°C/W on a standard JEDEC 51-7 four-layer thermal
test board. The maximum power dissipation at TA = 25°C
can be calculated by the following formula :
Layout Consideration
For the best performance of the RT8481A, the following
PCB Layout guidelines must be strictly followed.

Place the RSENSE resistor as close to IC as possible.

Keep the input/output traces as wide and short as
possible.
PD(MAX) = (125°C − 25°C) / (243.3°C/W) = 0.41W for
SOT-23-6 package
VOUT
Optocoupler
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 1 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
RLED
RIC1
LED
CIC1
:
:
:
VCC OUT CP
0.5
Maximum Power Dissipation (W)1
R1
RVC1 CVC1
Four-Layer PCB
6
5
4
2
3
0.4
CN
0.3
CN GND OVP
RIC2
0.2
R2
RSENSE
GND
0.1
GND
Figure 2. PCB Layout Guide
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 1. Derating Curve of Maximum Power Dissipation
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RT8481A
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
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot
assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be
accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
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DS8481A-00 January 2016
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