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RS2138
High Precision CC/CV Primary-Side PWM
Power Switch
DESCRIPTION
FEATURES
RS2138 is a high performance offline PWM Power
switch for low power AC/DC charger and adaptor
applications. It operates in primary-side sensing and
regulation. Consequently, opto-coupler and 431 could
be eliminated. Proprietary Constant Voltage (CV) and
Constant Current (CC) control is integrated as shown
in the figure 1.
In CC control, the current and output power setting can
be adjusted externally by the sense resistor RS at CS
pin. In CV control, multi-mode operations are utilized to
achieve high performance and high efficiency. In
addition, good load regulation is achieved by the
built-in cable drop compensation. Device operates in
PFM in CC mode as well at large load condition and it
operates in PWM with frequency reduction at
light/medium load.
RS2138 offers power on soft start control and
protection coverage with auto-recovery features
including Cycle-by-Cycle current limiting, VDD OVP,
VDD clamp and UVLO. Excellent EMI performance is
achieved with Frequency Jiggling.
• 5% Constant Voltage Regulation, 5% Constant
Current Regulation at Universal AC Input
• Primary‐ side Sensing and Regulation without 431
and Opto-coupler
• Power on Soft-start
• Built-in Leading Edge Blanking (LEB)
• Cycle-by-Cycle Current Limiting
• VDD Under Voltage Lockout with Hysteresis (UVLO)
• Programmable CV and CC Regulation
• Adjustable Constant Current and Output Power
Setting
• Built-in Secondary Constant Current Control with
Primary Side Feedback
• Built-in Adaptive Current Peak Regulation
• Built-in Primary Winding Inductance Compensation
• Program Cable Drop Compensation
• VDD OVP and VDD Clamp
• Available in an DIP-8 Package
• RoHS Compliant and 100% Lead (Pb)-Free and
Green (Halogen Free with Commercial Standard)
APPLICATIONS
•
•
•
•
•
Cell Phone Charger
Digital Cameras Charger
Small Power Adaptor
Auxiliary Power for PC, TV etc.
Linear Regulator/RCC Replacement
Figure 1. Typical CC/CV Curve
Tel: 886-66296288‧Fax: 886-29174598‧ http://www.princeton.com.tw‧2F, No. 233-1, Baociao Rd., Sindian Dist., New Taipei City 23145, Taiwan
RS2138
BLOCK DIAGRAM
APPLICAION CIRCUIT
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RS2138
ORDER INFORMATION
Device
RS2138 Y Z
Device Code
Y is package & Pin Assignments designator:
P: DIP-8
Z is Lead Free designator:
P: Commercial Standard, Lead (Pb) Free and Phosphorous (P) Free Package
G: Green (Halogen Free with Commercial Standard)
PIN ASSIGNMENTS
PIN DESCRIPTION
Pin Name
VDD
COMP
INV
CS
DRAIN
GND
V1.0
Description
Power supply
Loop compensation for CV stability
The voltage feedback from auxiliary winding. Connected to resistor divider from auxiliary
winding reflecting output voltage. PWM duty cycle is determined by EA output and current
sense signal at pin 4.
Current sense input
HV MOSFET drain pin. The Drain pin is connected to the primary lead of the transformer
Ground
3
Pin No.
1
2
3
4
5, 6
7, 8
January 2013
RS2138
FUNCTION DESCRIPTION
RS2138 is a cost effective PWM power switch optimized for off-line low power AC/DC applications including battery
chargers and adaptors. It operates in primary side sensing and regulation, thus opto-coupler and 431 are not required.
Proprietary built-in CV and CC control can achieve high precision CC/CV control meeting most adaptor and charger
application requirements.
STARTUP CURRENT AND START UP CONTROL
Startup current of RS2138 is designed to be very low so that VDD could be charged up above UVLO threshold and starts
up quickly. A large value startup resistor can therefore be used to minimize the power loss in application.
OPERATING CURRENT
The Operating current of RS2138 is as low as 2.5mA. Good efficiency is achieved with the low operating current together
with multi-mode control features.
SOFT START
RS2138 features an internal soft start to minimize the component electrical over-stress during power on startup. As soon
as VDD reaches UVLO (OFF), the control algorithm will ramp peak current voltage threshold gradually from nearly zero
to normal setting of 0.90V. Every restart is a soft start.
CC/CV OPERATION
RS2138 is designed to produce good CC/CV control characteristic as shown in the figure 1.
In charger applications, a discharged battery charging starts in the CC portion of the curve until it is nearly full charged
and smoothly switches to operate in CV portion of the curve.
In an AC/DC adapter, the normal operation occurs only on the CV portion of the curve. The CC portion provides output
current limiting. In CV operation, the output voltage is regulated through the primary side control. In CC operation mode,
RS2138 will regulate the output current constant regardless of the output voltage drop.
PRINCIPLE OF OPERATION
To support RS2138 proprietary CC/CV control, system needs to be designed in DCM mode for flyback system (Refer to
Application Circuit).
In the DCM flyback converter, the output voltage can be sensed via the auxiliary winding. During MOSFET turn-on time,
the load current is supplied from the output filter capacitor CO. The current in the primary winding ramps up. When
MOSFET turns off, the primary current transfers to the secondary at the amplitude of
N
IS = P × IP
NS
The auxiliary voltage reflects the output voltage as shown in figure 2 and it is given by
VAUX =
N AUX
× (VO + ΔV)
NS
Where ΔV indicates the drop voltage of the output diode.
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RS2138
Figure 2. Auxiliary voltage waveform
Via a resistor divider connected between the auxiliary winding and INV (pin 3), the auxiliary voltage is sampled at the end
of the demagnetization and it is hold until the next sampling.
The sampled voltage is compared with VREF (2.0V) and the error is amplified. The error amplifier output COMP reflects
the load condition and controls the PWM switching frequency to regulate the output voltage, thus constant output voltage
can be achieved.
When sampled voltage is below VREF and the error amplifier output COMP reaches its maximum, the switching
frequency is controlled by the sampled voltage thus the output voltage to regulate the output current, thus the constant
output current can be achieved.
ADJUSTABLE CC POINT AND OUTPUT POWER
In RS2138, the CC point and maximum output power can be externally adjusted by external current sense resistor RS at
CS pin as illustrated in Application Circuit. The output power is adjusted through CC point change. The larger RS, the
smaller CC point is, and the smaller output power becomes, and vice versa as shown in figure 3.
Figure 3. Adjustable output power by changing RS
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RS2138
OPERATION SWITCHING FREQUENCY
The switching frequency of RS2138 is adaptively controlled according to the load conditions and the operation modes.
No external frequency setting components are required. The operation switching frequency at maximum output power is
set to 60KHz internally.
For flyback operating in DCM, The maximum output power is given by
1
Po MAX = × L P × FSW × IP 2
2
Where LP indicate the inductance of primary winding and IP is the peak current of primary winding.
Refer to the equation 3, the change of the primary winding inductance results in the change of the maximum output
power and the constant output current in CC mode. To compensate the change from variations of primary winding
inductance, the switching frequency is locked by an internal loop such that the switching frequency is
1
FSW =
2TDEMAG
Since TDEMAG is inversely proportional to the inductance, as a result, the product LP and FSW is constant, thus the
maximum output power and constant current in CC mode will not change as primary winding inductance changes. Up to
±10% variation of the primary winding inductance can be compensated.
FREQUENCY JIGGLING FOR EMI IMPROVEMENT
The Frequency Jiggling (switching frequency modulation) is implemented in RS2138. The oscillation frequency is
modulated so that the tone energy is spread out. The spread spectrum minimizes the conduction band EMI and
therefore eases the system design.
CURRENT SENSING AND LEADING EDGE BLANKING
Cycle-by-Cycle current limiting is offered in RS2138 current mode PWM control. The switch current is detected by a
sense resistor into the CS pin. An internal leading edge blanking circuit chops off the sensed voltage spike at initial
internal power MOSFET on state so that the external RC filtering on sense input is no longer needed. The PWM duty
cycle is determined by the current sense input voltage and the EA output voltage.
GATE DRIVE
The internal power MOSFET in RS2138 is driven by a dedicated gate driver for power switch control. Too weak the gate
drive strength results in higher conduction and switch loss of MOSFET while too strong gate drive compromises EMI.
A good tradeoff is achieved through the built-in totem pole gate design with right output strength control.
PROGRAMMABLE CABLE DROP COMPENSATION
In RS2138, cable drop compensation is implemented to achieve good load regulation. An offset voltage is generated at
INV by an internal current flowing into the resister divider. The current is inversely proportional to the voltage across pin
COMP, as a result, it is inversely proportional to the output load current, thus the drop due to the cable loss can be
compensated. As the load current decreases from full-load to no-load, the offset voltage at INV will increase. It can also
be programmed by adjusting the resistance of the divider to compensate the drop for various cable lines used.
PROTECTION CONTROL
Good power supply system reliability is achieved with its rich protection features including Cycle-by-Cycle current
limiting (OCP), VDD clamp, Power on Soft Start, and Under Voltage Lockout on VDD(UVLO).
VDD is supplied by transformer auxiliary winding output. The output of RS2138 is shut down when VDD drops below
UVLO (ON) limit and Switcher enters power on start-up sequence thereafter.
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RS2138
ABSOLUTE MAXIMUM RATINGS
(Operating temperature range applies unless otherwise specified)
Parameter
Symbol
‐
Drain voltage (off state)
‐
VDD voltage
‐
VDD Zener clamp continuous current
‐
COMP voltage
‐
CS input voltage
‐
INV input voltage
Junction temperature
TJ
Operating temperature range
TOPR
Storage temperature range
TSTG
Lead temperature (soldering, 10secs)
TLEAD
Range
‐0.3V to Bvdss
‐0.3 to VDD_Clamp
10
‐0.3 to 7
‐0.3 to 7
‐0.3 to 7
‐20 to 150
‐20 to +85
‐40 to +150
260
Unit
V
V
mA
V
V
V
℃
℃
℃
℃
OUTPUT POWER TABLE
Part No.
RS2138P
Package
8-Pin, DIP
230VAC ±15%
20W
85 to 265VAC
16W
ELECTRICAL CHARACTERISTICS
(VDD=16V, TA=25℃, unless otherwise specified)
Parameter
Symbol
Supply Voltage (VDD) Section
Standby current
IDD ST
Operation current
VDD under voltage lockout enter
VDD under voltage lockout exit
Over voltage protection voltage
Maximum VDD operation voltage
Current Sense Input Section
LEB time
Over current threshold
OCP Propagation delay
Input impedance
Soft start time
CV Section
System nominal switch frequency
Frequency jitter range
Error Amplifier Section
Reference voltage for EA
DC gain of the EA
Max. cable compensation current
Power MOSFET Section
MOSFET drain‐source breakdown
voltage
Static drain to source on resistance
V1.0
Test Condition
Min.
Typ.
Max.
Unit
VDD=13V
Operation supply current
INV=2V,CS=0V,
VDD=VDDG=20V
VDD falling
VDD rising
Ramp up VDD until gate clock
is off
IDD=10mA
-
5
20
μA
‐
2.5
3.5
mA
13.5
7.5
14.5
8.5
16.0
10
V
V
27.5
29.5
31.5
V
30.5
32.5
34.5
V
TLEB
Vth_oc
Td_oc
ZSENSE IN
Tss
‐
870
‐
‐
‐
540
900
150
50
10
930
-
ns
mV
ns
KΩ
ms
Freq_Nom
Freq_startup
Δf/Freq
‐
‐
60
14
±4
-
KHZ
KHZ
%
1.97
‐
2
60
2.03
-
V
dB
-
42
-
μA
BVdss
‐
650
-
V
RDS(on)
‐
3.0
3.6
Ω
IDD_OP
UVLO(ON)
UVLO(OFF)
OVP
VDD
Clamp
VREF_EA
Gdc
I_COMP_MA
X
INV=0V, Comp=5V
INV=2V, COMP=0V
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January 2013
RS2138
PACKAGE INFORMATION
8-PIN, DIP
Notes:
1. Refer to JEDEC MS-001.
2. All dimensions are in millimeter.
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January 2013
RS2138
IMPORTANT NOTICE
Princeton Technology Corporation (PTC) reserves the right to make corrections, modifications, enhancements,
improvements, and other changes to its products and to discontinue any product without notice at any time.
PTC cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a PTC product. No
circuit patent licenses are implied.
Princeton Technology Corp.
2F, 233-1, Baociao Road,
Sindian Dist., New Taipei City 23145, Taiwan
Tel: 886-2-66296288
Fax: 886-2-29174598
http://www.princeton.com.tw
V1.0
9
January 2013
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