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RS2051
Novel Low Cost Green-Mode PWM Controller
with Soft Start & Latch Mode
Description
The RS2051 is a highly integrated low cost current mode PWM controller, which is ideal for small power current mode of offline
AC-DC fly-back converter applications. Making use of external resistors, the IC changes the operating frequency and
automatically enters the PFM/CRM (Cycle Reset Mode) under light-load/zero-load conditions. This can minimize standby power
consumption and achieve power-saving functions. With a very low start-up current, the RS2051 could use a large value start-up
resistor (2Mohms). Built-in synchronized slope compensation enhances the stability of the system and avoids sub-harmonic
oscillation. Dynamic peak current limiting circuit minimizes output power change caused by delay time of the system over a
universal AC input range. Leading edge blanking circuit on current sense input could remove the signal glitch due to snubber
circuit diode reverse recovery and thus greatly reduces the external component count and system cost in the design. Cycle-by-
Cycle current limiting ensures safe operation even during short-circuit.
Excellent EMI performance is achieved built-in soft start with 1.2mS、soft driver and frequency jitter.
The RS2051 offers perfect protection like OVP(Over Voltage Protection)、OLP(Over Load Protection)、SCP(Short circuit
protection)、OTP、Sense Fault Protection
、LM
(Latch Mode) and OCP(Over current protection). The RS2051’s output driver
is soft clamped to maximum 16.5V to protect the power MOSFET. RS2051 is offered in SOT-23-6L, SOT-8 and DIP-8 packages.
Features
Low Cost, PWM&PFM&CRM (Cycle Reset Mode)
Low Start-up Current (about 1.5μA)
Low Operating Current (about 2.2mA)
Current Mode Operation
Under Voltage Lockout (UVLO)
Built-in Synchronized Slope Compensation
Built-in fixed soft start with 1.2mS
Built-in Frequency jitter for better EMI Signature
Programmable PWM Frequency
Audio Noise Free Operation
Leading edge Blanking on Sense input
Constant output power limiting for universal AC input
Range
SOT-23-6L、SOP8 and DIP-8 Pb-Free Packaging
Compatible with SG6848 (6849) / SG5701 / SG5848 /
LD7535 (7550) / OB2262 (2263) / OB2278（2279）
Soft Clamped GATE output voltage 16.5V
VDD over voltage protect 25.6V
Cycle-by-cycle current limiting
Sense Fault Protect ion
OTP (Over Temperature Protection)
Output SCP (Short circuit Protection)
Output OLP (Over Load Protection)
Latch mode After OLP&SCP
High-Voltage CMOS Process with ESD
Applications
Switching AC/DC Adaptor
Battery Charger
Open Frame Switching Power Supply
Standby Power Supplies
Set-Top Box Power Supplies
384X Replacement
Pin Configurations
DS-RS2051-02
September, 2007
www.Orister.com
Page No. : 2/13
Name
GND
FB
RI
SEN
VDD
GATE
Description
GND Pin
Voltage feedback pin. Output current of this pin could controls the PWM duty
cycle、OLP and SCP.
This pin is to program the switching frequency. By connecting a resistor to ground
to set the switching frequency
Current sense pin, connect to sense the MOSFET current
Supply voltage pin
Totem output to drive the external Power MOSFET.
Typical Application Circuit
Block Diagram
DS-RS2051-02
September, 2007
www.Orister.com
Page No. : 3/13
Absolute Maximum Ratings
Symbol
V
DD
I
OVP
V
FB
V
SEN
P
D
ESD
Parameter
Supply voltage Pin Voltage
VDD OVP maximal enter current
Input Voltage to FB Pin
Input Voltage to SEN Pin
Power Dissipation
ESD Capability, HBM Model
ESD Capability, Machine Model
Lead Temperature
(Soldering)
20second:
SOT-23-6L
10second:
DIP-8
10second:
SOP-8
Rating
40
20
-0.3 to 6V
-0.3 to 6V
300
2500
250
220
260
℃
230
-55 to + 150
℃
Unit
V
mA
V
V
mW
V
V
℃
T
L
T
STG
Storage Temperature Range
RECOMMENDED OPERATION CONDITION
Symbol
VDD
RI
T
OA
P
O
F
PWM
Parameter
VDD Supply Voltage
RI PIN Resistor Value
Operation Ambient Temperature
Output Power
Frequency of PWM
Min ~ Max
11~20
65~130
-20~85
0~80
30~150
Unit
V
K ohm
℃
W
KHz
Electrical Characteristics
(Ta=25°C unless otherwise noted, V
DD
= 15V.)
Symbol
I
ST
I
SS
VDD
ON
VDD
OFF
VD
CLAMP
VDD
AIS
T
SS
T
OFF
T
RESTART
I
FB
V
FB
I
FB_0D
I
PFM
I
CRM
V
PFM
V
CRM
I
OLP&SCP
V
OLP&SCP
T
OLP&SCP
V
TH_L
V
TH_H
T
PD
R
CS
V
SCP
T
SCP
F
OSC
DS-RS2051-02
Parameter
Startup Current
Operating Current
Turn-on Threshold Voltage
Turn-off Threshold Voltage
VDD Clamp Voltage
Anti Intermission Surge
VDD Voltage
Soft start Time
Over temperature Protection
Temperature restart
V
FB
=0V
V
FB
=3V
V
FB
=Open
I
VDD
=20mA
Conditions
Min.
Supply Voltage (V
DD
Pin)
Typ.
1.5
3.2
2.2
1.65
12.8
7.8
25.6
12.7
1.2
130
100
1.57
4.6
1.47
1.37
1.45
0.51
0.30
152
4.2
50
0.65
0.85
150
50
177
1.2
60
65
Max.
Unit
μA
mA
mA
mA
V
V
V
V
mS
℃
℃
mA
V
mA
mA
mA
V
V
uA
V
mS
V
V
ns
KΩ
mV
mS
KHz
www.Orister.com
Voltage Feedback (FB Pin)
Short Circuit Current
V
FB
=0V
Open Loop Voltage
V
FB
=Open
Zero Duty Cycle FB current
Enter PFM&PWM, FB current
Enter CRM, FB current
PFM Threshold V
FB
Enter CRM Threshold V
FB
Enter OLP&SCP FB current
Enter OLP&SCP FB voltage
OLP&SCP min. delay Time
RI=100K
33
Current Sensing (SEN Pin)
Minimum Voltage Lever
Maximum Voltage Lever
Delay to Output
Input Impedance
Sense short protect voltage
Sen. short protect Delay Time
Oscillator (RI Pin)
Normal Frequency
RI=100Kohm
55
September, 2007
Page No. : 4/13
F
PFM
DC
MAX_W
DC
MAX_F
ΔF
TEMP
T
BLANK
F
JITTER
V
OL
V
OH
T
R1
T
F1
T
R2
T
F2
T
R3
T
F3
T
R4
T
F4
VG
CLAMP
PFM Frequency
RI=100Kohm
Maximum Duty Cycle PWM
RI=100Kohm
Maximum Duty Cycle PFM
RI=100Kohm
-30-85℃
Frequency Temp. Stability
Leading-Edge Blanking Time
Frequency jitter
RI=100Kohm
-4
GATE Drive Output (GATE Pin)
Output Low Level
V
DD
=15V, I
O
=20mA
Output High Level
V
DD
=15V, I
O
=20mA
9
Rising Time
C
L
=500pF
Falling Time
C
L
=500pF
Rising Time
C
L
=1000pF
Falling Time
C
L
=1000pF
Rising Time
C
L
=1500pF
Falling Time
C
L
=1500pF
Rising Time
C
L
=2000pF
Falling Time
C
L
=2000pF
Output Clamp Voltage
VDD=20V
22
77
14
5
300
4
0.8
123
71
248
116
343
153
508
209
16.5
KHZ
%
%
%
nS
%
V
V
ns
ns
ns
ns
ns
ns
ns
ns
V
Notice:
The drive current of GATE pin is a variable value, which is decided by
I
=
K
(
V
VDD
−
V
GATE
−
2.8)
2
(Among these, K is a invariable coefficient，V
GATE
is the voltage of GATE pin, V
VDD
is
the voltage of VDD pin)；So the higher the VDD voltage is and the lower the output voltage is, the bigger the drive
transient current is. When the GATE voltage is 0V and the VDD voltage is 13V, the output drive current would over
120mA. The output driver current would decrease with increasing of the GATE voltage.
DS-RS2051-02
September, 2007
www.Orister.com
Page No. : 5/13
OPERATION DESCRIPTION
Current Mode
Compared to voltage mode control, current mode control has a current feedback loop. When the voltage of the Sense resistor
peak current of the primary winding reaches the internal setting value V
TH
, the register resets and the power MOSFET cuts off.
So, to detect and modulate the peak current cycle-by-cycle could control the output of the power supply. The current feedback
has a good linear modulation rate and a fast input and output dynamic impact, and avoid the pole that the output filter
inductance brings and the two-class system descends to the one-class. So it widens the frequency range and optimizes
overload protection and short circuit protection.
Startup Current and Under Voltage Lockout
The startup current of RS2051 is set to be very low so that a large value startup resistor can be used to minimize the power loss.
For AC to DC adaptor with universal input range design, a 2 MΩ, 1/8 W startup resistor and a 10uF/25V VDD hold capacitor
could be used.
The turn-on and turn-off threshold of the RS2051 is designed to 12.8V/7.8V. During startup, the hold-up capacitor must be
charge to 12.8V through the startup resistor. The hysteresis is implemented to prevent the shutdown from the voltage dip during
startup.
Internal Bias and OSC Operation
A resistor connected between RI pin and GND pin sets the internal constant current source to charge or discharge the internal
fixed capacitor. The charge time and discharge time determines the internal clock speed and the switching frequency.
Increasing the resistance will reduce the value of the input current and reduce the switching frequency. The relationship
between RI and PWM switching frequency follows the below equation within the RI allowed range.
F
OSC
=
6000
(
kHz
)
RI
(
K
Ω
)
For example, a 100kΩ resistor RI could
generate a 13uA constant current and a 60kHz PWM switching frequency.
The suggested operating frequency range of RS2051 is within 30KHz to 150KHz.
Green Power Operation
The power dissipation of switching mode power supply is very important in zero load or light load condition. The major
dissipation results from conduction loss、switching loss and consume of the control circuit. However, all of them relates to the
switching frequency. There are many difference topologies has been implemented in different chip. The basic operation theory
of all these approaches intends to reduce the switching frequency under light-load or no-load condition.
The RS2051`s green power function adapts PWM、PFM and CRM combining modulation. When RI resistor is 100kΩ, the
PWM frequency is 60kHz in medium or heavy load operation. Through modifying the pulse width, The RS2051 could control
output voltage. The current of FB pin increases when the load is in light condition and the internal mode controller enters
PFM&PWM when the feedback current is over 1.37mA. The operation frequency of oscillator is to descend gradually. When the
feedback current is over 1.43mA, the frequency of oscillator is invariable, namely 22kHz.
To decrease the standby consumption of the power supply, Chip-Rail introduces the Cycle Reset Mode technology (CRM). If the
feedback current is over 1.45mA, mode controller of the RS2051 would reset internal register all the time and cut off the GATE
pin. While the output voltage is lower than the set value, the register would be set, the GATE pin operate again. So the
frequency of the internal OSC is invariable, the register would reset some pulses so that the practical frequency is decreased at
the GATE pin.
RS2051 Green-Power Function
DS-RS2051-02
September, 2007
www.Orister.com
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