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SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
September 2009
SG6858 Low-Cost, Green-Mode, PWM Controller for Flyback Converters
Features
Green-Mode PWM Supports the "Blue Angel" Standard Low Startup Current: 10A (Maximum) Low Operating Current: 2.5mA Leading-Edge Blanking (LEB) Constant Output Power Limit Built-in Synchronized Slope Compensation Current-Mode Operation Cycle-by-Cycle Current Limiting Under-Voltage Lockout (UVLO) Programmable PWM Frequency VDD Over-Voltage Protection with Auto-Restart Gate Output Voltage Clamped at 17V Few External Components Required SSOT-26 and DIP-8 Packages Available
Description
This highly integrated PWM controller provides several special enhancements designed to meet the low standby-power needs of low-power SMPS. To minimize standby power consumption, the proprietary greenmode function provides off-time modulation to linearly decrease the switching frequency under light-load conditions. This green-mode function enables the power supply to meet even the strictest power conservation requirements. The BiCMOS fabrication process enables reducing the startup current to 10A and the operating current to 2.5mA. To further improve power conservation, a large startup resistance can be used. Built-in synchronized slope compensation ensures the stability of peakcurrent-mode control. Proprietary internal compensation provides a constant output power limit over a universal AC input range (90VAC to 264VAC). Pulse-by-pulse current limiting ensures safe operation even during short circuits. To protect the external power MOSFET from being damaged by supply over voltage, the output driver is clamped at 17V. SG6858 controllers can improve the performance and reduce the production cost of power supplies. The SG6858 replaces linear and RCC-mode power adapters. It is available in 8-pin DIP and 6-pin SSOT-26 packages.
Applications
Battery chargers for cellular phones, cordless phones, PDAs, digital cameras, and power tools Power adapters for ink jet printers, video game consoles, and portable audio players Open-frame SMPS for TV/DVD standby and other auxiliary supplies, home appliances, PC 5V standby power, and consumer electronics Replacements for linear transformers and RCC SMPS Offline High Brightness (HB) LED drivers
Ordering Information
Part Number
SG6858TZ SG6858DZ
Operating Temperature Range
-40 to +125C -40 to +125C
Eco Status
RoHS RoHS
Package
6-Pin SSOT-26 8-Pin DIP-8
Packing Method
Tape & Reel Tube
For Fairchild's definition of Eco Status, please visit: http://www.fairchildsemi.com/company/green/rohs_green.html.
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4 www.fairchildsemi.com
SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
Typical Application
L EMI AC Input N Filter Bridge Rectifier Diode + CBulk RStart RSN DVDD + CVDD
5
RSN2 CSN2 CSN NP DSN NS DR + CO1 CO2 VO + ZDO
VO-
NA
VDD
3 RI
GATE 6 SG6858 SENSE 4 GND
1
RRI
2 FB
RG RLF RSENSE Rd R1
CFB
CLF
PC817 RF TL431 R2 CF
Figure 1. Typical Application
Block Diagram
GND 1(8) OVP 25V
OSC S R Q Soft Driver
RI 3(5)
VDD
6(1) GATE
VDD 5(2)
Internal BIAS Vlimit ramp + UVLO Slope Compensation 16.5V/11.5V Green Mode Controller 3R 2R Blanking Circuit 5V
4(4) SENSE
2(7) FB
SOT(DIP)
Figure 2. Block Diagram
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4
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SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
Marking Information
XXX:AAI=SG6858 TT: Die run code : Year code -- : Week code
Figure 3. SSOT-26
F: Fairchild Logo Z: Assembly Plant Code X: Year Code Y: Week Code TT: Die Run Code T: N=DIP P: Y=Green Package M: Manufacture flow code
Figure 4. DIP-8
Pin Configuration
GND FB RI 1 2 3 SOT - 26 6 5 4 GAT E
GATE VDD 1 2 3 4 DIP-8 8 7 6 5 GND FB NC RI
VDD
NC
SENSE
SE NSE
Figure 5. Pin Configurations
Pin Definitions
Pin # DIP-8 SSOT-26
1 2 3 4 4 6 5 GATE VDD NC Totem-pole output driver for the power MOSFET. Power supply. No connection.
Name Description
Current sense. This pin senses the voltage across a resistor. When the voltage SENSE reaches the internal threshold, PWM output is disabled. This activates over-current protection. This pin provides current amplitude information for current-mode control. A resistor connected from the RI pin to ground generates a constant current source. This current is used to charge an internal capacitor to determine the switching frequency. Increasing the resistance reduces the amplitude of the current source and reduces the switching frequency. A 95k resistor, RI, results in a 50A constant current, II, and a 70kHz switching frequency. No connection. Feedback. The FB pin provides the output voltage regulation signal, and feedback to the internal PWM comparator, so the PWM comparator can control the duty cycle. Ground.
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5
3
RI
6 7 8 2 1
NC FB GND
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4
SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
Absolute Maximum Ratings
Stresses exceeding the absolute maximum ratings may damage the device. The device may not function or be operable above the recommended operating conditions and stressing the parts to these levels is not recommended. In addition, extended exposure to stresses above the recommended operating conditions may affect device reliability. The absolute maximum ratings are stress ratings only. All voltage values, except differential voltages, are given with respect to GND pin.
Symbol
VDD VFB VSENSE RJC TJ TSTG TL ESD Supply Voltage Input Voltage to FB Pin Input Voltage to SENSE Pin
Parameter
Min.
-0.3 -0.3 SSOT DIP -40 -55
Max.
30 7.0 7.0 208.4 82.5 +125 +150 260 3.0 0.2
Unit
V V V C/W C C C kV
Thermal Resistance (Junction-to-Case) Operating Junction Temperature Storage Temperature Range Lead Temperature, Wave Soldering, 10 Seconds Electrostatic Discharge Capability
Human Body Model, JESD22-A114 Machine Model, JESD22-A115
Recommended Operating Conditions
The Recommended Operating Conditions table defines the conditions for actual device operation. Recommended operating conditions are specified to ensure optimal performance to the datasheet specifications. Fairchild does not recommend exceeding them or designing to Absolute Maximum Ratings.
Symbol
VDD TA DC Supply Voltage
Parameter
Operating Ambient Temperature
Min.
-40
Max.
22 +125
Unit
V C
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4
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SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
Electrical Characteristics
VDD=15V, TA=-40C ~+125C (TA= TJ), unless otherwise specified.
Symbol VDD Section
VDD-OP VDD-ON VDD-OFF IDD-ST IDD-OP VDD-OVP tD-VDDOVP VDD-G OFF
Parameter
Continuously Operating Voltage Turn-On Threshold Voltage Turn-Off Voltage Startup Current Operating Supply Current VDD Over-Voltage Protection Level VDD OVP Debounce Time VDD Low Threshold Voltage to Exit Green Off Mode
Test Condition
Min.
Typ.
Max.
22
Unit
V V V A mA V sec V
15.5 10.5 VDD=VDD-ON - 0.1V VDD=15V, GATE with 1nF to GND 23 50
16.5 11.5 10 2.5 25 125
17.5 12.5 15 3.5 26 200
VDD-OFF VDD-OFF VDD-OFF + 0.95 + 1.10 + 1.25
Feedback Input Section
AV ZFB VFB-OPEN VFB-N VFB-G SG FB Input to Current Comparator Attenuation Input Impedance FB Pin Open High Voltage Green Mode Entry FB Voltage Green Mode Ending FB Voltage Green Mode Modulation Slope RI=95k 4.5 2/5 5 5.0 2.85 2.20 75 5.5 V/V k V V V Hz/mV
Current Sense Section
ZSENSE tPD VSTHFL VSTHVA tLEB DCYSAW Input Impedance Delay to Output Flat Threshold Voltage for Current Limit Valley Threshold Voltage for Current Limit Leading-Edge Blanking Time Duty Cycle of SAW Limit Maximum Duty Cycle 0.75 240 10 60 0.96 0.80 300 45 0.85 360 110 k ns V V ns %
Oscillator Section
fOSC fOSC-G fDV fDT Frequency Green Mode Frequency Frequency Variation vs. VDD Deviation Frequency Variation vs. Temperature Deviation RI=95k RI=95k VDD=13.5V to 22V TA=-40C ~+125C 0 65 70 22 0.02 2.00 2 75 kHz kHz % %
Output Section
DCYMAX VGATE-L VGATE-H tR tF VGATE-CLAMP Maximum Duty Cycle Output Voltage Low Output Voltage High Rising Time Falling Time Output Clamp Voltage VDD=15V, IO=20mA VDD=13.5V, IO=20mA VDD=13.5V, CL=1nF VDD=13.5V, CL=1nF VDD=13.5V, TA=25C 8 100 35 16 170 55 17 240 75 18 70 75 80 1.5 % V V ns ns V
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4
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SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
Typical Performance Characteristics
Turn-On Threas hold Voltage (V DD-O N ) v s Tem perature
17.5 12.5 12.1
Turn-off Thres hold Voltage (V DD-OF F ) v s Tem perature
17.0
V DD-O N (V)
V DD-O FF (V)
-40 -25 -10 5 20 35 50 65 80 95 110 125
16.5
11.7 11.3 10.9 10.5
16.0
15.5
Tem perature ()
-40
-25
-10
5
Tem perature ()
20
35
50
65
80
95
110
125
Figure 6. Turn-On Threshold Voltage (VDD-ON) vs. Temperature
Startup Current(IDD-S T ) v s Temperature
20
Figure 7. Turn-Off Threshold Voltage (VDD-OFF) vs. Temperature
Operating Supply Current (IDD-O P ) v s Tem perature
3.5
15
3.0
IDD-S T (uA)
10
IDD-O P (m A)
-40 -25 -10 5 20 35 50 65 80 95 110 125
2.5
5
2.0
0
1.5 -40 -25 -10 5 20 35 50 65 80 95 110 125
Temperature ()
Temperature ()
Figure 8. Startup Current (IDD-ST) vs. Temperature
Figure 9. Operating Supply Current (IDD-OP) vs. Temperature
M ax im um Duty Cy c le (DCY MA X ) v s Tem perature
80 78
Frequenc y (F O S C ) v s Tem perature
75 73 71 69 67 65 -40 -25 -10 5 20 35 50 65 80 95 110 125
DCY MA X (%)
F O S C (KHz )
76 74 72 70 -40 -25 -10 5 20 35 50 65 80 95 110 125
Temperature ()
Temperature ()
Figure 10. Frequency (fOSC) vs. Temperature
Figure 11. Maximum Duty Cycle (DCYMAX) vs. Temperature
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4
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SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
Typical Performance Characteristics (Continued)
Green-M ode Entry FB Voltage(V FB -N ) v s Temperature
3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5 -40 -25 -10 5 20 35 50 65 80 95 110 125
Green-M ode Ending FB Voltage (V FB -G ) v s Tem perature
2.50 2.40
V FB -G (V)
V FB -N (V)
2.30 2.20 2.10 2.00 1.90 -40 -25 -10 5 20 35 50 65 80 95 110 125
VDD=15V
VDD=15V
Tem perature ()
Tem perature ()
Figure 12. Green Mode Entry FB Voltage (VFB-N) vs. Temperature
Figure 13. Green Mode Ending FB Voltage (VFB-G) vs. Temperature
Operation Current (IDD-OP) vs VDD Voltage
3.5
Leading-Edge Blank ing Time (t L E B ) v s Tem perature
350 330 310 290 270 250 -40 -25 -10 5 20 35 50 65 80 95 110 125
3.0
IDD-OP (m A)
t L E B (ns ec )
2.5
VDD=15V
2.0
1.5 12 13 14 15 16 17 18 19 20 21 22 23 24
Tem perature ()
VDD Voltage (V)
Figure 14. Leading-Edge Blanking Time (tLEB) vs. Temperature
Figure 15.
Operating Current (IDD-OP) vs. Temperature
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4
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SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
Operation Description
SG6858 devices integrate many useful designs into one controller for low-power switch-mode power supplies. The following descriptions highlight some of the features of the SG6858 series.
Startup Operation
The startup current is only 10A. Low startup current allows a startup resistor with a high resistance and low wattage to supply the startup power for the controller. A 1.5M, 0.25W, startup resistor and a 10F/25V VDD hold-up capacitor would be sufficient for an AC-to-DC power adapter with a wide input range (100VAC to 240VAC).
Operating Current
The operating current has been reduced to 2.5mA. The low operating current results in higher efficiency and reduces the VDD hold-up capacitance requirement. Figure 16. Current Sense R-C Filter
Constant Output Power Limit Green-Mode Operation
The proprietary green-mode function provides off-time modulation to linearly decrease the switching frequency under light-load conditions. On-time is limited to provide stronger protection against brownouts and other abnormal conditions. The feedback current, which is sampled from the voltage feedback loop, is taken as the reference. Once the feedback current exceeds the threshold current, the switching frequency starts to decrease. This green-mode function dramatically reduces power consumption under light-load and zeroload conditions. Power supplies using the SG6858 meet the strictest regulations regarding standby power consumption. When the SENSE voltage across the sense resistor RS reaches the threshold voltage (around 0.96V), the output GATE drive is turned off following a short propagation delay tPD. This propagation delay introduces an additional current proportional to tPD*VIN/LP. The propagation delay is nearly constant regardless of the input line voltage VIN. Higher input line voltages result in larger additional currents. At high input line voltages, the output power limit is higher than at low input line voltages. To compensate for this output power limit variation across a wide AC input range, the threshold voltage is adjusted by adding a positive ramp (VLIMIT_RAMP). This ramp signal rises from 0.80V to 0.96V, then flattens out at 0.96V. A smaller threshold voltage forces the output GATE drive to terminate earlier. This reduces the total PWM turn-on time and makes the output power equal to that of low line input. This proprietary internal compensation ensures a constant output power limit for a wide AC input voltage range (90VAC to 264VAC).
Oscillator Operation
A resistor connected from the RI pin to ground generates a constant current source for the SG6858. This current is used to charge an internal capacitor. The charge-time determines the internal clock speed and the switching frequency. Increasing the resistance reduces the amplitude of the input current and reduces the switching frequency. A 95k resistor RI results in a 50A constant current II and a 70kHz switching frequency. The relationship between RI and the switching frequency is:
Under-Voltage Lockout (UVLO)
The turn-on and turn-off thresholds are fixed internally at 16.5V/11.5V. During startup, the hold-up capacitor must be charged to 16.5V through the startup resistor, so that the SG6858 is enabled. The hold-up capacitor continues to supply VDD until power can be delivered from the auxiliary winding of the main transformer. VDD must not drop below 11.5V during this startup process. This UVLO hysteresis window ensures that the hold-up capacitor is adequate to supply VDD during startup.
f PWM =
6650 R I (k )
(kHz )
(1)
Leading-Edge Blanking (LEB)
Each time the power MOSFET is switched on, a turn-on spike occurs at the sense-resistor. To avoid premature termination of the switching pulse, a 300ns leadingedge blanking time is built in. Conventional RC filtering can therefore be omitted. During this blanking period, the current-limit comparator is disabled and it cannot switch off the gate driver.
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4 8
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SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
VDD Over-Voltage Protection (OVP)
Integrated VDD over-voltage protection prevents damage due to over-voltage conditions. When the VDD exceeds the internal threshold due to abnormal conditions, PWM output is turned off until the VDD voltage drops below the UVLO, then starts again. Over-voltage conditions are usually caused by open feedback loops.
Built-in Slope Compensation
The sensed voltage across the current sense resistor is used for current mode control and pulse-by-pulse current limiting. Built-in slope compensation improves stability and prevents sub-harmonic oscillations due to peak-current mode control. The SG6858 has a synchronized, positively-sloped ramp built-in at each switching cycle. The slope of the ramp is:
Gate Output
The BiCMOS output stage is a fast totem-pole gate driver. Cross conduction has been avoided to minimize heat dissipation, increase efficiency, and enhance reliability. The output driver is clamped by an internal 17V Zener diode to protect power MOSFET transistors against undesired over-voltage gate signals.
0.36 x Duty Duty (max .)
(2)
Noise Immunity
Noise from the current sense or the control signal can cause significant pulsewidth jitter, particularly in continuous-conduction mode. While slope compensation helps alleviate this problem, further precautions should be taken. Good placement and layout practices should be followed. Avoiding long PCB traces and component leads, locating compensation and filter components near the SG6858, and increasing the power MOS gate resistance are advised.
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4
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Typical Application Circuit (Flyback Converter for Printer Application)
Application
Adapter
SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
Fairchild Devices
SG6858
Input Voltage Range
90~264VAC
Output
12V/1.25A (15W)
L
F1 1A/250V
IN4007*4 22F/400V + CBulk RStart 1.5M RSN 66k 47 RSN DSN UF1003 10 RG
5
R XC1 XC 1M AC Input RXC2 0.22F/300V 1M N RVDD NA 1.2 FR103 DVDD
10nF/1KV CSN NP
47 1nF RSN2 CSN2 470F/16V DR + + NS MBR10100CT CO1 CO2 VO +
VO680F/16V
10F/50V + CVDD
SSP2N60
100k
3 RI
VDD GATE 6 SG6858
2 FB
RSENSE
1.5
560
Rd
38.3k R1
RRI
SENSE 4 GND
1
PC817 RF CY 2.2nF TL431 1% CF 10k R2
20k 2.2nF
Figure 17.
Schematic of Application Circuit
Transformer
Core: EF-20 Primary-Side Inductance: 2mH (Pin1 to Pin3)
1
N1 105Turns 0.27*1 N3 22Turns 0.25*1
N2 18Turns 0.3*2
TP+
Shielding lead to Pin 2 N3: 2-4 Shielding lead to Pin 2 N2: TP - - TP+ Shielding lead to Pin 2 N1: 1-3
3 4
TP-
2
Figure 18.
Transformer Structure
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4
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SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
Physical Dimensions
Figure 19. 6-Pin, SSOT-6 Package
Package drawings are provided as a service to customers considering Fairchild components. Drawings may change in any manner without notice. Please note the revision and/or date on the drawing and contact a Fairchild Semiconductor representative to verify or obtain the most recent revision. Package specifications do not expand the terms of Fairchild's worldwide terms and conditions, specifically the warranty therein, which covers Fairchild products. Always visit Fairchild Semiconductor's online packaging area for the most recent package drawings: http://www.fairchildsemi.com/packaging/.
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4
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SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
Physical Dimensions (Continued)
9.83 9.00
6.67 6.096
8.255 7.61
5.08 MAX
3.683 3.20
7.62
0.33 MIN (0.56) 2.54
3.60 3.00 0.56 0.355 1.65 1.27
7.62
0.356 0.20 9.957 7.87
NOTES: UNLESS OTHERWISE SPECIFIED A) THIS PACKAGE CONFORMS TO JEDEC MS-001 VARIATION BA B) ALL DIMENSIONS ARE IN MILLIMETERS. C) DIMENSIONS ARE EXCLUSIVE OF BURRS, MOLD FLASH, AND TIE BAR EXTRUSIONS. D) DIMENSIONS AND TOLERANCES PER ASME Y14.5M-1994 E) DRAWING FILENAME AND REVSION: MKT-N08FREV2.
Figure 20. 8-Pin, Dual-Inline Package (DIP-8)
Package drawings are provided as a service to customers considering Fairchild components. Drawings may change in any manner without notice. Please note the revision and/or date on the drawing and contact a Fairchild Semiconductor representative to verify or obtain the most recent revision. Package specifications do not expand the terms of Fairchild's worldwide terms and conditions, specifically the warranty therein, which covers Fairchild products. Always visit Fairchild Semiconductor's online packaging area for the most recent package drawings: http://www.fairchildsemi.com/packaging/.
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4
www.fairchildsemi.com 12
SG6858 -- Low-Cost, Green-Mode, PWM Controller for Flyback Converters
(c) 2008 Fairchild Semiconductor Corporation SG6858 * Rev. 1.2.4
www.fairchildsemi.com 13

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