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Bias/Primary Side PWM Controller with Ultra Low Start-up Current
POWER MANAGEMENT Description
The SC4911 is a highly integrated bias/primary side PWM controller designed for off-line or isolated dc-dc converters in telecommunication and data communication applications. The high degree of integration minimizes the amount of discrete parts needed and leads to substantial cost and space saving converter designs. The circuitry of the SC4911 includes line voltage UVLO featuring startup current less than 50A and programmable switching frequency. The device supply is derived from an auxiliary winding on the transformer. The auxiliary winding could also provide output feedback information if an optocoupler is not used.
SC4911
Features
Programmable switching frequency up to 500kHz Line UVLO Protection Undervoltage Lockout Low startup current (45A) Cycle by cycle current limit 8 pin MSOP. Available in Lead-free, fully WEEE and RoHS compliant
Applications
Telecom isolated converters Offline isolated power supplies Instrumentation power supplies Battery chargers
Typical Application Circuit
D1 C1 C2 C3 R2 R3 R4 C4 C6 U1 SC4911 Q1 1 8 T1 D2 R1 Vin Vout
C5 R5
LUVLO
3
VCC
FB
OUT
7
4 R7 C7
COMP GND RT
ILIM
5
R6
C8
R8
2
R9
Revision: April 5, 2006
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SC4911
POWER MANAGEMENT Absolute Maximum Ratings
Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not implied.
Parameter Supply Voltage Supply Current Line UVLO Voltage FB, ILIM and COMP Output Current Source or Sink Storage Temperature Range Junction Temperature Range Lead Temperature (Soldering) 10 Sec. ESD Rating (Human Body Model)
Symbol V CC ICC
Maximum 18 20 VCC -0.3 to 6 250
Units V mA V V mA C C C kV
TSTG TJ TLEAD ESD
-60 to +150 -40 to +150 260 2
Electrical Characteristics
Unless specified: TA = T = -40C to +125C, VCC = 12V, R = 100K, VFB = 2.4V, LUVLO = 1.5V.
J T
Parameter Pow er Supply Vcc Turn-On Voltage Operating Current Startup Current VCC Zener Shunt Voltage Line UVLO Lockout Threshold UVLO Hysteresis Bias Current Error Amplifier Feedback Input Voltage Feedback Input Voltage Regulation Input Bias Current Amplifier Transconductance Amplifier Souce Current Amplifier Sink Current Open Loop Voltage Gain
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Test Conditions
Min
Typ
Max
Unit
6.0 fs = 200kHz, No load LUVLO = 0.6V ICC = 10mA 15 3.4 45 16
7.0 5.0 80 17
V mA A V
1.22
1.25 70
1.28
V mV nA
VLUVLO < 1.25V
100
V C C = 12V VCC = 7V to 12V
2.44 2.425
2.50
2.56 2.575
V V nA mho A A dB
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170 1000 500 600 50
2
SC4911
POWER MANAGEMENT Electrical Characteristics (Cont.)
Unless specified: TA = T = -40C to +125C, VCC = 12V, R = 100K, VFB = 2.4V, LUVLO = 1.5V.
J T
Parameter Oscillator Frequency Range
Test Conditions
Min
Typ
Max
Unit
RT = 200k RT = 43 k
100 500 170 200 230 kHz kHz
Frequency Duty Cycle Maximum Duty Cycle Minimum Duty Cycle Current Limit Current Limit Threshold Propagation Delay to Gate Output Output Voltage Low State Output Voltage High State Rise Time Fall Time Sink 5mA Source 5mA COUT = 100pF COUT = 100pF V FB = 3V
57
65
72 0
% %
500
525 70
550
mV nS
0.1 9.5 10.2 25 10
0.5
V V nS nS
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SC4911
POWER MANAGEMENT Pin Configuration
TOP VIEW
Ordering Information
Part Number SC4911IMSTR P ackag e MSOP-8(1) Temp. Range (TJ) -40C to +125C
LUVLO RT FB COMP
(MSOP- 8 PIN)
VCC OUT GND ILIM
SC4911IMSTRT(2)
Notes: (1) Only available in tape and reel packaging. A reel contains 2500 devices. (2) Lead free product. This product is fully WEEE and RoHS compliant.
Pin Descriptions
Pin # 1 2 3 4 5 6 7 8 Pin Name LUVLO RT FB COMP ILIM GND OUT VC C Line under voltage lockout. For programming oscillator frequency. F e e d b a ck. Feedback compensation. Current limit sense. Ground. Gate drive output. Low during UVLO or LUVLO. Supply voltage. Pin Function
Marking Information
Part Number Code (Example: AAxx) yyww = Date Code (Example: 0012) xxxx = Semtech Lot No. (Example: E901 xxxx 01-1)
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SC4911
POWER MANAGEMENT Block Diagram
Vcc
RT Startup & Uvlo Comp
L-Uvlo
Oscillator
Output Driver
Logic BandGap
Out
+
FB
+ PWM Comparator PWM Latch
Error Amplifier
Comp ILIM
OR gate
Ilim Comparator
Vt 0 .5 V 0.525V
Gnd
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SC4911
POWER MANAGEMENT Applications Information
The SC4911 is a voltage mode controller designed mainly for the isolated or non-isolated flyback converters. As shown in the Block Diagram, it includes start-up circuit, bandgap, error amplifier, PWM comparator, logic and output stage. Line undervoltage has dual functions of line voltage lockout and shutdown. When LUVLO is pull down to ground, the SC4911 is disabled and the quiescent current is reduced to typical 30A. When it is connected to a resistor voltage divider between input voltage and ground, it monitors the line voltage. Oscillator The switching frequency of the SC4911 is set by an external resistor connected between RT and GND. Figure 1 shows the relationship between the resistance of the external resistor and the switching frequency.
Figure 1. Timing Resistor vs Oscillator Frequency
Timing Resistor vs Switching Frequency 250
Resistance ( Kohms )
The bias current consumed by the device during this period is only typical 30A. Once Vcc exceeds the turn-on voltage, OUT starts driving the MOSFET, transferring energy to the secondary and the bias output. If the bias output voltage builds to and is above the Vcc turnoff voltage, the start-up is completed and normal operation begins. The size of the start-up resistor not only affects power supply start-up time, but also power supply efficiency. The resistor dissipates continuous power in normal operation. Due to ultra low start-up current of the SC4911, large value resistor (several hundred Kohm to 1Mohm) could be used, particularly for off-line applications to improve efficiency considering reasonable start-up time. Current Limit The current sense resistor is selected by dividing the current sense threshold voltage 0.525V by the primary peak current at the desired current limit point, typically 120% of the primary peak current.
R SENSE = 0.525 V 1.2 x IPK
200 150 100 50 0 100 200 300
Frequency ( KHz )
400
500
Care must be taken to ensure proper selection of sense resistor and good layout to prevent erratic operation. The non-inductive resistor must be used as the sense resistor. Parasitic inductance in series with the sense resistor must be minimized. Additional RC filter may be necessary to eliminate the narrow spike time on the leading edge of the primary current. The RC time constant can be chosen to be equal to the time constant of the sense resistor and the parasitic inductance. PCB Layout Long power supply and ground traces should be avoided. A 0.1F ceramic capacitor closely placed between Vcc and GND is recommended. The timing resistor used to program switching frequency should be located close to pin RT and pin GND in order to have a stable switching frequency. MOSFET should be located near the device and a resistor in the range of several ohms could be used in series with the gate drive to damp the ringing if the trace between the drive output and MOSFET is not short enough. Star ground connection is recommended to avoid ground loops. The Power path for input filter, MOSFET and transformer should be separated from signal path for timing resistor and feedback resistor, and both should be brought to a single ground point.
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Line UVLO and VCC UVLO The Line UVLO monitors the input line voltage and VCC UVLO monitors the supply voltage to the SC4911. If either input line or VCC is below the lockout threshold, the output is held low and the supply current to the chip is typically 30A. A resistor divider from the input line to GND determines the desired undervoltage lockout level. To prevent noise coupling to affect proper UVLO operation, a small capacitor, one hundred to a few hundred pF, is recommended to be used from Line UVLO pin to GND. Start-up As shown in Typical Applications Circuit on the front page, when input voltage is applied, C1 is trickle charged through the start-up resistor R1 until the SC4911 Vcc reaches the turn-on voltage.
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SC4911
POWER MANAGEMENT Typical Applications
48V TO 12V FLYBACK CONVERTER AS AUXILIARY SUPPLY Figure 2 shows a simple bias supply using the SC4911. This 36 - 72V input flyback power supply employs primary side regulation from the transformer bias winding, which eliminates the need of opto coupler and secondary reference error amplifier. This is best for low cost applications requiring isolation and narrow load variation range on the secondary. During the start-up, the input source trickle charges the capacitor C1 through start-up resistor R1. As long as voltage across C1 reaches the Vcc turn-on threshold, the SC4911 turns on abruptly and start driving the gate of MOSFET Q1. After the SC4911 starts up the primary bias winding supplies the primary circuitry. The output of the T1 bias winding is rectified, filtered by C1 and is fed to the FB through feedback resistor R5 and R7. The output voltage on the secondary is proportional to the bias voltage by the turns ratio of the output to the bias winding. C7 and R9 are for frequency compensation. An additional small capacitor could be used from COMP to GND if it is needed. The LUVLO pin monitors the input voltage via resistor R2 and R4 to determine turn-on and turn-off of the converter. Capacitor C5 helps filter out the noise from the input line.
Vpri = 12V R1 100K
D1 1N5819 C1 10uF
2 1 3
T1 D2 1N5819 8 10 Vo = 12V C2 100uF
Vin = 48V R2 2.7MEG R4 100K
C3 10uF C5 100pF
R3 68 C4 0.1uF 1 8 U1 SC4911 C6 150pF
5
R5 383K
LUVLO 3 FB
VCC OUT 7
Q1 Si2320DS
4 R7 100K C7 47nF R9 4.7K
COMP RT 2 6 R10 100K
ILIM GND
5
R6 51 C8 1000pF R8 1.00
Figure 2. 48V to 12V Flyback Converter as Auxiliary Supply
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SC4911
POWER MANAGEMENT Typical Applications (Cont.)
Isolated feedback 48V to 12V application Figure 3 gives an example of isolated feedback and low power flyback application of the SC4911. T1 is a three winding transformer. The winding 1-2 is a bias winding which supplies the primary circuitry after the SC4911 starts up. During the startup, the input source trickle charges the capacitor C1 through the start-up resistor R1. As long as voltage across C1 reaches the Vcc turnon threshold, the SC4911 turns on abruptly and start driving the gate of MOSFET Q1. As Q1 starts switching, regenerative feedback from the bias winding will provide current via D1 to capacitor C1. In this configuration, the input voltage is in the range of 36V to 72V for telecom applications, and the LUVLO pin monitors the input voltage via resistor R2 and R5 to determine turn-on and turn-off of the converter. Capacitor C5 helps filter out the noise from the input line. Timing resistor R13 sets switching frequency. R3 and C6 are used to suppress the voltage spikes across the switch due to leakage inductance of T1. R11 is current sense resistor to set current limit of the convertor. R10 and C9 filter spikes caused by parasitic capacitance during the turning on of Q1. The U3 SC431 is shunt regulator integrating an accurate 2.5V bandgap reference, op amp and driver into a single device. It compares sensed output voltage divided by R7 and R9 to its internal reference and drives the amplified error signal to opto-coupler U1. C7 keeps ripple and noise from over driving the device. U1 delivers the isolated feedback signal to the SC4911. C8 and R12 are for compensation. An additional small capacitor could be used from COMP to GND if needed.
D1 1N5819 R1 100K C1 10uF
2 1 3
T1 D2 1N5819 Vo = 5V C2 100uF R4 330 R6 1K C7 U3 SC431 R8 1K 220pF R7 10K
Vin = 48V C3 10uF R2 2.7MEG R5 100K 1
8
10 R3 68 C4 0.1uF 8 U2 SC4911 Q1 Si2320DS C6 150pF 5 U1 NEC2501
C5 100pF
LUVLO 3 FB
VCC OUT 7
4 C8 47nF R12 4.7K
COMP
ILIM GND 6 R13 100K
5
R10 51 C9 1000pF R11 1.00
R9 10K
RT 2
Figure 3. Isolated 48V to 12V Flyback Converter
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SC4911
POWER MANAGEMENT Typical Applications (Cont.)
Non-Isolated feedback 48V to 12V application Figure 4 is an example of low cost, non-isolated feedback, low power application of the SC4911. T1 is a two winding transformer. The output voltage is directly fed to the primary circuitry. During the start-up, the input source trickle charges the capacitor C3 through start-up resistor R1. As long as the voltage across C3 reaches the Vcc turn-on threshold, the SC4911 turns on abruptly and starts driving the gate of MOSFET Q1. After the SC4911 starts up the secondary winding supplies the primary circuitry. In this 36V to 72V telecom application, LUVLO monitors the input voltage via resistor R2 and R4 to determine turn-on and turn-off of the converter. Capacitor C4 helps filter out the noise from the input line. Voltage feedback is directly brought from the output to the input error amplifier of SC4911 through R5 and R7. C6 and R9 are for frequency compensation. An additional small capacitor could be used from COMP to GND if needed.
D1 1N5819 R1 100K R2 2.7MEG R4 100K C3 10uF 8 U1 SC4911 1 LUVLO VCC 3 FB OUT 7
Vin = 48V
T1
D2 1N5819 C1 47uF
Vo = 12V
C2 10uF C4 100pF
R3 68 C5 150pF
R5 383K
Q1 Si2320DS
4 R7 100K C6 47nF
COMP RT 2 6 R10 100K
ILIM GND
5
R6 51 C7 1000pF R8 1.00
R9 4.7K
Figure 4. Non-isolated 48V to 12V Flyback Converter
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SC4911
POWER MANAGEMENT Typical Applications (Cont.)
48V to 12V FLYBACK CONVERTER AS BATTERY CHARGER Figure 5 is another example of non-isolated feedback, low power and compact application of the SC4911 as a battery charger. Battery charging rate is determined by current sense resistor R8. LUVLO is used to not only turn on or turn off the charger by battery charge controller, but also monitors the input line voltage via resistor R2 and R5 to determine turn-on and turnoff of the converter. During the start-up, the input source trickle charges the capacitor C3 through start-up resistor R1. As long as voltage across C3 reaches the Vcc turn-on threshold, the SC4911 turns on abruptly and starts driving the gate of MOSFET Q1. After the SC4911 starts up the secondary winding supplies the primary circuitry. In this application, voltage feedback is directly brought from the output to the input error amplifier of SC4911 through R4 and R7. C5 and R9 are for frequency compensation.
D1 1N5819 R1 100K R2 2.7MEG R5 100K 1 3 8 C3 10uF
Vin = 48V
T1
D2 1N5819 C1 47uF
To 12V Battery
C2 10uF
R4 619K
R3 68 C4 150pF
To Battery Charger Controller
U1 SC4911 Q1 Si2320DS
LUVLO VCC FB OUT 7
4 R7 100K C5 47nF R9 4.7K
COMP RT 2
ILIM GND 6 R10 100K
5
R6 51 C6 1000pF R8 1.00
Figure 5. 48V to 12V Flyback Converter as Battery Charger
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SC4911
POWER MANAGEMENT Evaluation Board Schematic
J3 Vb=12V 1 2 C1 47uF
D1 1N5819HW D3 1N5819HW
T1 2 31414R 1 3 5 8 D2 B150 C2 47uF C3 47uF J1 1 2 Vo=12V
J2 Vin=48V 1 2 C4 10uF/100V
R2 2.7MEG R4 100K 1 8
R1 100K C5 10uF U1 SC4911 R6 0 5
R3 68 C7 150pF/100V
4
R5 383K
C6 470pF
LUVLO VCC 3 FB OUT 7
Q1 Si2320DS
4 R8 100K C8 47nF R10 4.99K
R7 51 C9 1000pF R9 1.00
COMP RT 2
ILIM GND 6 R11 100K
C10 470pF
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SC4911
POWER MANAGEMENT Evaluation Board Schematic- BOM
Item 1 2 3 4 6 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Quantity Reference 3 1 1 1 1 2 1 2 1 1 4 1 1 1 1 1 1 1 1 1 C 1, C 2, C 3 C4 C5 C6 C7 C 8, C 10 C9 D 1, D 3 D2 Q1 R1, R4, R8, R11 R2 R3 R5 R6 R7 R9 R10 T1 U1 Part/Description 47uF 10uF/100V 10uF 470pF 150pF/100V 47nF 1000pF 1N5819HW B150-13 S i 2320D S 100K 2.7 MEG 68 383K 0 51 1 4.99K 31414R SC4911IMSTR MIDCOM Semtech Corp. 805-498-2111 Si2320DS-T1 Manufacturer # 16TPB47M EEU-FC2A100 GRM32ER61C106KC31L Foot Print SM/CT 7343 CYL/D.250/LS.100/.031 SM/C 1210 SM/C 0805 SM/C 0805 SM/C 0805 SM/C 0805 SOD123 SM/SMA SOT-23 SM/R 0805 SM/R 0805 SM/R 0805 SM/R 0805 SM/R 0805 SM/R 0805 SM/R 0805 SM/R 0805 31414R MSOP-8
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SC4911
POWER MANAGEMENT Typical Characteristics
Vcc Turn-On Voltage vs Temperature
5.80 Turn On Voltage (V) 5.70 5.60 5.50 5.40 5.30 5.20 5.10 -40 -20 0 20 40 60 80 100 120 Temperature (C)
Oscillator Frequency vs Temperature
200.00 199.00 198.00 197.00 196.00 195.00 194.00 193.00 192.00 191.00 190.00 -40
Rt = 100k
Frequency (kHz)
-20
0
20
40
60
80
100
120
Temperature (C)
Vcc Zener Shunt Voltage vs Temperature
16.04 Zener Shunt Voltage (V) Lockout Threshold (V) 16.02 16.00 15.98 15.96 15.94 15.92 15.90 -40 -20 0 20 40 60 80 100 120 Temperature (C) 1.245 1.244 1.243 1.242 1.241 1.240 1.239 1.238 1.237 1.236
Line UVLO Lockout Threshold vs Temperature
-40
-20
0
20
40
60
80
100
120
Temperature (C)
Max. Duty Cycle vs Temperature
66.50 Max. Duty Cycle (%) 66.00 65.50 65.00 64.50 64.00 63.50 63.00 -40 -20 0 20 40 60 80 100 120 Temperature (C)
Feedback Input Voltage (V) 2.525 2.520 2.515 2.510 2.505 2.500 2.495 2.490
Error Amplifier Feedback Voltage vs Temperature
-40
-20
0
20
40
60
80
100
120
Temperature (C)
Current Limit Threshold vs Temperature
528 527 Threshold (mV) 526 525 524 523 522 521 520 -40 -20 0 20 40 60 80 100 120 Temperature (C)
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SC4911
POWER MANAGEMENT Outline Drawing - MSOP-8
e/2 A N 2X E/2 PIN 1 INDICATOR ccc C 2X N/2 TIPS 12 e B E1 E D
DIM
A A1 A2 b c D E1 E e L L1 N 01 aaa bbb ccc
DIMENSIONS MILLIMETERS INCHES MIN NOM MAX MIN NOM MAX
.043 .000 .006 .030 .037 .009 .015 .009 .003 .114 .118 .122 .114 .118 .122 .193 BSC .026 BSC .016 .024 .032 (.037) 8 0 8 .004 .005 .010 1.10 0.00 0.15 0.95 0.75 0.22 0.38 0.08 0.23 2.90 3.00 3.10 2.90 3.00 3.10 4.90 BSC 0.65 BSC 0.40 0.60 0.80 (.95) 8 0 8 0.10 0.13 0.25
aaa C SEATING PLANE
D A2 A GAGE PLANE 0.25
H c
C
A1 bxN bbb C A-B D
L (L1) DETAIL
01
A
SIDE VIEW
SEE DETAIL
A
NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. DATUMS -A- AND -B- TO BE DETERMINED AT DATUM PLANE -H3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. 4. REFERENCE JEDEC STD MO-187, VARIATION AA.
Land Pattern - MSOP-8
X
DIM
(C) G Z C G P X Y Z
DIMENSIONS INCHES MILLIMETERS
(.161) .098 .026 .016 .063 .224 (4.10) 2.50 0.65 0.40 1.60 5.70
Y P
NOTES: 1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET.
Contact Information
Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805)498-2111 FAX (805)498-3804
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