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Buck or Buck-Boost(Inverting) Current Mode Controller
POWER MANAGEMENT Description
The SC4508 is a low voltage current mode switching regulator controller that drives a P-channel power MOSFET with programmable switching frequency. It can be configured in either buck or buck boost (inverting) converters. The converters can be operated from 2.7V to 15V input voltage range. The typical operating supply current is 3mA and a shutdown pin allows the user to turn the controller off reducing it to less than 200A. The output voltage can adjusted by external resistor divider. The switching frequency is programmable up to 1.5MHz, allowing small inductor and capacitor values to minimize PCB space. The operating current level is programmable via an external sense resistor. Accessible reference voltage allows users to make output voltage as low as they want.
SC4508
Features
Wide input voltage range 2.7V to 15V Programmable output voltage Programmable switching frequency up to 1.5MHz Buck or buck boost(inverting) configuration Current mode control with slope compensation Very low quiescent current in shutdown mode Accessible reference voltage MLPQ-12 package
Applications
Low power point of use converters Single or multiple output low power converters Positive and/or negative output voltage DSL cards Graphic cards I/O cards Negative bias supplies
Typical Application Circuits
Vin Vin
VDD VDD VREF VREF FB+ FBCOMP CS FB+ OUT FB-
SC4508
SC4508
CS
COMP SS/SHDN GND OSC SS/SHDN GND OSC
OUT
Revision: November 30, 2004
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SC4508
POWER MANAGEMENT Absolute Maximum Rating
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. Exposure to Absolute Maximum rated conditions for extended periods of time may affect device reliability.
Parameter VDD to GND SS/SHDN to GND FB+, FB-, COMP, OSC to GND VREF Current OUT Source or Sink Thermal Resistance, Junction to Ambient Thermal Resistance, Junction to Case Storage Temperature Range Junction Temperature Range Peak IR Reflow Temperature 10 - 40s
Symbol
Maximum -0.3 to 16 3.2 5 1 250
Units V V V mA mA C/W C/W C C C
JA J C TSTG TJ TPKG
48 3 -60 to +150 -40 to +150 260
Electrical Characteristics
Unless specified: VDD = 2.7V to 15V , TA = TJ = -40C to 125C
Parameter Pow er Supply Input Voltage Range Quiescent Current Operating Current Undervoltage Lockout Start Threshold UVLO Hysteresis Oscillator Frequency Range Frequency Charge Current Error Amplifier Feedback Input Voltage Input Bias Current Transconductance Output Source or Sink Current
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Test Conditions
Min
Typ
Max
Unit
2.7 SHDN = low SHDN = high, No load 200 3
15 500
V A mA
VDD rising
2.5 100
2.55
V mV
100 C T = 300pF 450 500 100
1500 550
KHz KHz A
0.75 100 5 50
2
V nA mS A
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100
SC4508
POWER MANAGEMENT Electrical Characteristics
Unless specified: VDD = 2.7V to 15V , TA = TJ = -40C to 125C
Parameter PWM Comparator Delay to Output VREF Reference Output Voltage Output Current Line Regulation Load Regulation Soft Start/Shutdow n Charge Current Discharge Current SHDN Logic Hight Voltage SHDN Logic Low Voltage Current Limit Cycle by Cycle Threshold Shutdown Threshold Delay to Output Output Gate Drive On-Resistance(H) Gate Drive On-Resistance(L) Rise Time Fall Time
Test Conditions
Min
Typ
Max
Unit
50
ns
1.231
1.25
1.269 1
V mA mV mV
VDD = 2.7 to 15V, IVREF = 1mA VDD = 5V, IVREF = 0 to 1mA
4 4
20 12 2 0.25
A mA V V
V D D = 5V
90
110 0.16 50
130
mV V nS
VDD = 5V, ISOURCING = 250mA VDD = 5V, ISINK = 250mA COUT = 200pF COUT = 200pF
20 20 20 20
Ohm Ohm nS nS
Note: (1) This device is ESD sensitive. Use of standard ESD handling precautions is required.
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SC4508
POWER MANAGEMENT Pin Configuration
TOP VIEW
VDD OSC COMP
Ordering Information
DEVICE SC4508IMLTRT(2) PACKAGE MLPQ-12(1) Temp. Range (TJ) -40C to 150C
12 OUT CS SS/SHDN 1 2 3 4
11
10 9 8 7 FB+ FBPGND
Notes: (1) Only available in tape and reel packaging. A reel contains 3000 devices. (2) Lead free product. This product is fully WEEE and RoHS compliant.
5
6
VREF
NC
AGND
(MLPQ-12 4x4)
Pin Descriptions
Pin # 1 2 3 Pin Name OUT CS SS/SHDN Pin Function Gate driver output for external P-MOSFET. OUT swings from VDD to PGND. Current sense input pin. Connect a current sense resistor between VDD and CS. Soft start pin. Connects an external capacitor between this pin and AGND. The ramp up time is defined by the capacitor. The device goes into shutdown when VSS/SHDN is pulled below 0.25V. 1.25V reference output. VREF can source up to 1mA. Bypass with a 0.1uF ceramic capacitor from VREF to AGND. No connection. Analog ground. Power ground. Error amplifier inverting input. Error amplifier non-inverting input. Compensation pin for the internal transconductance error amplifier. Connect loop compensation network from COMP to AGND. Oscillator frequency set input. Connect a ceramic capacitor from OSC to AGND to set the internal oscillator frequency from 100KHz to 1.5MHz. Use equation to set the 100A f= oscillator frequency. C is the capacitor from OSC to AGND. C * 0.65 Supply voltage. Bypass a 1uF ceramic capacitor from VDD to PGND. Pad for heatsinking purposes. Connect to ground plane using multiple vias. Not connected internally.
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4 5 6 7 8 9 10 11
VREF NC AGND PGND FB FB + COMP OSC
12 -
VD D THERMAL PAD
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SC4508
POWER MANAGEMENT Block Diagram
Marking Information
Top View
yyww = Date Code (Example: 0012)
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SC4508
POWER MANAGEMENT Application Information
The SC4508 is designed to control buck (step down) or buck-boost (inverting) converter with P-channel MOSFET as a switch using current mode, programmable switching frequency architecture. During steady state operation, the switch is turned on each cycle and turned off when the voltage across current sense resistor exceeds the voltage level at COMP pin set by voltage loop error amplifier. A fixed 0.5V artificial ramp is added internally to the sensed current signal for operations when dutycycle is larger than 50%. In over load or output shortage condition, if the sensed current signal reaching typical 120mV, the switch is turned off immediately in the same cycle. If the sensed current signal further increases to typical 160mV, not only the switch is turned off but also the soft start capacitor is discharged by a internal MOSFET to ground then charging back to threshold 250mV during which the switch is held off. With the "hiccup" mode over current protection, the thermal stress is reduced in the faulty conditions. Current Sense and Current Limit The typical cycle-by-cycle current limit threshold in the current sense pin of the SC4508 is 120mV. The over current limit is assumed typical 120% of full load current. Then the current sense resistor can be calculated by the following equation: Rs = 120% * I (pk ) L
IRMS _ INPUT _ CAP IO
IN O O D IL (pk) = IO + 2 f * L ( V + V ) for Buck s IN D
Inductor Selection A inductor is chosen based on the required output ripple current which is usually 20-30% of load current. The ripple current I decreases with higher inductance and increases with higher input voltage VIN or output voltage VO.
L= VIN - VO VO + VD ( ) for Buck fs * IL VIN + VD VO + VD VIN ( ) for Buck-Boost fs * IL VIN + VO + VD
L=
Specify the maximum inductor current larger than IL(pk) set by the current sense resistor RS to avoid the inductor core saturation. Input Capacitor CIN and Output Capacitor COUT Selection Both input and output capacitors need to be sized to handle the ripple current safely. Buck converters have high ripple current in the input side while buck-boost converters have high ripple current in the both input and output capacitors. Therefore, the RMS value of the current must be less than the high frequency ripple rating of the capacitors. In continuous mode operation,
VO ( VIN - VO ) VIN
120mV
for Buck
1V -V
V +V
IRMS _ CAP IO IL (pk ) = IO VIN + VO + VD VIN + VO + VD 1 VIN ( ) 2 fs * L VIN + VO + VD
VO + VD for Buck-Boost input and output VIN
capacitors. The C is selected for Buck converter is by required OUT output ripple voltage and converter loop stability. The output ripple is determined by:
VO IL (ESR + 1 ) 8fs C OUT
for Buck - Boost
IO - full load current VO - output voltage VIN - input voltage VD - diode forward voltage drop fS - switching frequency L - inductor
where f is converted switching frequency, C is output S OUT capacitance and I is inductor ripple current.
L
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SC4508
POWER MANAGEMENT Application Information (Cont.)
In Buck converter, the worst case input RMS ripple current occurs at 50% duty cycle, or at VIN = 2VOUT. Under this condition, the input capacitors RMS ripple current is about half of the load current. MOSFET Selection The selection criteria for the power MOSFET is its operating junction temperature not exceeding the maximum junction temperature. Therefore, from a specified ambient temperature, the maximum junction to ambient temperature rise has to be determined, which is related to the MOSFET power dissipation or power loss. The power loss includes conduction loss and switching loss. The conduction loss is given by:
PON = ( VO + VD 2 )IOR DS( ON) for Buck VIN + VD
LAYOUT GUIDELINES Careful attention to layout requirements are necessary for successful implementation of the SC4508 PWM controller. High switching currents are present in the application and their effect on ground plane voltage differentials must be understood and minimized. 1). The high power parts of the circuit should be laid out first. A ground plane should be used, the number and position of ground plane interruptions should be such as to not unnecessarily compromise ground plane integrity. Isolated or semi-isolated areas of the ground plane may be deliberately introduced to constrain ground currents to particular areas, for example, the input capacitor and output capacitor ground. 2). The loop formed by the Input Capacitor(s) (Cin), the MOSFET and the Schottky diode or inductor must be kept as small as possible. This loop contains all the high current, fast transition switching. Connections should be as wide and as short as possible to minimize loop inductance 3). The connection between the junction of MOSFET, Schottky diode and the output inductor should be a wide trace or copper region. It should be as short as practical. Since this connection has fast voltage transitions, keeping this connection short will minimize EMI. 4) The Output Capacitor(s) (Cout) should be located as close to the load as possible, fast transient load currents are supplied by Cout only, and connections between Cout and the load must be short, wide copper areas to minimize inductance and resistance. 5) A separate analog ground plane connects to the SC4508 AGND pin. All analog grounding path including decoupling capacitors, feedback resistors, compensation components, and current-limit setting resistors should be connected to this plane.
PON = (
VIN + VO + VD VIN
2 )( VO + VD )IORDS( ON) for Buck-Boost
Freewheeling Diode Selection The Schottky diode is recommended as freewheeling diode in the both Buck and Buck-Boost applications. The diode conducts during the off-time. The diode voltage and current ratings are selected based upon the peak reverse voltage, the peak current and average power dissipation.
VD(REV ) = VIN , ID(PEAK ) = IO + IL V - VO ,ID( AVG ) = IO IN for Buck 2 VIN + VD
VIN + VO + VD VIN IL ,ID( AVG ) = IO 2
VD(REV ) = VIN + VO , ID(PEAK ) = IO ( for Buck - Boost
)+
The most stressful condition for the diode occurs when the output is shorted. Under this condition, due to the VOUT = 0, the diode conducts at close to 100% duty cycle. Therefore, attention should be paid to the thermal condition when laying out a board.
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SC4508
POWER MANAGEMENT Evaluation Board Schematic, Buck
12V GND
U1 SC4508 C4 1uF 12 3 C5 0.1uF 11 C6 330pF 5 6 7 VDD SS/SHDN OSC NC AGND PGND U1 C3 1nF 2 1 4 9 8 10 R4 20K C11 2.7nF TP4 C7 TP2 0.1uF R7 10K R2 10 R1 0.09 TP1 4 3 2 1 U2 FDFS2P102A 5 6 7 8 C8 470uF/6.3V C9 470uF/6.3V TP3 L1 10uH C1 47uF/16V C2 47uF/16V
CS OUT VREF FB+ FBCOMP
f s=440KHz
6V/1A GND
R5 3.74K
C10 2nF
TP5 TP6 TP7 TP8
R6 1.24K
Bill of Materials
Item 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Quantity 2 1 1 2 1 2 1 1 1 1 1 1 1 1 1 1 1 C1,C2 C3 C 4, C5,C7 C6 C8,C9 C 10 C11 L1 R1 R2 R4 R5 R6 R7 U1 U2 Reference Part 47uF/16V 1nF 1uF 0.1uF 330pF 470uF/6.3V 2nF 2.7nF 10uH 0.09 10 20K 4.75K 1.24K 10K S C 4508 F D F S 2P 102A Semtech Corp. Fairchild P/N: FDFS2P102A Sanyo P/N: 16TPB470M Manufacturer Sanyo P/N: 16TPB47M
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SC4508
POWER MANAGEMENT Evaluation Board Schematic, Buck-Boost
TP9
12V GND
U1 SC4508 C4 1uF 12 3 C5 0.1uF C6 680pF fs =2 5 0K H z 7 11 5 6 VDD SS/SHDN OSC NC AGND PGND U1 C3 1nF 2 1 4 9 8 10 R9 1.0K R4 20K D1 1N5819HW C11 2.7nF R8 249 C7 0.1uF TP4 TP10 R2 100 R7 10K R1 0.1 C1 47uF/16V C2 47uF/16V
CS OUT VREF FB+ FBCOMP
TP1 U2 Si4831DY 4 5 3 2 1 6 7 8 L1 33uH TP3
C10 100pF
C8 47uF/16V
C9 100uF/6.3V
-5V/0.1A
R5 6.04K
R6 240
GND
TP5 TP6 TP7 TP8
Bill of Materials
Item 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 19 20 Quantity 3 1 1 2 1 1 1 1 1 1 1 1 1 1 2 1 1 1 1 C1,C2,C8 C3 C4 C 5, C 7 C6 C9 C 10 C11 D6 L1 R1 R2 R4 R5 R6, R8 R7 R9 U1 U2 Reference Part 47uF/16V 1nF 1uF 0.1uF 680pF 100uF/6.3V 100pF 2.7nF 1N5819HW 3.3uH 0.1 100 20K 6.04K 249 10K 1.0K S C 4508 S i 4831D Y Semtech Corp. Vishay Manufacturer Sanyo P/N: 16TPB47M
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SC4508
POWER MANAGEMENT Outline Drawing - MLPQ-12, 4 x 4
A D
B
DIM
A A1 A2 b D D1 E E1 e L N aaa bbb
DIMENSIONS INCHES MILLIMETERS MIN NOM MAX MIN NOM MAX
.031 .040 .002 .000 (.008) .010 .012 .014 .153 .157 .161 .074 .085 .089 .153 .157 .161 .074 .085 .089 .031 BSC .018 .022 .026 12 .003 .004 1.00 0.80 0.05 0.00 (0.20) 0.25 0.30 0.35 3.90 4.00 4.10 1.90 2.15 2.25 3.90 4.00 4.10 1.90 2.15 2.25 0.80 BSC 0.45 0.55 0.65 12 0.08 0.10
PIN 1 INDICATOR (LASER MARK)
E
A2 A aaa C A1 D1 LxN E/2 E1 2 1 N e D/2
NOTES: 1. 2.
SEATING PLANE C
bxN bbb
CAB
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
Land Pattern - MLPQ-12, 4 x 4
K
DIM
2x (C) H 2x G 2x Z C G H K P X Y Z
DIMENSIONS INCHES MILLIMETERS
(.148) .106 .091 .091 .031 .016 .041 .189 (3.75) 2.70 2.30 2.30 0.80 0.40 1.05 4.80
Y X 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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