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SC283
POWER MANAGEMENT Features

Dual Channel 2.5MHz, 1.8A Synchronous Step-Down Regulator
Description
The SC283 is a dual channel 1.8A synchronous stepdown regulator designed to operate with an input voltage range of 2.9 to 5.5 Volts. Each channel offers fifteen pre-determined output voltages via four control pins programmable from 0.8 to 3.3 Volts. The control pins allow for on-the-fly voltage changes, enabling system designers to implement dynamic power savings. The SC283 is also capable of adjusting the output voltage via an external resistor divider. The device operates with a fixed 2.5MHz oscillator frequency, allowing the use of small surface mount external components. Connecting CTL0 -- CTL3 to logic low forces the device into shutdown mode reducing the supply current to less than 1A. Connecting any of the control pins to logic high enables the converter and sets the output voltage according to Table 1. Other features include undervoltage lockout, soft-start to limit inrush current, and over-temperature protection. The SC283 is available in a thermally-enhanced, 2mm x 3mm x 0.8mm MLPQ-W18 package and has a rated temperature range of -40 to +85C.
VIN Range: 2.9 - 5.5V VOUT Selectable: 0.8 - 3.3V Up to 1.8A Output Current for Each Channel Ultra-Small Footprint, <1mm Height Solution 2.5MHz Switching Frequency Efficiency Up to 93% Low Output Noise Across Load Range Excellent Transient Response Start Up into Pre-Bias Output 100% Duty-Cycle Low Dropout Operation <1A Shutdown Current Internal Soft Start Input Under-Voltage Lockout Output Over-Voltage, Current Limit Protection Over-Temperature Protection Adjustable Output Voltage 2mm x 3mm x 0.8mm thermally enhanced MLPQ-W18 package -40 to +85C Temperature Range Pb-Free product. RoHS/WEEE and Halogen Free compliant
Applications
Desktop Computing Set-Top Box LCD TV Network Cards Printer

Typical Application Circuit
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SC283
Pin Configuration Ordering Information
Device
SC283WLTRT(2)(3) SC283EVB(4)
Package
2mm x 3mm x 0.8mm MLPQ-W18 Evaluation Board
Notes: (1) Calculated from package in still air, mounted to 3" x 4.5", 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards. (2) Available in tape and reel only. A reel contains 3,000 devices. (3) Pb-Free product. RoHS/WEEE and Halogen Free compliant. (4) Please specify the default VOUTA & VOUTB when ordering.
Table 1 - Output Voltage Settings
CTL3_ 0 CTL2_ 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 CTL1_ 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 CTL0_ 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Output Voltage Shutdown 0.80 1.00 1.025 1.05 1.20 1.25 1.30 1.50 1.80 2.20 2.50 2.60 2.80 3.00 3.30
2mm x 3mm x 0.8mm MLPQ-W18 JA = 65C/W (1)
0 0 0 0 0 0 0 1 1 1 1 1 1 1 1
Marking Information
Marking for 2mm x 3mm MLPQ-W 18 Lead Package: yw = Datecode (Reference Package Marking Design Guidelines, Appendix A) xxx = Semtech Lot number (Example: 901)
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SC283
Absolute Maximum Ratings
VINA and VINB Supply Voltages ..................... -0.3 to 6.0V LXA, LXB Voltage .... . CTLxA/B pins Voltages ESD Protection Level(6) -1 to VIN+1V, -3V (20ns Max), 6V Max -0.3 to VIN+0.3V -0.3 to VIN+0.3V 3.5kV ..................... VOUTA, VOUTB Voltage ........................
Recommended Operating Conditions
Supply Voltage VINA and VINB ........................ 2.9 to 5.5V Maximum Output Current for each channel ............ 1.8A Temperature Range ................................. -40 to +85C
Peak IR Reflow Temperature ............................... 260C ....................................
Thermal Information
Thermal Resistance, Junction to Ambient(5) ............ 65 C/W Maximum Junction Temperature ........................ +150C Storage Temperature Range ..................... -65 to +150 C
Exceeding the absolute maximum ratings may result in permanent damage to the device and/or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended. Notes: (5) Calculated from package in still air, mounted to 3" x 4.5", 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards. (6) Tested according to JEDEC standard JESD22-A114-B.
Electrical Characteristics
Unless specified: VINA= VINB= 5.0V, VOUTA= VOUTB=1.50V, CINA=CINB=10F, COA=COB= 22F, L= 2.2H, -40C TJ +125 C. Unless otherwise noted typical values are TA= +25 C.
Parameter
Under-Voltage Lockout Output Voltage Tolerance(7) Current Limit Supply Current Shutdown Current High Side Switch Resistance(8) Low Side Switch Resistance(8) LX Leakage Current(8) Load Regulation Oscillator Frequency Soft-Start Time Foldback Holding Current CTLx Input Current(8) CTLx Input High Threshold
Symbol
UVLO
Conditions
Rising VINA, VINB Hysteresis
Min
2.65 240 -2.0 2.25
Typ
2.75 300
Max
2.85
Units
V mV
VOUT ILIMIT IQ ISHDN RDSON_P RDSON_N ILK(LX) VLOAD-REG fOSC tSS ICL_HOLD ICTL_ VCTLx_HI
Channel A & B; VIN= 2.9 - 5.5V; IOUT=0A Channel A & B; Peak LX current Channel A & B; No load, Per channel CTL0-3= GND, Per channel Channel A & B; ILX= 100mA, TJ= 25 C Channel A & B; ILX= -100mA, TJ= 25 C Channel A & B; VIN= 5.5V; LX= 0V; CTL0-3= GND Channel A & B; VIN= 5.5V; LX= 5.0V; CTL0-3= GND Channel A & B; VIN= 5.0V; IOUT=1mA - 1.8A Channel A & B Channel A & B; IOUT= 1.8A Average LX Current, VOUT=1.5V Average LX Current, VOUT=3.3V Channel A & B; CTL0-3=VIN or GND Channel A & B
+2.0 3.0 10 1 95 65 1 10 10 3.75
% A mA A m
-10
-1 0.5
A %
2.125
2.500 850 240 130
2.875
MHz s mA mA
-2.0 1.2
2.0
A V
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SC283
Electrical Characteristics (continued)
Parameter
CTLx Input Low Threshold VOUT Over Voltage Protection Thermal Shutdown Temperature Thermal Shutdown Hysteresis
Symbol
VCTLx_LO VOVP TSD TSD_HYS
Conditions
Channel A & B Channel A & B Channel A & B(9) Channel A & B
(9)
Min
Typ
Max
0.4
Units
V % C C
115 160 10
Notes: (7) The "Output Voltage Tolerance" includes output voltage accuracy, voltage drift over temperature and the line regulation. (8) The negative current means the current flows into the pin and the positive current means the current flows out from the pin. (9) The thermal shutdown for both Channel A and B is independent from each other.
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SC283
Typical Characteristics
Circuit Conditions: CIN= 10uF/6.3V; COUT= 22uF/6.3V, Unless otherwise noted, L= 2.2uH (TOKO: 1127AS-2R2M).
Efficiency vs. Load Current Efficiency
100%
Total Loss (Per Channel) vs. Load Current Total Loss
1000
VIN=5.0V;VOUT=3.3V
95% 90% 800
TA=25 C
VIN=5.0V;VOUT=3.3V
Efficiency (%)
Loss (mW)
85% 80% 75% 70%
600
VIN=3.3V;VOUT=1.5V
400
VIN=3.3V;VOUT=1.5V
VIN=5.0V;VOUT=1.5V
65% 60% 0.0 0.3 0.6 0.9 1.2 Output Current (A) 1.5 1.8
200
TA=25 C
0 0.0 0.3 0.6
VIN=5.0V;VOUT=1.5V
0.9 1.2 Output Current (A) 1.5 1.8
Load Regulation Load Regulation
1.0% 0.8% 0.6% 0.4%
DropoutDropout Voltage of 100% Duty Cycle Operation Voltage in 100% Duty Cycle Operation
500 450 400
TA=25 C
TA= 25 C
Dropout Voltage (mV)
VIN=3.3V;VOUT=1.5V VIN=5.0V;VOUT=1.5V
350 300 250 200 150 100
Load Regulation
L= 1071AS-2R2M (DCR= 60m_max)
0.2% 0.0% -0.2% -0.4% -0.6% -0.8% -1.0% 0.0 0.3 0.6 0.9
VIN=5.0V;VOUT=3.3V
50 0
L= 1127AS-2R2M (DCR=48m_max)
1.2
1.5
1.8
0.0
0.3
0.6
0.9 Output Current (A)
1.2
1.5
1.8
Output Current (A)
UVLOUVLO Rising Threshold Variation Rising Threshold Variation
1.0% 0.8% 0.6% 0.4% Variation Variation 0.2% 0.0% -0.2% -0.4% -0.6% -0.8% -1.0% -40 -15 10 35 60 85 Ambient Temperature ( C) 5% 4% 3% 2% 1% 0% -1% -2% -3%
UVLO Hysteresis Variation UVLO Hysteresis Variation
IOUT= 0A
-4% -5% -40
IOUT= 0A
-15 10 35 60 85
Ambient Temperature ( C)
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SC283
Typical Characteristics (continued)
Circuit Conditions: CIN= 10uF/6.3V; COUT= 22uF/6.3V, Unless otherwise noted, L= 2.2uH (TOKO: 1127AS-2R2M).
RDS(ON)RDSON (P & N) Variation over Line Variation vs. Input Voltage
30% 25% 20% 15% 20% 15%
RDSON (P & N) Variation Over Temperature RDS(ON) Variation vs. Temperature
VIN= 5.0V ILX= 100mA
P-Channel
10% 5%
Variation
Variation
10% 5% 0% -5% -10% 2.5 3.0 3.5 4.0 Input Voltage (V) 4.5 5.0 5.5
0% -5% -10%
N-Channel
ILX= 100mA TA= 25 C
N-Channel
-15% -20% -40 -15
P-Channel
10
35
60
85
Ambient Temperature ( C)
Switching Frequency Variation vs. Input Switching Frequency Variationover Line Voltage 5% 4% 3% 2%
Switching Frequency Variation vs. Temperature Switching Frequency Variation
1.0% 0.8%
VOUT= 3.3V
0.6% 0.4%
Variation
0% -1% -2% -3% -4% -5% 2.5 3.0 3.5 4.0 Input Voltage (V) 4.5 5.0 5.5
Variation
VOUT= 1.5V IOUT= 0A TA= 25 C
1%
0.2% 0.0% -0.2% -0.4% -0.6% -0.8% -1.0% -40 -15 10 35 60 85 Ambient Temperature ( C)
VIN= 5.0V IOUT= 0A
LineRegulation ove Line Line Regulation
1.0% 0.8% 0.6% 0.4% 1.0% 0.8% 0.6%
Line Regulation vs. Temperature Line Regulation over Temperature
VOUT= 1.5V
0.4%
Regulation
0.0% -0.2% -0.4% -0.6% -0.8% -1.0% 2.5 3.0 3.5 4.0 Input Voltage (V) 4.5 5.0 5.5
Regulation
VOUT= 3.3V IOUT= 0A TA= 25 C
0.2%
0.2% 0.0% -0.2% -0.4% -0.6% -0.8% -1.0% -40 -15 10 35 60 85 Ambient Temperature ( C)
VOUT= 1.5V IOUT= 0A
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SC283
Typical Waveforms
Circuit Conditions: CIN= 10uF/6.3V; COUT= 22uF/6.3V, L= 2.2uH (TOKO: 1127AS-2R2M).
Output Voltage Ripple (VOUT=1.5V)
VOUT 10mV/div
Output Voltage Ripple (VOUT=1.5V)
Output Voltage Ripple (VOUT=1.5V)
VOUT 10mV/div
Output Voltage Ripple (VOUT=1.5V)
ILX 1A/div
ILX 1A/div
VLX 2V/div
VIN=3.3V IOUT=1.8A
VLX 2V/div 500ns/div
VIN=5.0V IOUT=1.8A
500ns/div
Output Voltage Ripple (VOUT=3.3V)
VOUT 10mV/div
Output Voltage Ripple (VOUT=3.3V)
Output Voltage Ripple (VOUT=3.3V)
Output Voltage Ripple (VOUT=3.3V)
VOUT 10mV/div ILX 0.5A/div ILX 1A/div
VLX 2V/div
VIN=5.0V IOUT=0A
VLX 2V/div 500ns/div
VIN=5.0V IOUT=1.8A
500ns/div
Transient Response (VOUT=1.5V; 0A to 1A to 0A)
Transient Response (VOUT=1.5V)
Transient Response (VOUT=3.3V; 0A to 1A to 0A)
Transient Response (VOUT=3.3V)
100mV/div
VOUT
100mV/div
VOUT
1A/div
IOUT
500mA/div
IOUT
VIN=5.0V IOUT=0A to 1A
50s/div
VIN=5.0V IOUT=0A to 1A
50s/div
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SC283
Typical Waveforms (continued)
Circuit Conditions: CIN= 10uF/6.3V; COUT= 22uF/6.3V, L= 2.2uH (TOKO: 1127AS-2R2M).
Start Up (Enable)(VOUT=1.5V)
Start Up (VOUT=1.5V)
Start Up (Enable)(VOUT=1.5V)
Start Up (VOUT=1.5V)
2V/div
VIN
2V/div
VIN
2V/div
VCTLx
2V/div
VCTLx
0.5V/div
VOUT
0.5V/div
VOUT
VIN=5.0V ROUT=1k
50s/div
VIN=5.0V ROUT=0.83 (1.8A)
200s/div
Start Up (Power up VIN=VCTLx) (VOUT=1.5V)
Start Up (VOUT=1.5V), EN=VIN
Start Up (Power up VIN=VCTLx) (VOUT=1.5V)
Start Up (VOUT=1.5V), EN=VIN
2V/div
VIN
2V/div
VIN
0.5V/div
VOUT
0.5V/div
VOUT
VIN=5.0V ROUT=1k
200s/div
VIN=5.0V ROUT=0.83 (1.8A)
200s/div
Start Up (Enable)(VOUT=3.3V)
Start Up (VOUT=3.3V)
Start Up (Enable)(VOUT=3.3V)
Start Up (VOUT=3.3V)
2V/div
VIN
2V/div
VIN
2V/div
VCTLx
2V/div
VCTLx
1V/div
VOUT
1V/div
VOUT
VIN=5.0V ROUT=1k
100s/div
VIN=5.0V ROUT=1.83 (1.8A)
200s/div
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SC283
Typical Waveforms (continued)
Circuit Conditions: CIN= 10uF/6.3V; COUT= 22uF/6.3V, L= 2.2uH (TOKO: 1127AS-2R2M).
Start Up (Power up VIN=VCTLx) (VOUT=3.3V)
Start Up (VOUT=3.3V), EN=VIN
Start Up (Power up VIN=VCTLx) (VOUT=3.3V)
Start Up (VOUT=3.3V), EN=VIN
2V/div
VIN
2V/div
VIN
1.5V/div
VOUT
1.5V/div
VOUT
VIN=5.0V ROUT=1k
200s/div
VIN=5.0V ROUT=1.83 (1.8A)
200s/div
Shutdown-Disable (1.5V)
Shutdown (Disable)(VOUT=1.5V)
Shutdown (Disable)(VOUT=3.3V)
Shutdown-Disable (3.3V)
2V/div
VIN
2V/div
VIN
2V/div
VCTLx
2V/div
VCTLx
1V/div
VOUT
1.5V/div
VOUT
VIN=5.0V ROUT=1.5
200s/div
VIN=5.0V ROUT=3.3
200s/div
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SC283
Pin Descriptions
Pin #
1 2 3, 13, T1 4, 12, T2 5
Pin Name
VINA LXA GNDA GNDB CTL3B
Pin Function
Channel A. Input supply voltage for the converter power stage and internal circuitry. Switching node of Channel A - connect an inductor between this pin and the output capacitor. Channel A. Ground connection for converter power stage and internal circuitry. Channel B. Ground connection for converter power stage and internal circuitry. Channel B. Control bit 3 - see Table 1 for decoding. This pin has a 1 M internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. Channel B. Control bit 2 - see Table 1 for decoding. This pin has a 1 M internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. Channel B. Control bit 1 - see Table 1 for decoding. This pin has a 1 M internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. Channel B. Control bit 0 - see Table 1 for decoding. This pin has a 1 M internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. Output voltage sense pin of Channel B. Channel B. Input supply voltage for the converter power stage and internal circuitry. Switching node of Channel B - connect an inductor between this pin and the output capacitor. Channel A. Control bit 3 - see Table 1 for decoding. This pin has a 1 M internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. Channel A. Control bit 2 - see Table 1 for decoding. This pin has a 1 M internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. Channel A. Control bit 1 - see Table 1 for decoding. This pin has a 1 M internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. Channel A. Control bit 0 - see Table 1 for decoding. This pin has a 1 M internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. Output voltage sense pin of Channel A.
6
CTL2B
7
CTL1B
8 9 10 11 14
CTL0B VOUTB VINB LXB CTL3A
15
CTL2A
16
CTL1A
17 18
CTL0A VOUTA
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SC283
Block Diagram
Current Amp
AVIN
Plimit Comp Plimit Amp
PVIN VINA
Oscillator & Slope Generator Control Logic
VOUT VOUTA
LXA LX
CTL0A CTL0 CTL1A CTL1
CTL2 CTL2A Voltage Select
Error Amp 500mV Ref PWM Comp
CTL3A CTL3
GNDA PGND
AGND
Current Amp
AVIN
Plimit Comp Plimit Amp
PVIN VINB
Oscillator & Slope Generator Control Logic
VOUT VOUTB
LXB LX
CTL0B CTL0 CTL1B CTL1
CTL2 CTL2B Voltage Select
Error Amp 500mV Ref PWM Comp
CTL3B CTL3
GNDB PGND
AGND
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SC283
Applications Information
Detailed Description
The SC283 is a two channel synchronous step-down converter. Both channels on this device are designed to operate in fixed-frequency PWM mode at 2.5MHz and provide the same current capacity of up to 1.8A. The switching frequency is chosen to minimize the size of the external inductor and capacitors while maintaining high efficiency. Both channels of SC283 are independent. enough in value for the current through the resistor chain to be at least 20A in order to ignore the VOUT pin current.
RFB1 = VOUT - VOSTD RFB 2 VOSTD
(1)
where VOSTD is the pre-determined output voltage via the CTL pins. CFF is needed to maintain good transient response performance. The correct value of CFF can be found using Equation 2.
C FF [nF ] = 2.5 x
Operation
During normal operation, the PMOS MOSFET is activated on each rising edge of the internal oscillator. The voltage feedback loop uses an internal feedback resistor divider. The period is set by the internal oscillator. The device has an internal synchronous NMOS rectifier and does not require a Schottky diode on the LX pin. The device operates as a buck converter in PWM mode with a fixed frequency of 2.5MHz.
(VOUT - 0.5)2 VOSTD x( ) RFB1[k] (VOUT - VOSTD ) VOSTD - 0.5
(2)
Programmable Output Voltage
Both channels on SC283 have fifteen pre-determined output voltage values which can be individually selected by programming the CTL input pins (see Table 1 -- Output Voltage Settings). Each CTL pin has an active 1 M internal pulldown resistor. The 1M resistor is switched in circuit whenever the CTL input voltage is below the input threshold, or when the part is in undervoltage lockout. It is recommended to tie all high CTL pins together and use an external pull-up resistor to VIN if there is no enable signal, or if the enable input is an open drain/collector signal. The CTL pins may be driven by a microprocessor to allow dynamic voltage adjustment for systems that reduce the supply voltage when entering sleep states. Avoid all zeros being present on the CTL pins when changing programmable output voltages as this would disable the device. SC283 is also capable of regulating a different (higher) output voltage, which is not shown in the Table 1, via an external resistor divider. There will be a typical 2A current flowing into the VOUT pin. The typical schematic for an adjustable output voltage option from the standard 1.0V with CTLx=[0010], is shown in Figure 1. RFB1 and RFB2 are used to adjust the desired output voltage. If the RFB2 current is such that the 2A VOUT pin current can be ignored, then RFB1 can be found by Equation 1. RFB2 needs to be low
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To simplify the design, it is recommended to program the desired output voltage from a standard 1.0V as shown in Figure 1 with the correct CFF calculated from Equation 2. For programming the output voltage from other standard voltages, RFB1, RFB2 and CFF need to be adjusted to meet Equations 1 and 2.
L_ VIN CIN_ 10F VIN LX_
VOUT_ COUT_
(Channel A or B) VOUT_ CTL0_
SC283
RFB1
CFF
RFB2
RFB1 = (VOUT - 1) x RFB 2
for CTLx= 0010 (1.0V)
Enable
CTL1_ CTL2_ CTL3_ GND
Figure 1 -- Typical Schematic for Adjusting the Output Voltage Up from an Output Voltage of 1.0V (CTLx=[0010])
Maximum Power Dissipation
Each channel of SC283 has its own JA of 65C/W when only one channel is in operation. Since both channels are within same package, there is about 50% heat which will be transferred to the adjacent channel. The equivalent total thermal impedence will be higher when the neighboring channel is also in operation. To guarantee an operating junction temperature of less than 125C, Figure 2 shows the maximum allowable power loss of each channel. The curve is based upon the junction temperature of either channel reaching a maximum of 125C. Each channel of SC283 can support up to 1.8A load current. Figures 3a
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SC283
Applications Information (continued)
and 3b show the maximum allowable load current based upon the limit of maximum loss for VIN=3.3V and VIN=5.0V, respectively. The curves are drawn for high duty-cycle operation. If the operating duty-cycle is lower, the loss is SC283 Maximum Loss for lower allowing higher load current. TJ=125C
1.8 1.6 2.0 1.8
SC283 Maximum Load Current for T J=125 C
TA 42 C
Load Current of Channel B (A)
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 1.8
TA= 60 C
TA= 25 C
Loss of Channel B (W)
1.4 1.2
TA= 55 C
1.0 0.8
VIN= 3.3V VOUT= 2.5V
TA= 85 C
TA= 85 C
0.6 0.4 0.2 0.0 2.0 1.8
Load Current of Channel A (A)
(a) VIN= 3.3V, Current for T SC283 Maximum LoadVOUT=2.5V J=125 C
TA 47 C TA= 70 C
Load Current of Channel B (A)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6
Loss of Channel A (W)
Figure 2 -- Maximum allowable loss for each channel for a maximum junction temperature of 125C
Protection Features
The SC283 provides the following protection features: Current Limit Over-Voltage Protection Soft-Start Operation Thermal Shutdown
TA= 85 C
* * * *
VIN= 5.0V VOUT= 3.3V
0.8 1.0 1.2 1.4 1.6 2.0
Load Current of Channel A (A)
Current Limit and Protection
The internal PMOS power device in the switching stage is protected by a current limit feature. If the inductor current is above the PMOS current limit for 16 consecutive cycles, the part enters foldback current limit mode and the output current is limited to the current limit holding current (ICL_HOLD) of a few hundred milliampere. Under this condition, the output voltage will be the product of ICL_HOLD and the load resistance. The current limit holding current will decrease when the output voltage increases. The load presented must fall below the current limit holding current for the part to exit foldback current limit mode. Figure 4 shows how the typical current limit holding current varies with output voltage. The SC283 is capable of sustaining an indefinite short circuit without damage and will resume normal operation when the fault is removed. The foldback current limit mode is disabled during soft-start. Current limit functionality is shown in Figure 6.
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(b) VIN= 5.0V, VOUT=3.3V Figure 3 -- Maximum allowable Load Current for each channel for a maximum junction temperature of 125C
Current Limit Holding Current over Vout 300
TA= 25 C
Current Limit Holding Current (mA) 250
VIN= 3.6V VIN= 5.0V
200
150
100
50
VIN= 3.3V
0 1.0 1.5 2.0 2.5 3.0 3.5 Output Voltage (V)
Figure 4 -- Typical Current Limit Holding Current
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2.0
SC283
Applications Information (continued)
vs. Output Voltage
Over-Voltage Protection
In the event of a 15% over-voltage on the output, the PWM drive is disabled leaving the LX pin floating.
corner frequency of the output filter is shown in Equation 3.
1 2 L COUT
fC =
(3)
Soft-Start
The soft-start mode is activated after VIN reaches its UVLO and one or more CTL pins are set high to enable the part. A thermal shutdown event will also activate the soft start sequence. Soft-start mode controls the maximum current during startup thus limiting inrush current. The PMOS current limit is stepped through four soft start levels of approximately 20%, 25%, 40%, & 100%. Each step is maintained for 200s following an internal reference start up duration of 50s giving a total nominal startup period of 850s. During startup, the chip operates by controlling the inductor current swings between 0A and current limit. If at any time VOUT reaches 86% of the target or at the end of the soft-start period, the SC283 will switch to PWM mode operation. Figure 5 shows the typical diagram of soft start operation. The SC283 is capable of starting up into a pre-biased output. When the output is precharged by another supply rail, the SC283 will not discharge the output during the soft start interval.
Values outside this range may lead to instability, malfunction, or out-of-specification performance. In general, the inductance is chosen by making the inductor ripple current to be less than 30% of maximum load current. When choosing an inductor, it is important to consider the change in inductance with DC bias current. The inductor saturation current is specified as the current at which the inductance drops a specific percentage from the nominal value. This is approximately 30%. Except for short-circuit or other fault conditions, the peak current must always be less than the saturation current specified by the manufacturer. The peak current is the maximum load current plus one half of the inductor ripple current at the maximum input voltage. Load and/or line transients can cause the peak current to exceed this level for short durations. Maintaining the peak current below the inductor saturation specification keeps the inductor ripple current and the output voltage ripple at acceptable levels. Manufacturers often provide graphs of actual inductance and saturation characteristics versus applied inductor current. The saturation characteristics of the inductor can vary significantly with core temperature. Core and ambient temperatures should be considered when examining the core saturation characteristics. When the inductance has been determined, the DC resistance (DCR) must be examined. The efficiency that can be achieved is dependent on the DCR of the inductor. The lower values give higher efficiency. The RMS DC current rating of the inductor is associated with losses in the copper windings and the resulting temperature rise of the inductor. This is usually specified as the current which produces a 40C temperature rise. Most copper windings are rated to accommodate this temperature rise above maximum ambient. Magnetic fields associated with the output inductor can interfere with nearby circuitry. This can be minimized by the use of low noise shielded inductors which use the
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Shut Down
When all CTL pins of each channel are low, the channel will run in shutdown mode, drawing less than 1A from the input power supply. The internal switches and bandgap voltage will be immediately turned off.
Thermal Shutdown
The device has a thermal shutdown feature to protect the SC283 if the junction temperature exceeds 160C. During thermal shutdown, the on-chip power devices are disabled, tri-stating the LX output. When the temperature drops by 10C, it will initiate a soft start cycle to resume normal operation.
Inductor Selection
The SC283 converter has internal loop compensation. The compensation is designed to work with an output filter corner frequency of less than 40kHz for a VIN of 5V and 50KHz for a VIN of 3.3V over any operating condition. The
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SC283
Applications Information (continued)
minimum gap possible to limit the distance that magnetic fields can radiate from the inductor. However shielded inductors typically have a higher DCR and are thus less efficient than a similarly sized non-shielded inductor. Final inductor selection depends on various design considerations such as efficiency, EMI, size, and cost. Table 2 lists the manufacturers of recommended inductor options. The saturation characteristics and DC current ratings are also shown.
DCR Max () Rated Current (A) 1.80 2.70 2.50 2.20 L at Rated Current (H) 1.54 0.70 1.54 0.70 Dimensions LxWxH (mm) 2.8x3.0x1.5 2.8x3.0x1.5 3.5x3.7x1.8 3.2x3.2x1.5
mined by the capacitance of the ceramic output capacitor. The ceramic capacitor supplies the load current initially until the loop responds. Within a few switching cycles the loop will respond and the inductor current will increase to match the required load. The output voltage droop during the period prior to the loop responding can be related to the choice of output capacitor by the relationship from Equation 4.
COUT = 3 I LOAD VDROOP f OSC
(4)
Manufacturer Part Number TOKO 1071AS-2R2M TOKO 1071AS-1R0N TOKO 1127AS-2R2M Panasonic ELLVGG1R0N
L (H)
2.2020% 0.060 1.0030% 0.040 2.2020% 0.048 1.0023% 0.062
The output capacitor RMS ripple current may be calculated from Equation 5.
I COUT ( RMS ) = 1 VOUT (VIN ( MAX ) - VOUT ) L f OSC VIN 2 3
(5)
Table 2 - Recommended Inductors
Table 3 lists the manufacturers of recommended output capacitor options.
COUT Selection
The internal voltage loop compensation in the SC283 limits the minimum output capacitor value to 22F if using a 2.2H inductor or 44F if using a 1H inductor. This is due to its influence on the the loop crossover frequency, phase margin, and gain margin. Increasing the output capacitor above this minimum value will reduce the crossover frequency and provide greater phase margin. The total output capacitance should not exceed 50F to avoid any start-up problems. For most typical applications it is recommended to use an output capacitance of 22F to 44F. When choosing the output capacitor's capacitance, verify the voltage derating effect from the capacitor vendor's data sheet. Capacitors with X7R or X5R ceramic dielectric are recommended for their low ESR and superior temperature and voltage characteristics. Y5V capacitors should not be used as their temperature coefficients make them unsuitable for this application. The output voltage droop due to a load transient is deter(c) 2010 Semtech Corp. 15
Manufacturer Part Nunber Murata GRM21BR60J106K Murata GRM219R60J106K Murata GRM21BR60J226M Murata GRM31CR60J476M
Value (F)
Type
Rated Voltage (VDC)
Value at 3.3V (F) 4.74 4.05 6.57 20.3
Dimensions LxWxH (mm) 2.0x1.25x1.25 (EIA:0805) 2.0x1.25x0.85 (EIA:0805) 2.0x1.25x1.25 (EIA:0805) 3.2x1.6x1.6 (EIA:1206)
1010% 1010% 2220% 4720%
X5R X5R X5R X5R
6.3 6.3 6.3 6.3
Table 3 - Recommended Capacitors
CIN Selection
The SC283 source input current is a DC supply current with a triangular ripple imposed on it. To prevent large input voltage ripple, a low ESR ceramic capacitor is required. A minimum value of 10F should be used. It is important to consider the DC voltage coefficient characteristics when determining the actual required value. It should be noted a 10F, 6.3V, X5R ceramic capacitor with 5V DC applied may exhibit a capacitance as low as 4.5F.
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SC283
Applications Information (continued)
To estimate the required input capacitor, determine the acceptable input ripple voltage and calculate the minimum value required for CIN from Equation 6.
VOUT 1 - VIN = V - ESR f OSC I OUT VOUT VIN
C IN
(6)
The input capacitor RMS ripple current varies with the input and output voltage. The maximum input capacitor RMS current is found from Equation 7.
VOUT 1 - VIN
I CIN ( RMS ) =
VOUT VIN
(7)
The input voltage ripple and RMS current ripple are at a maximum when the input voltage is twice the output voltage or 50% duty cycle. The input capacitor provides a low impedance loop for the edges of pulsed current drawn by the PMOS switch. Low ESR/ESL X5R ceramic capacitors are recommended for this function. To minimise stray inductance ,the capacitor should be placed as closely as possible to the VIN and GND pins of the SC283.
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SC283
Applications Information (continued)
SC4633 Soft Start
A
Stage 0
Stage 1
B
Stage 2
C
G F H
Stage 3
Stage 5
D
Stage 4
I
Stage 6
E
Figure 5 -- Typical Diagram of Soft Start Operation
SC183C/SC283/SC4633 Over Current Protection
J
Stage 6
Stage 7
K
Stage 8
M
L
Figure 6 -- Typical Diagram of Current Limit Protection
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SC283
Applications Information (continued)
PCB Layout Considerations
The layout diagram in Figure 7 shows a recommended top-layer PCB for the SC283 and supporting components. Figure 8 shows the bottom layer for this PCB. Fundamental layout rules must be followed since the layout is critical for achieving the performance specified in the Electrical Characteristics table. Poor layout can degrade the performance of the DC-DC converter and can contribute to EMI problems, ground bounce, and resistive voltage losses. Poor regulation and instability can result. The following guidelines are recommended when developing a PCB layout: 1. The input capacitor, CIN, should be placed as close to the VIN and GND pins as possible. This capacitor provides a low impedance loop for the pulsed currents present at the buck converter's input. Use short wide traces to connect as closely to the IC as possible. This will minimize EMI and input voltage ripple by localizing the high frequency current pulses. 2. Keep the LX pin traces as short as possible to minimize pickup of high frequency switching edges to other parts of the circuit. COUT and L should be connected as close as possible between the LX and GND pins, with a direct return to the GND pin from COUT. 3. Route the output voltage feedback/sense path away from the inductor and LX node to minimize noise and magnetic interference. 4. Use a ground plane referenced to the SC283 GND pin. Use several vias to connect to the component side ground to further reduce noise and interference on sensitive circuit nodes. 5. If possible, minimize the resistance from the output and GND pin to the load. This will reduce the voltage drop on the ground plane and improve the load regulation. It will also improve the overall efficiency by reducing the copper losses on the output and ground planes.
LB VIN VOUTB
CINB
CTLxA GND U1
COUTB
GND CTLxB
COUTA
GND
CINA VIN LA GND
VOUTA
Figure 7 -- Recommended PCB Layout (Top Layer)
VIN
GND
Figure 8 -- Bottom Layer Detail
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SC283
A D B
DIM
Outline Drawing - 2x3 MLPQ-W18
A D B
DIM
PIN 1 DIMENSIONS INDICATOR MILLIMETERS (LASER MARK)
E
PIN 1 INDICATOR (LASER MARK)
E
A aaa C A2 D1 LxN E/2 1.700 0.850 2X E1 A1 C
SEATING PLANE
0.70 0.80 A 0.05 A1 0.00 (0.20) A2 b 0.15 0.20 0.25 D 1.90 2.00 2.10 A D1 0.136 0.286 0.386 Eaaa2.90 3.00 3.10 C E1 0.55 0.70 0.80 A2 e 0.40 BSC L 0.375 0.425 0.475 18 N D1 ND 2 7 NE aaa 0.08 bbb 0.10
MIN NOM MAX
A1
C
SEATING PLANE
0.70 0.80 A 0.05 A1 0.00 (0.20) A2 b 0.15 0.20 0.25 D 1.90 2.00 2.10 D1 0.136 0.286 0.386 2.90 3.00 3.10 E E1 0.55 0.70 0.80 e 0.40 BSC L 0.375 0.425 0.475 18 N ND 2 7 NE aaa 0.08 bbb 0.10
DIMENSIONS MILLIMETERS MIN NOM MAX
LxN E/2
1.700 0.850 2X E1
2 1 N
e/2 e D/2
NOTES:
bxN bbb CAB
2 1 N
1.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
e/2 e D/2
bxN bbb CAB
2.
Land Pattern - 2x3 MLPQ-W18
NOTES: 1. 2.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
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SC283
(c) Semtech 2010 All rights reserved. Reproduction in whole or in part is prohibited without the prior written consent of the copyright owner. The information presented in this document does not form part of any quotation or contract, is believed to be accurate and reliable and may be changed without notice. No liability will be accepted by the publisher for any consequence of its use. Publication thereof does not convey nor imply any license under patent or other industrial or intellectual property rights. Semtech assumes no responsibility or liability whatsoever for any failure or unexpected operation resulting from misuse, neglect improper installation, repair or improper handling or unusual physical or electrical stress including, but not limited to, exposure to parameters beyond the specified maximum ratings or operation outside the specified range. SEMTECH PRODUCTS ARE NOT DESIGNED, INTENDED, AUTHORIZED OR WARRANTED TO BE SUITABLE FOR USE IN LIFE-SUPPORT APPLICATIONS, DEVICES OR SYSTEMS OR OTHER CRITICAL APPLICATIONS. INCLUSION OF SEMTECH PRODUCTS IN SUCH APPLICATIONS IS UNDERSTOOD TO BE UNDERTAKEN SOLELY AT THE CUSTOMER'S OWN RISK. Should a customer purchase or use Semtech products for any such unauthorized application, the customer shall indemnify and hold Semtech and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs damages and attorney fees which could arise.
Contact Information
Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805) 498-2111 Fax: (805) 498-3804 www.semtech.com
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