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Micro Power Constant-Current DC-DC Converter
POWER MANAGEMENT Description
The SC104 is a micro power dc-dc step-up converter which converts an input voltage, in the range of 1.55V to 10V, to a constant current. The part features a range of user programmable voltages and currents including dynamic adjustment of the "constant" current. The converter is capable of generating output voltages as low as the input supply voltage and up to 38V. During shutdown, the part draws a typical 500nA standby current. The output current is set by the external resistor R1. Dynamic adjustment of the output current can be made by the application of an analog voltage to the ADJ input, or by PWMing this pin. The peak switch current is programmable through the external resistor RLIM enabling the use of a wide range of inductors and battery technologies.
SC104
Features
Adjustable output current using pulse width modulation or analog voltage input 38V output capability supports up to 10 white LEDs Wide range of input voltages 1.55V to 10V Low quiescent current 0.5A standby current in shutdown Programmable cycle by cycle current limit Surface mount packaging (3x3mm 8pin MLP) Output voltage and over-temperature protection
Applications
White LED supplies Color LED supplies Cellular phones PDAs Electronic books Handheld computers Wireless web appliances
Typical Application Circuit - 6 LEDs
VIN = 3V to 5V
IOUT ADJUST
1 2
U1 ADJ FB GND OUT
SC104 EN LIM IN LX
CIN 4.7uF 8 7 6 5 L1 12uH RLIM 7.50k ENABLE
IOUT = 15mA
LED4
LED5
LED6
3 4
LED3
LED2
LED1 RSET 23.2R
COUT 0.47uF D1
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SC104
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 Input Supply Voltage LX Pin Voltage (Power switch OFF) FB Pin Voltage EN Pin Voltage LIM Pin Voltage ADJ Pin Voltage OUT Pin Voltage Thermal Impedance Junction to Ambient Operating Ambient Temperature Range Operating Junction Temperature Range Storage Temperature Range Lead Temperature SC104IMLTR (Soldering) 10s - 30s SC104IMLTRT (Soldering) 20s - 40s ESD Rating (Human Body Model)
Symbol VIN V LX V FB V EN VLIM V AD J VOUT J A TA TJ TSTG TLEAD V ESD
Maximum -0.3 to 12 -0.3 to 55 -0.3 to 6 -0.3 to 6 -0.3 to 6 -0.3 to 2 -0.3 to 55 84 -40 to +85 -40 to +125 -65 to +150 240 260 2
Units V V V V V V V C/W C C C C kV
Electrical Characteristics
Unless specified: VIN = VEN = 2V, -40 TA 85C, typical values are at room temperature.
Parameter Input Supply Voltage
Symbol VIN
Test Conditions TA = 25oC
Min 1.55 1.75
Typ
Max 10 10
Units V
Shutdown Current Feedback Voltage Feedback Input Current Power Switch Saturation Voltage Junction Temperature at Thermal Shutdown(1) Over-Voltage Protection Adjust Voltage Adjust Voltage Switching Inhibit Threshold
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IQ(OFF) V FB IFB VCE(SAT) TTSD VOVP V AD J VTH(ADJ)
2
V E N = 0V 340 VFB < 350mV ILX = 300mA 140 Test current = 10A IADJ= 0A 36 0.97 200
0.5 350 700 150 150 38 1.00
3 360 1115 250 160 44 1.03 100
A mV nA mV
o
C
V V mV
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SC104
POWER MANAGEMENT Electrical Characteristics (Cont.)
Unless specified: VIN = VEN = 2V, -40 TA 85C, typical values are at room temperature.
Parameter Adjust Input Current
Symbol IADJ
Test Conditions V A D J = 0V V A D J = 1V VADJ = 1.25V
Min 6 -500 -1.3 -920 0.60 1.00 1.8 70
Typ 10 0 -2.3
Max 20 500 -3.8
Units A nA A mA
Maximum Switch Current Switch Off Time
ILX tOFF
ILI M= 150A VFB = 300mV V FB = 0V
0.76 1.25 2.25
1.00 1.60 2.7
s
Switch On Time Maximum Duty Cycle Switch Leakage Voltage Reference for Current Limit Setting Resistor ILX to ILIM Ratio Logic Inputs Enable Input Voltage
tON %DC ILX VLIM ILX / ILIM
excludes current limited conditions
s %
switch off, VLX = 5V ILIM = 50A ILIM = 50A 385 7100
-0.01 400 8150
-1.0 410 8800
A mV A/A
VIH VIL
IEN > 300nA
1.5 0.2
V
Enable Input Bias Current
IIL IIH
VEN = 0.3V VEN = 1.3V
0.3 0.7
A
Note: (1) Guaranteed by design.
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SC104
POWER MANAGEMENT Pin Configuration
Top View
Ordering Information
Part Number SC104IMLTR(1) SC104IMLTRT(2) S C 104E V B Evaluation Board P ackag e MLP-8
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. MLP-8
Block Diagram
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SC104
POWER MANAGEMENT Pin Descriptions
Pin 1 2 3 4 5 6 7 8 Pin Name AD J FB GND OUT LX IN LIM EN Pin Function Apply a PWM or analog voltage to this pin for dynamic output current adjustment. Connects to the resistor in the ground leg of the series LEDs (feedback for the constant load current). Ground pin. Connect to the boost output at the cathode of the schottky diode. This pin is the input for the over-voltage protection circuit. Internal switch connection. Connect the inductor and the anode of the schottky diode to this pin. Input supply pin. Connect to a battery or power supply. Current limit set pin. A resistor (RLIM) connected from LIM to GND sets the peak inductor current limit threshold. Digital input for enable. Connect this pin to ground for shutdown or connect a voltage between 1.2V to 5.0V for enable. A 1M pull-up resistor connecting to IN is also acceptable, provided 10A > IEN > 300nA.
Marking Information
104 yyww
Marking for the MLP 8 Lead package: yyww = Date Code (Example: 0012)
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SC104
POWER MANAGEMENT Applications Information
Component Selection - Introduction Referring to the 6 LED typical schematic below, there are three components that depend upon the application that need to be determined: RSET - this resistor sets the output current for the device RLIM - this resistor sets the peak inductor current L - the output inductor All the other components can be mostly generalized and are addressed below the following design steps.
VIN = 3V to 5V
Vf = 0.35V VCE(SAT) = 0.25V thus DC = 0.87 Since this value is greater than the guaranteed minimum value for maximum duty cycle, the device will be operating in discontinuous mode to provide the desired output. Note that the duty cycle does not depend upon the output current, and that unless the output to input ratio is low, the device will usually need to be in discontinuous mode, so we will cover that first (Step 1 through Step 5). Continuous mode calculations start at Step 6. Step 2: Calculating the Inductor for Discontinuous Mode Having determined that we need to be operating in discontinuous mode, we next need to calculate the maximum inductor value allowed that will permit the part to output the correct power. The maximum discontinuous inductor value, L(D) is given by:
L(D ) = 2 * 1.4 * VOUT * IOUT * (t ON(MIN ) + t OFF(MIN ) ) t ON(MIN ) * VIN * (VIN - VCE ( SAT ) )
2
IOUT ADJUST
1 2
U1 ADJ FB GND OUT
SC104 EN LIM IN LX
CIN 4.7uF 8 7 6 5 L1 12uH RLIM 7.50k ENABLE
IOUT = 15mA
LED4
LED5
LED6
3 4
LED3
LED2
LED1 RSET 23.2R
COUT 0.47uF D1
Step 1: Continuous or Discontinuous? The first thing to do when designing with the SC104 is to determine whether the output inductor will be operating in continuous mode (where the inductor current does not drop to zero while the device is switching) or discontinuous mode (where the inductor current drops to zero while switching). This determination can be made simply by calculating the required duty cycle needed for the target output voltage, and comparing it to the guaranteed minimum value for the maximum duty cycle from the Electrical Characteristics on Page 3. %DC(MIN) = 70% (or 0.7 duty). If DC is greater than 0.7 then discontinuous mode is required. The required duty cycle is calculated as follows:
DC =
*
(V
OUT
(VOUT - VIN + Vf )
- VCE ( SAT ) + Vf )
Where: tON(MIN) = minimum switch on-time = 1.8s IOUT = required output current tOFF(MIN) = minimum switch off-time = 0.6s Using our 6 LED example: IOUT = 15mA thus L(D) = 14.4H Selecting the next lower standard value gives us L(D) = 12H. Of course a lower value inductor may be used if desired, but may not necessarily be the most efficient choice. Step 3: Calculating the Current Limit Required with this Inductor for Discontinuous Mode Having determined the inductor value we are going to use, we next need to calculate the current limit required to meet the necessary output power. The discontinuous mode current limit, ILIM(D), is given by:
ILIM(D ) =
(V
(VOUT - VIN + Vf )
OUT
- VCE( SAT ) + Vf )
Where: VOUT = output voltage, the sum of the total LED (max.) forward voltage drop at the required output voltage plus the feedback voltage, 0.35V. VIN = minimum input voltage Vf = Schottky diode (D1) forward voltage drop VCE(SAT) = power switch saturation voltage Using the 6 LED example above: VOUT = (6 * 3.475) + 0.35 = 21.2V VIN = 3V
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(V
IN
- VCE( SAT ) ) * t ON(MIN) L (D )
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SC104
POWER MANAGEMENT Applications Information (Cont.)
Step 3: (Cont.) Using our 6 LED example: L(D) = 12H thus ILIM(D) = 412mA Step 4: Calculating the Current Limit Resistor for Discontinuous Mode The current limit resistor value is calculated based upon the minimum ratio of the switch current to the current out of the LIM pin. It also takes into account the fact that there is a propagation delay during which time the inductor current ramps beyond the current limit trip point. Since ILIM increases as RLIM decreases, this value is a maximum. The maximum current limit resistor, RLIM(D), for discontinuous mode is therefore:
RLIM(D ) = t * (VIN - VCE(SAT ) ) ILIM(D ) - plh L (D ) VLIM(MIN) * Ratio (MIN)
equation:
R SET = VFB IOUT
Using our 6 LED example: IOUT = 15mA thus RSET = 23.3 We will select the 1% resistor value 23.2. Note: this calculation is applicable to both continuous and discontinuous modes. Step 6: Calculating the Inductor for Continuous Mode Having determined that we need to be operating in continuous mode, we next need to calculate the maximum inductor value allowed that will permit the part to output the correct power. The maximum continuous inductor value, L(C) is given by:
L(C) = 2 * VIN * t OFF(MIN) * (VOUT + Vf - VIN ) 1.4 * VOUT * IOUT
Where: VLIM(MIN) = the minimum value of the current limit voltage reference = 385mV Ratio (MIN) = the minimum value of the I LX to I LIM ratio = 7100 A/A tplh = propagation delay from reaching the current limit trip point to the power switch turning off = 200ns Using our 6 LED example: ILIM(D) = 412mA thus RLIM(D) = 7.5k Selecting this value or the next lower standard value in this case gives us 7.5k. Of course a lower value resistor may be used if desired, but may not necessarily be the most efficient choice. Step 5: Calculating the Current Set Resistor The current set resistor is in series with the series LED string. Thus the voltage developed across it is proportional to the current flowing through the LEDs. The device will regulate this voltage so that its average value equals the feedback voltage, VFB, which is typically 350mV. Thus the current set resistor value is given by the following
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Where: IOUT = required output current tOFF(MIN) = minimum switch off-time = 0.6s Selecting the next lower standard value gives us a safe value for this inductor. Step 7: Calculating the Current Limit Required with this Inductor for Continuous Mode Having determined the inductor value we are going to use, we next need to calculate the current limit required to meet the necessary output power. The continuous mode current limit, ILIM(C), is given by:
ILIM( C ) =
(VOUT + Vf - VIN ) * t OFF(MIN)
0.4 * L( C )
Step 8: Calculating the Current Limit Resistor for Continuous Mode The current limit resistor value is calculated based upon the minimum ratio of the switch current to the current out of the LIM pin. It also takes into account the fact that there is a propagation delay during which time the
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SC104
POWER MANAGEMENT Applications Information (Cont.)
Step 8: (Cont.) inductor current ramps beyond the current limit trip point. Since ILIM increases as RLIM decreases, this value is a maximum. The maximum current limit resistor, RLIM(C), for continuous mode is therefore:
RLIM( C ) = t * (VIN - VCE(SAT ) ) ILIM( C ) - plh L(C) VLIM(MIN ) * Ratio (MIN)
Schottky diode: any schottky diode rated for the average and peak currents being seen in the circuit will suffice. However a higher current rated schottky diode will result in lower forward voltage drops and hence higher efficiency. The selection of the schottky will depend upon the optimum choice between efficiency, board space, and cost. Inductor: similarly, any inductor rated for the average and peak currents required by the design and capable of operating at the fixed off-time of 760ns will suffice, but inductors with lower series resistance will result in lower losses. The selection of the inductor will depend upon the optimum choice between efficiency, board space, component height, and cost. Toko D62LCB and D63LCB series work very well. LED Dimming Dimming the LEDs (i.e. reducing the output current from the set level to reduce brightness) can be achieved a couple of ways: 1) PWMing the ADJ pin using an open drain or open collector (with no pull-up). Using a PWM signal at this pin will reduce the output current by alternating between OFF (ADJ < 100mV) and switching normally. The lower the duty cycle, the lower the output current. A PWM frequency of 1kHz maximum is recommended. 2) analog voltage applied to the ADJ pin: apply a DC voltage between 0V to 0.1V (OFF) and 1V (full current) to the ADJ pin. This pin should not be pulled above 1V under normal operation, and should never exceed the absolute maximum rating. Using a DC voltage will result in the modulation frequency of the inductor current ramp reducing and care must be taken to ensure that this does not become audible in sensitive applications.
Where: VLIM(MIN) = the minimum value of the current limit voltage reference = 385mV Ratio (MIN) = the minimum value of the I LX to I LIM ratio = 7100 A/A tplh = propagation delay from reaching the current limit trip point to the power switch turning off = 200ns Selecting the calculated value or the next lower standard value is recommended. Component Selection - General Capacitor Selection: the SC104 has been designed to be used with ceramic input and output capacitors. The input to the device should be bypassed using a 4.7F ceramic capacitor rated for the maximum input voltage. The output capacitor should be a ceramic capacitor in the range of 0.22F to 1F. Care should be taken that the voltage rating of this capacitor meets the output voltage requriements, and if the part is going to be run open circuit during assembly testing using the OVP feature, then the capacitor should be rated 50V.
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SC104
POWER MANAGEMENT Typical Characteristics
Shutdown Current vs. Junction Temperature vs. Input Voltage
2.0 1.8 1.6 1.4 IQ(OFF) (A) VFB (mV) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -50 -25 0 25 TA (C) 50 75 100 V IN = 2V VIN = 5V VEN = 0V VIN = 10V 360 358 356 354 352 350 348 346 344 342 340 -50 -25 0 25 TJ (C) 50 75 100 125 V IN = 2V V IN = 5V V IN = 10V
Average Feedback Voltage vs. Junction Temperature vs. Input Voltage
Power Switch Saturation Voltage vs. Junction Temperature
250 225 200 175 VCE(SAT) (mV) 150 125 100 75 50 25 0 -50 -25 0 25 TJ (C) 50 75 100 125 VLIM (mV) VIN = 2V ILX = 300mA 410.0 407.5 405.0 402.5 400.0 397.5 395.0 392.5 390.0 387.5 385.0 -50
Current Limit Reference Voltage vs. Junction Temperature
VIN = 2V ILIM = 50A
-25
0
25 TJ (C)
50
75
100
125
ILX to ILIM Ratio vs. Junction Temperature
9000 8800 8600 8400 ILX/ILIM (A/A) 8200 8000 7800 7600 7400 7200 7000 -50 -25 0 25 TJ (C) 50 75 100 125 VIN = 2V ILIM = 50A
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SC104
POWER MANAGEMENT Typical Application Circuit - 3 LEDs
VIN = 3V to 5V
U1 IOUT ADJUST 1 2 IOUT = 15mA LED3 3 4 LED2 LED1 RSET 23.2R COUT 1uF D1 ADJ FB GND OUT
SC104 EN LIM IN LX 8 7 6 5 L1 27uH
CIN 4.7uF ENABLE
RLIM 15k
Typical Application Circuit - 10 LEDs
VIN = 3V to 5V
IOUT ADJUST
1 2
U1 ADJ FB GND OUT
SC104 EN LIM IN LX
CIN 4.7uF 8 7 6 5 L1 6.2uH RLIM 3.9k ENABLE
IOUT = 15mA
LED6
LED7
LED8
LED9
LED10
3 4
LED5
LED4
LED3
LED2
LED1 RSET 23.2R
COUT 0.22uF D1
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SC104
POWER MANAGEMENT Outline Drawing - MLP-8
A N
D
B
DIM
A A1 A2 A3 b b2 D E e L L1 L2 N 01 aaa bbb
DIMENSIONS MILLIMETERS INCHES MIN NOM MAX MIN NOM MAX
.031 .039 .000 .002 .026 .030 (.008) .011 .012 .015 .007 .012 .114 .118 .122 .114 .118 .122 .026 BSC .008 .011 .018 .008 .011 .018 .005 8 0 12 .003 .004 1.00 0.80 0.00 0.05 0.65 0.75 (0.20) 0.29 0.31 0.39 0.17 0.30 2.90 3.00 3.10 2.90 3.00 3.10 0.65 BSC 0.20 0.29 0.45 0.20 0.29 0.45 0.13 8 0 12 0.08 0.10
PIN 1 INDICATOR (LASER MARK) 12 01 aaa C C bxN bbb
E
A3 A2 A A1 CAB
SEATING PLANE
e e/2 L2
L b2 L1
NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. DIMENSIONS "D" AND "E" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
L2
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SC104
POWER MANAGEMENT Land Pattern - MLP-8
X
DIM
(C) G Y P Z C G P X Y Z
DIMENSIONS INCHES MILLIMETERS
(.114) .079 .026 .015 .035 .150 (2.90) 2.00 0.65 0.38 0.90 3.80
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 Visit us at: www.semtech.com
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