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SC4538
High Efficiency Boost Converter for up to 10 LEDs
POWER MANAGEMENT Features
Input voltage range -- 2.8V to 5.5V Programmable LED current up to 30mA Current-mode PWM control -- 800kHz Soft-start to reduce in-rush current PWM dimming -- 100Hz to 50kHz PWM dimming at ISET -- Analog and filtered Over-voltage protection -- 38V (minimum) Under-voltage lockout (UVLO) Thermal shutdown Shutdown current -- <0.1A (typical) Ultra-thin package -- 2mm x 2mm x 0.6mm Lead-free package, WEEE and RoHS compliant
Description
The SC4538 is designed to regulate current for a series string of white LEDs in LCD backlighting applications where small size and high efficiency are priorities. This device integrates an 800kHz current-mode PWM boost converter and a 30mA programmable low dropout current sink regulator. An external resistor sets the LED current up to 30mA. PWM dimming (100Hz to 50kHz) may be applied directly to the enable (EN) pin, or dimming can be controlled by applying an analog signal to the ISET circuit. The boost circuit can output up to 38V (guaranteed) to drive up to 10 LEDs in series. The current regulator protects against shorts between the BL and OUT pins and also eliminates backlight glow during shutdown when using LEDs with high leakage. Under-voltage lockout and thermal shutdown provide additional protection. A small external capacitor and series resistor control soft-start and loop compensation. Over-voltage detection protects the SC4538 if the BL pin is floating or shorted to ground. The SC4538 is available in an MLPD-8 pin 2mm x 2mm x 0.6mm package, with a rated temperature range of -40C to +85C.
Applications
DSLR, DSC, and Video Cameras Cellular handsets Portable media players Personal navigation systems Satellite radio Handheld video games
Typical Application Circuit
D L1
2.8 to 5.5V
LX IN
CIN
COUT
OUT
SC4538
Enable and PWM dimming RISET CCOMP
EN ISET
RCOMP
COMP
BL GND
June 20, 2008
(c) 2008 Semtech Corporation
1
SC4538
Pin Configuration Ordering Information
Device
SC4538ULTRT(1) (2) SC4538EVB
Package
MLPD-UT-8 2x2 Evaluation Board
LX EN ISET COMP
1 2 3 4
TOP VIEW
8 7 6
IN GND BL OUT
Note: (1) Available in tape and reel only. A reel contains 3,000 devices. (2) Lead-free package only. Device is WEEE and RoHS compliant.
T
5
MLPD: 2mm x 2mm 8 Lead JA = 74C/W
Marking Information
FE0 yw
FE0 = Marking code for SC4538 yw = Date Code
2
SC4538
Absolute Maximum Ratings
Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +20.0 LX Voltage, Output Voltage (V) . . . . . . . . . . . . . -0.3 to +45 Current Sink Voltage (V) . . . . . . . . . . . . . . . . . . . . -0.3 to +45 Enable Voltage (V) . . . . . . . . . . . . . . . . . . . . . . -0.3 to (VIN + 0.3) Compensation Voltage (V) . . . . . . . . . . . . . . . . . . -0.3 to +2.0 Current Set Voltage (V) . . . . . . . . . . . . . . . . . . . . . . -0.3 to +2.0 ESD Protection Level(1) (kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Recommended Operating Conditions
Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 to 5.5 Output Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 to 38 Ambient Temperature Range (C) . . . . . . . . . . . . . -40 to +85
Thermal Information
Thermal Resistance, Junction to Ambient(2) (C/W) . . . . . 74 Maximum Junction Temperature (C) . . . . . . . . . . . . . . +150 Storage Temperature Range (C) . . . . . . . . . . . . -65 to +150 Peak IR Reflow Temperature (10s to 30s) (C) . . . . . . . +260
Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended. NOTES: (1) Tested according to JEDEC standard JESD22-A114-B. (2) Calculated from package in still air, mounted to 3 x 4.5 (in), 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards.
Electrical Characteristics
Unless otherwise noted: VIN = 3.6V, CIN = 2.2F, COUT = 1F, CCOMP = 47nF, RCOMP = 1.27k, RISET = 5.76k, L = 22H, TA = -40 to 85C, typical values are at TA = 25C.
Parameter
UVLO Threshold UVLO Hysteresis Quiescent Supply Current Shutdown Supply Current EN Logic High Voltage EN Logic Low Voltage EN Logic Input Current Thermal Shutdown Temperature Thermal Shutdown Hysteresis Boost Converter Characteristics Switching Frequency Maximum Duty Cycle Minimum On-Time Switch Over-Current Protection
Symbol
VUVLO VUVLO-HYS IQ ISHDN VIN VIL IIL, IIH TSD
Conditions
VIN rising
Min
2.40
Typ
2.60 120
Max
2.79
Units
V mV mA
Not switching EN tied to GND 1.80
1.8 0.1 1
A V
0.8 VIN = 5.5V, VEN = 0V or 5.5V TJ rising 0.01 155 20 1
V A C C
fSW DMAX tON(MIN) IOCP VIN = 3.2V, TA = 25C
680 92
800
920
kHz %
100 425 725
ns mA
3
SC4538
Electrical Characteristics (continued)
Parameter
Boost Converter Characteristics (continued) Switch Leakage Current Switch Saturation Voltage COMP Sourcing Current ICOMP COMP Sinking Current OUT Over-Voltage Protection OUT Internal Pull-Down Current OUT Bias Current IOUT OUT Leakage Current PWM Dimming Frequency Range(1)(2) PWM Dimming Duty Cycle Range(1)(2) Current Sink Characteristics BL Current Setting Range(3) IBL BL Current Setting Accuracy BL Leakage Current BL Current Line Regulation BL Voltage ISET Bias Voltage ISET-to-IBL Gain Start-Up Time IL(BL) IBL/VIN VBL VISET AISET tstart-up TA= 25C VEN = 0V, VBL = 2V VIN = 3.0 to 5.5V -3.5 0.01 0.05 0.35 0.5 230 1.3 3.5 0.1 % A mA/V V V A/A s 1 30 mA fEN DEN VEN = 0V, VOUT = VIN = 5.5V Applied to EN pin 200Hz on EN pin 100 0 0.01 1 50k 100 A Hz % VOVP IOVP During OVP condition VEN = VIN, VOUT = 20V VCOMP = 0.9V, TA = 25C 38 1 50 70 6 43 V mA A IL(LX) VSAT VLX = 5.5V ILX = 0.3A VCOMP = 0.9V, TA = 25C 0.01 250 5 A 1 450 A mV
Symbol
Conditions
Min
Typ
Max
Units
Notes: (1) Guaranteed by design. (2) See PWM Dimming description in the Applications Information section for limitations at high PWM dimming frequencies and low PWM dimming duty cycles. (3) Not recommended to program below 1mA with RISET due to tolerance stackup. To produce output current less than 1mA, set the current > 1mA and use PWM dimming.
4
SC4538
Typical Characteristics
All data taken with VOUT = 20V (6 white LEDs), RISET = 5.76k (IBL = 20mA), VIN = 3.6V, L = 22H, and efficiency () = PLED/PIN unless otherwise noted.
Efficiency vs. IBL
100 3.6VIN 80 2.8VIN 60 5.5VIN 40
Efficiency vs. Output Voltage
90
86 5.5VIN
Efficiency (%)
Efficiency (%)
82 2.8VIN 78 3.6VIN
20
74
0 0 6 12 18
70
IBL (mA)
24
30
0
8
16
24
32
40
VOUT (V)
Efficiency vs. Input Voltage
90
90
Efficiency vs. Input Voltage
6 LEDs
4 LEDs
86
86
Efficiency (%)
82 VOUT 78 LED
Efficiency (%)
82 10 LEDs 78
74
74
70 2.8 3.3 3.9
4.4
70
5.0
5.5
2.8
3.3
3.9
4.4
5.0
5.5
VIN (V)
VIN (V)
Efficiency vs. Input Voltage
90 20mA 86
Normalized IBL vs. Output Voltage
6
4
Normalized IBL (%)
5.0 5.5
Efficiency (%)
2
82
0
78 30mA 10mA 74
-2
-4
70 2.8 3.3 3.9 4.4
-6 0 8 16
VIN (V)
VOUT (V)
24
32
40
5
SC4538
Typical Characteristics (continued)
PWM Operation at 200Hz and 1% Duty Cycle PWM Operation at 50kHz and 15% Duty Cycle
VLX (20V/div)
VLX (20V/div)
IOUT (20mA/div) VOUT (10V/div)
IOUT (20mA/div) VOUT (20V/div)
VEN (2V/div) 2ms/div
VEN (2V/div) 4s/div
PWM Operation at 32kHz and 50% Duty Cycle
PWM Operation at 32kHz and 10% Duty Cycle
VLX (20V/div)
VLX (20V/div)
IOUT (20mA/div) VOUT (20V/div)
IOUT (20mA/div) VOUT (20V/div)
VEN (2V/div) 10s/div
VEN (2V/div) 10s/div
IBL vs. Duty Cycle
100
IBL Error vs. PWM Frequency at 50% Duty Cycle
6.0
Percentage of Maximum IBL (%)
80
4.0
2.0
IBL Error (%)
50kHz 32kHz 20 40 60 80 100
60
0.0
40 200Hz
-2.0
20
-4.0
0 0
-6.0 0.1 1 10 100
PWM Duty Cycle (%)
PWM Frequency (kHz)
6
SC4538
Typical Characteristics (continued)
Typical Waveforms at VIN = 2.8V Typical Waveforms at VIN = 5.5V
VLX (20V/div)
VLX (20V/div)
VIN (50mV/div)
VIN (50mV/div)
VOUT (50V/div) VOUT (50V/div)
1s/div
1s/div
Typical Waveforms at VIN = 3.6V
Start-Up Response
VLX (20V/div)
VLX (20V/div)
VIN (50mV/div)
IOUT (20mA/div)
VOUT (10V/div) VOUT (50mV/div) VEN (2V/div) 1s/div 100s/div
Waveforms During Over-Voltage Protection
3.5
Normalized IBL vs. Input Voltage
VLX (50V/div) IOUT (20mA/div)
2.3
1.2
IBL Error (%)
0.0
VOUT (20V/div)
-1.2
-2.3
VEN (2V/div)
-3.5
400s/div
2.8
3.25
3.7
4.15
4.6
5.05
5.5
VIN (V)
7
SC4538
Pin Descriptions
Pin #
1 2 3 4
Pin Name
LX EN ISET COMP
Pin Function
Collector of the internal power transistor -- connect to the boost inductor and rectifying Schottky diode. Enable and brightness control pin for LED string Output current set pin -- connect a resistor from this pin to GND to set the maximum current. Output of the internal transconductance error amplifier -- this pin is used for loop compensation and soft-start. Connect a 1.27k resistor and 47nF capacitor in series to GND. Boost output voltage monitor pin -- internal over-voltage protection monitors the voltage at this pin. Connect this pin to the output capacitor and the anode of the LED string. LED constant current sink -- connect this pin to the cathode of the LED string Ground Power supply pin -- bypass this pin with a capacitor close to the pin Pad for heatsinking purposes -- connect to the ground plane using multiple vias. Not connected internally.
5 6 7 8 T
OUT BL GND IN Thermal Pad
8
SC4538
Block Diagram
OUT 5 Thermal Shutdown
UVLO
OVP
1 IN 8 Reference and Internal Regulator S R Comparator RSENSE 7 OSC Sense Amp Q
LX
GND
Error Signal Selection and Summation
4
COMP
EN
2 EN ADJ ADJ LED Setpoint
BL
6
3 ISET
9
SC4538
Applications Information
General Description
The SC4538 contains an 800kHz fixed-frequency currentmode boost converter and an independent LED current regulator. The LED current set point is chosen using an external resistor, while the PWM controller operates independently to keep the current in regulation. The SC4538 receives information from the internal LED current regulator and drives the output to the proper voltage with no user intervention. The current flowing through the LED string is independently controlled by an internal current regulator, unlike the ballasting resistor scheme that many LED current regulators use. The internal current regulator can be shut off entirely without leaking current from a charged output capacitor or causing false-lighting with low LED count and high VIN. The backlight current (IBL) is programmed using an external resistor. The path from the EN pin to the output control is a high bandwidth control loop. This feature allows the PWM dimming frequency to range between 100Hz and 50kHz. In shutdown mode, leakage through the current regulator output is less than 1A. This keeps the output capacitor charged and ready for instant activation of the LED string. The 800kHz switching speed provides high output power while allowing the use of a low profile inductor, maximizing efficiency for space constrained and cost-sensitive applications. The converter and output capacitor are protected from open-LED conditions by over-voltage protection. cycle PWM signal used for a few milliseconds provides the additional advantage of reduced in-rush at start up. The start-up delay time between the enable signal going high and the activation of the internal current regulator causes nonlinearity between the IBL current and the duty cycle of the PWM frequency seen by the EN pin. As the PWM signal frequency increases, the total on time per cycle of the PWM signal decreases. Since the start up delay time remains constant, the effect of the delay becomes more noticeable, causing the average IBL to be less predictable at lower duty cycles. Recommended minimum duty cycles are 20% for 50kHz PWM frequency, 15% for 32kHz PWM frequency and 2% for 200Hz PWM frequency. Refer to the IBL vs. Duty Cycle in the Typical Characteristics section for PWM performance across duty cycle for different PWM frequencies.
Zero Duty Cycle Mode
Zero duty cycle mode is activated when the voltage on the BL pin exceeds 1.3V. In this mode, the COMP pin voltage is pulled low, suspending all switching. This allows the VOUT and VBL voltages to fall. The COMP voltage is held low until the VBL falls below 1V, allowing VCOMP to return to its normal operating voltage and switching to resume.
Protection Features
The SC4538 provides several protection features to safeguard the device from catastrophic failures. These features include:
PWM Dimming
The enable pin can be toggled to allow PWM dimming. In a typical application, a microcontroller sets a register or counter that varies the pulse width on a GPIO pin. The SC4538 provides dimming between 100Hz and 50kHz. The SC4538 is compatible with a wide range of devices, allowing dimming strategies that avoid the audio band by using high frequency PWM dimming. A wide range of illumination can be generated while keeping the instantaneous LED current at its peak value for luminescent efficiency and color purity. The SC4538 can accommodate any PWM duty cycle between 0 and 100%. A low duty
* * * *
Over-voltage Protection (OVP) Soft-start Thermal Shutdown Current Limit
Over-Voltage Protection (OVP) A built-in over-voltage protection circuit prevents damage to the IC and output capacitor in the event of an opencircuit condition. The output voltage of the boost converter is detected at the OUT pin and divided internally. If the voltage at the OUT pin exceeds the OVP limit, the boost converter is shut down and a strong pull down is applied to the OUT pin to quickly discharge the output capacitor. This additional level of protection prevents a condition where the output capacitor and Schottky diode
10
SC4538
Applications Information (continued)
must endure high voltage for an extended period of time. Soft-Start The soft-start mode reduces in-rush current by utilizing the external compensation network. As the error amplifier slowly charges the COMP node voltage, the duty cycle of the boost switch ramps from 0% to its final value once in regulation. The gradual increase of the duty cycle slowly charges the output capacitor and limits in-rush current during start up. Soft-start is implemented only when the input power is cycled. Thermal Shutdown A thermal shutdown system is included for protection in the event the junction temperature exceeds 155C. In thermal shutdown, the on-chip power switch is disabled. Switching and sinking resumes when the temperature drops by 20C. Current Limit The power switch of the boost converter is protected by an internal current limit function. The switch is opened when the current exceeds the maximum switch current value.
D 1 VIN VOUT
IIN
VOUT IOUT VIN
D is the duty cycle for continuous operation. Efficiency () can be approximated by using the curves provided in the Typical Characteristics section. Table 1 lists inductors that have been proven to work with SC4538. Table 1 -- Recommended Inductors
Part Number Value (H) DCR () Rated Current (A) Tolerance Dimensions (L x W x H) (mm) 3.9 x 3.9 x 1.7
Coilcraft LPS4018-223ML Murata LQH43CN150K03 Murata LQH32CN150K53
22
0.360
0.70
20%
15 15
0.320 0.580
0.570 0.300
10% 10%
4.5 x 3.2 x 2.6 3.2 x 2.5 x 1.55
Capacitor Selection
The input capacitor should be at least 2.2F. A larger capacitor will reduce the voltage ripple on the input. The output capacitor values can range from 0.22F to 1F. The compensation capacitor value should be 47nF. Capacitors of X5R type material or better can be used for any of the capacitors. See Table 2 for the recommended capacitors. Table 2 -- Recommended Capacitors
Part Number Input Capacitor Value (F) Rated Voltage (V) Type Case Size
Inductor Selection
The inductor value should be within the range of 4.7H to 22H. The DCR needs to be considered when selecting the inductor to ensure optimum efficiency. The largest inductor package that can be tolerated in the circuit area should be used since the DCR generally decreases with increasing package size. The saturation current of the inductor should be much higher than the peak current of the internal boost switch to ensure that the inductor never enters saturation during normal operation of the part. The equation to calculate the peak inductor current is:
IL 2
Murata GRM188C70J225KE20 Output Capacitor Murata GRM21BR71H105KA12L Compensation Capacitor Taiyo Yuden EMK105BJ473KV-F
2.2
6.3
X7S
0603
1.0
50
X7R
0805
IL(Peak)
IIN
0.047
16
X7R
0402
where
IL VIN D L f osc
11
SC4538
Applications Information (continued)
Diode Selection
A Schottky diode with a reverse voltage of 40V and a forward current rating of 1A should be used with this device for optimum per formance. The Central Semiconductor Corporation CMOSH-4E is a suitable diode for this application.
PCB Layout Considerations
Poor layout can degrade the performance of the DC-DC converter and can be a contributory factor in EMI problems, ground bounce, thermal issues, and resistive voltage losses. Poor regulation and instability can result. A typical layout is shown in Figure 2. The following design rules are recommended:
Selection of Other Components
RISET sets the maximum load current for the SC4538. Use the following equation to select the proper value: RISET = 230 x VISET/ILOAD where VISET = 0.5V (typ). Refer to Figure 1 for selecting values for other current settings. Notice that the error increases as the desired IBL current decreases.
100
* * *
Place the inductor and filter capacitors as close to the device as possible and use short, wide traces between the power components. Route the output voltage feedback path away from the inductor and LX node to minimize noise and magnetic interference. Use a ground plane to further reduce noise interference on sensitive circuit nodes.
IBL (mA)
10
RISET Typical
Current Tolerance
1 1 10 RISET (k) 100
Figure 1 - Set Resistor Value Selection Graph
Figure 2- Layout
12
SC4538
Outline Drawing - MLPD-UT-8 2x2
A
D
B
DIMENSIONS INCHES MILLIMETERS DIM MIN NOM MAX MIN NOM MAX
PIN 1 INDICATOR (LASER MARK) E A A1 A2 b D D1 E E1 e L N aaa bbb .020 . 000 . 024 . 002 (.006) .007 . 010 . 012 . 075 .079 . 083 . 061 .067 . 071 . 075 .079 . 083 . 026 .031 . 035 . 020 BSC . 012 .014 . 016 8 .003 .004 0.60 0.50 0.00 0.05 (0. 1524) 0.18 0.25 0.30 1.90 2.00 2.10 1.55 1.70 1.80 1.90 2.00 2.10 0.65 0.80 0.90 0. 50 BSC 0.30 0.35 0.40 8 0.08 0.10
A aaa C A1 A2 C
SEATING PLANE
D1 12 LxN E/2 E1
N e e/2 D/2 bxN bbb CAB
NOTES:
1. 2. CONTROLLING DIMENSIONS ARE IN MILLIMETERS ( ANGLES IN DEGREES). COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
13
SC4538
Land Pattern - MLPD-UT-8 2x2
H R DIM C G (C) K G Z H K P R
Y
DIMENSIONS INCHES (.077) .047 .067 .031 .020 .006 .012 .030 .106 MILLIMETERS (1.95) 1.20 1.70 0.80 0.50 0.15 0.30 0.75 2.70
X Y Z
P X
NOTES:
1.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET.
2.
3.
THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD SHALL BE CONNECTED TO A SYSTEM GROUND PLANE. FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR FUNCTIONAL PERFORMANCE OF THE DEVICE.
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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