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STCS2A
2 A max constant current LED driver
Features

Up to 40 V input voltage Less than 0.5 V voltage overhead Up to 2 A output current PWM dimming pin Shutdown pin LED disconnection diagnostic Slope control with external cap
10 1
PowerSO-10
Applications

LED constant current supplying for varying input voltages Low voltage lighting Small appliances LED lighting Car LED lights
solution for driving LEDs in low voltage applications such as 5 V, 12 V or 24 V giving benefits in terms of precision, integration and reliability. The current is set with external resistor up to 2 A with a 10 % precision; a dedicated pin allows implementing PWM dimming. An external capacitor allows setting the slope for the current rise from tens of microseconds to tens of milliseconds allowing reduction of EMI. An open-drain pin output provides information on load disconnection condition.
Description
The STCS2A is a BiCMOS constant current source designed to provide a precise constant current starting from a varying input voltage source. The main target is to replace discrete components
Table 1.
Device summary
Order code STCS2ASPR Package PowerSO-10 Packaging 600 parts per reel
July 2008
Rev 3
1/18
www.st.com 18
Contents
STCS2A
Contents
1 2 3 4 5 6 7 Application diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Pin configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Typical performance characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Detail description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1 7.2 7.3 7.4 Current setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Enable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 PWM dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Diagnostic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1 8.2 Reverse polarity protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9 10
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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STCS2A
Application diagram
1
Figure 1.
Application diagram
Typical application diagram for 2 A LED current
VIN 4.5V up to 40V BAT46ZFILM RIN 100 ohm CBYP 0.1F
VCC PWM EN DISC SLOPE GND
DRAIN
ON OFF ON OFF
STCS2A
FB SOURCE
CDRAIN 0.47F
Load disconnection (Open Drain output)
CSLOPE 10nF
RFB 0.05 ohm
3/18
Pin configuration
STCS2A
2
Figure 2.
Pin configuration
Pin connections (top view)
Table 2.
Pin n 1 2 3 4 5 6
Pin description
Symbol VCC PWM EN NC DRAIN SOURCE Supply voltage PWM dimming input Shutdown No connect Internal N-MOSFET drain Internal N-MOSFET source. Reference voltage is 100 mV. An external resistor between SOURCE and GND pins sets different current levels for different application needs Feedback input. The control loop regulates the current in such a way that the average voltage at the FB input is 100 mV (nominal). The cathode of the LED and a resistor to ground to set the LED current should be connected at this point Ground Capacitor for slope control Load disconnection flag (open drain) Internally connected to ground Note
7 8 9 10
FB GND SLOPE DISC exp-pad
4/18
STCS2A
Maximum ratings
3
Table 3.
Maximum ratings
Absolute maximum ratings
Parameter DC supply voltage Drain pin Source pin Logic pins Configuration pins Human body model (all pins) Junction temperature Storage temperature range Value -0.3 to +45 V -0.3 to +45 -0.3 to + 3.3 -0.3 to + VCC + 3.3 -0.3 to + 3.3 2 -40 to 150 -55 to 150 V V V kV C C Unit
Symbol VCC DRAIN SOURCE PWM, EN, DISC SLOPE, FB ESD TJ
(1)
TSTG
1. TJ is calculated from the ambient temperature TA and the power dissipation PD according the following formula: TJ = TA + (PD x RthJA). See Figure 16. for details of max power dissipation for ambient temperatures higher than 25 C.
Note:
Absolute maximum ratings are those values beyond which damage to the device may occur. Functional operation under these conditions is not implied. Thermal data
Parameter Thermal resistance junction-case Thermal resistance junction-ambient (1) Thermal resistance junction-ambient (2) Thermal resistance junction-ambient
(3)
Table 4.
Symbol RthJC RthJA RthJA RthJA
PowerSO-10 2 50 35 12
Unit C/W C/W C/W C/W
1. FR4 with using the recommended pad-layout 2. FR4 with heat sink on board (6 cm2). 3. FR4 with copper-filled through holes and external heat sink applied.
5/18
Electrical characteristics
STCS2A
4
Table 5.
Symbol VCC
Electrical characteristics
Electrical characteristics (VCC = 12 V; IO = 100 mA; TJ = -40 C to 125 C; VDRAIN = 1 V; CDRAIN = 1 F; CBYP = 100 nF typical values are at TA = 25C, unless otherwise specified)
Parameter Supply voltage range Output current range IO Output current Regulation (percentage with respect to VCC = 12 V) VFB Feedback voltage RFB = 50 m VCC = 4.5 to 40 V, IO = 100 mA; VDRAIN = 1 V IO = 0 to 2A On Mode ICC Quiescent current (Measured on VCC pin) Shutdown Mode; VCC = 5 to 12V Shutdown Mode; VCC = 12 to 40V VDROP Dropout voltage (VDRAIN to GND) IO = 100 mA IO = 2 A Shutdown; VDRAIN = 40 V CSLOPE = 10 nF, TJ = -40 C to 105 C VPWM rising, VCC = 12 V CSLOPE = floating VPWM falling, VCC = 12 V CSLOPE = floating Low level voltage Leakage current DISC Load disconnection threshold (VDRAIN-GND) ISINK = 5 mA VDISC = 5 V DISC Turn-ON DISC Turn-OFF 75 mV 110 155 C Hysteresis 25 800 3 s 1.2 0.2 0.5 1 V A 0.12 0.58 -1 90 100 450 Test conditions Min. 4.5 1 2 +1 110 750 1 3 0.16 V 0.9 10 A s A Typ. Max. 40 2000 Unit V mA A % mV
LEAKDRAIN Drain leakage current TR/TF Rise/Fall time of the current on PWM transition
TD
Delay on PWM signal (Figure 1)
Thermal Protection
Shutdown temperature
Logic inputs (PWM and EN) VL VH Input low level Input high level EN, PWM leakage current EN input leakage current PWM input leakage current VEN = 5 V; VPWM = 5 V VEN = 40 V VPWM = 40 V 1.2 2 60 120 A 0.4 V V
Note:
6/18
All devices 100 % production tested at TA = 25 C. Limits over the operating temperature range are guaranteed by design.
STCS2A
Timing
5
Figure 3.
Timing
PWM and output current timing
PWM
90%
Current
10%
TD
Trise
TD
Tfall
Figure 4.
Block diagram
High Voltage 45 V Preregulator 3.3 V Bandgap 1.23 V Shutdown all blocks
75 mV
Low Voltage 3.3 V Thermal Shutdown
H.V. 45 V
VCC
DISC + Disc comp
EN PWM
Enable Input PWM Input
Enable, PWM & Slope controll Logic
DRAIN
+
Logic
100 mV
Comp
Driver
GND
Slope Control
SOURCE
SLOPE
FB
7/18
Typical performance characteristics
STCS2A
6
Figure 5.
Typical performance characteristics
IDRAIN vs VCC, TA = 25C Figure 6.
1000
IDRAIN vs RSET
100 IDRAIN [mA] 10 1 0.1
1 RFB []
10
100
Figure 7.
IDRAIN vs temperature
Figure 8.
VDROP (including VFB) vs temperature
Figure 9.
ICC vs temperature
Figure 10. ICC vs VCC
8/18
STCS2A
Typical performance characteristics
Figure 11. Trise/Tfall vs CSLOPE
Figure 12. Turn-on time
Figure 13. Dimming operation: Rise
Figure 14. Dimming operation: Fall
CSLOPE = 1 nF
CSLOPE = 1 nF
9/18
Detail description
STCS2A
7
Detail description
The STCS2A is a BiCMOS constant current source designed to provide a precise constant current starting from a varying input voltage source. The main target is to replace discrete components solution for driving LEDs in low voltage applications such as 5 V, 12 V or 24 V giving benefits in terms of precision, integration and reliability.
7.1
Current setting
The current is set with an external sensing resistor connected to the FB pin. The feedback voltage is 100 mV, then a low resistor value can be chosen reducing power dissipation. A value between 1 mA and 2 A can be set according to the resistor value the resulting output current has a tolerance of 10%. For instance, should one need a 700 mA LEDs current, RF should be selected according to the following equation: RF = VFB / ILEDs = 100 mV / 700 mA = 142 m
7.2
Enable
When the enable pin is low the device completely off thus reducing current consumption to less than 1 A. When in shutdown mode, the internal main switch is off.
7.3
PWM dimming
The PWM input allows implementing PWM dimming on the LED current; when the PWM input is high the main switch will be on and vice versa. A typical frequency range for the input is from few Hertz to 50 kHz. The maximum dimming frequency is limited by the minimum rise/fall time of the current (obtained with CSLOPE=0) which is around 4 s each. Above 50 kHz the current waveforms starts assuming a triangular shape. While the PWM input is switching, the overall circuitry remains on, this is needed in order to implement two important features: short delay time and controlled slope for the current. Since the PWM pin is controlling just the main switch, the overall circuitry is always on and it is able to control the delay time between the PWM input signal and the output current in the range of few s, this is important to implement synchronization among several light LED sources. The rise and fall slope of the current is controlled by the CSLOPE capacitor. The rise and fall time are linear dependent from the CSLOPE capacitor value (see graph in typical characteristics). A controlled rise time has two main benefits: reducing EMI noise and avoid current spike at turn on. When CSLOPE is left floating, the internal switch is turned on at maximum speed, in this condition an overshoot can be present on the LED current before the system goes into regulation.
10/18
STCS2A
Detail description
7.4
Diagnostic
When STCS2A is in on mode (EN is high), the device is able to detect disconnection or fail of the LED string monitoring VDRAIN pin. If VDRAIN is lower than 75 mV the DISC pin is pulled low regardless the PWM pin status. This information can be used by the system to inform that some problem happens in the LEDs.
11/18
Application information
STCS2A
8
8.1
Application information
Reverse polarity protection
STCS2A must be protected from reverse connection of the supply voltage. Since the current sunk from VCC pin is in the range of 450 A a small diode connected to VCC is able to protect the chip. Care must be taken for the whole application circuit, especially for the LEDs, in fact, in case a negative voltage is applied between VIN and GND, a negative voltage will be applied to the LED string that must have a total breakdown voltage higher than the negative applied voltage in order to avoid any damage.
Figure 15. Reverse polarity condition
VIN
BAT46 or similar
VCC PWM
DRAIN DISC
EN
+
SLOPE
GND
FB
SOURCE
CSLOPE
RSENSE
8.2
Thermal considerations
The STCS2A is able to control a LED current up to 2 A and able to sustain a voltage on the drain pin up to 40 V. Those operating conditions are however limited by thermal constraints. The poor thermal conduction of epoxy FR4 boards does not permit to benefit of the outstanding thermal performance of the PowerSO-10. In any case one way to improve the thermal conduction is the use of large heat spreader areas at the copper layer of the PC board. This leads to a reduction of thermal resistance to 30 - 36 C/W for 3 to 6 cm2 on-board heatsink. Use of copper-filled through holes on conventional FR4 techniques increases the metallization and decreases thermal resistance accordingly. Using a configuration with 16 holes under the spreader of the package with a pitch of 1.8 mm and a diameter of 0.7 mm, the thermal resistance (junction - heatsink) can be reduced to 12 C/W. The thermal resistances shown in the Error! Reference source not found. section are the typical ones. The power dissipation in the device can be calculated as follow:
12/18
STCS2A PD = (VDRAIN - VFB) x ILED + (VCC x ICC)
Application information
basing on this and on the thermal resistance and ambient temperature, the junction temperature can be calculated as: TJ = RthJA x PD + TA A typical application could be: - Input voltage: 12 V; - 3 white LEDs with an typical VF = 3.6 V; - LEDs current: 1000 mA; - Package: PowerSO-10; - TA = 50 C; In this case the drain voltage is given by: VDRAIN = 12 - 3 x 3.6 = 1.2 V end the power dissipated in the IC is the following: PD = (1.2 - 0.1) x 1 + 12 x 0.5 x 10-3 = 1.1 W With a thermal resistance junction-ambient equal to 12 C/W the junction temperature is: TJ = 12 x 1.1 + 50 = 63 C. The following pictures show the maximum power dissipation according to the ambient temperature:
Figure 16. Maximum power dissipation vs TA for PowerSO-10
12.00 10.00
PDMAX = (TJMAX-TA)/RthJA
RthJA [C/W]
12 35
PDMAX [W]
8.00 6.00 4.00 2.00 0.00 25 35 45 55 65 75 [C] 85
95 105 115 125
13/18
Package mechanical data
STCS2A
9
Package mechanical data
In order to meet environmental requirements, ST offers these devices in ECOPACK(R) packages. These packages have a lead-free second level interconnect. The category of second Level Interconnect is marked on the package and on the inner box label, in compliance with JEDEC Standard JESD97. The maximum ratings related to soldering conditions are also marked on the inner box label. ECOPACK is an ST trademark. ECOPACK specifications are available at: www.st.com.
14/18
STCS2A
Package mechanical data
PowerSO-10 Mechanical Data
Dim. A A1 A2 A3 b c D D1 E E1 E2 E3 e L 0.95 0 3.40 1.25 0.40 0.35 9.40 7.40 13.80 9.30 7.20 5.90 1.27 1.65 8 0.037 0 mm. Min. Typ. Max. 3.70 0.10 3.60 1.35 0.53 0.55 9.60 7.60 14.40 9.50 7.60 6.10 0.134 0.049 0.016 0.014 0.370 0.291 0.543 0.366 0.283 0.232 0.050 0.065 8 Min. inch. Typ. Max. 0.146 0.004 0.142 0.053 0.021 0.022 0.378 0.299 0.567 0.374 0.299 0.240
0068039-E
15/18
Package mechanical data
STCS2A
Tape & Reel PowerSO-10 Mechanical Data
mm. Dim. Min. A C D N T Ao Bo Ko Po P W 14.9 9.9 4.15 3.9 23.9 23.7 12.8 20.2 60 30.4 15.1 10.1 4.35 4.1 24.1 24.3 0.587 0.390 0.163 0.153 0.941 0.933 Typ. Max. 330 13.2 0.504 0.795 2.362 1.197 0.594 0.398 0.171 0.161 0.949 0.957 Min. Typ. Max. 12.992 0.519 inch.
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STCS2A
Revision history
10
Table 6.
Date
Revision history
Document revision history
Revision 1 2 3 Initial release. Modified: Table 3 on page 5. Modified: Table 5 on page 6. Changes
19-Feb-2008 28-May-2008 02-Jul-2008
17/18
STCS2A
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