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 10

■ PWM dimming pin 1

■ Shutdown pin
PowerSO-10
■ LED disconnection diagnostic
■ Slope control with external cap

Applications
solution for driving LEDs in low voltage
■ LED constant current supplying for varying
applications such as 5 V, 12 V or 24 V giving
input voltages
benefits in terms of precision, integration and
■ Low voltage lighting reliability.
■ Small appliances LED lighting The current is set with external resistor up to 2 A
■ Car LED lights with a ± 10 % precision; a dedicated pin allows
implementing PWM dimming. An external
capacitor allows setting the slope for the current
Description rise from tens of microseconds to tens of
The STCS2A is a BiCMOS constant current milliseconds allowing reduction of EMI.
source designed to provide a precise constant An open-drain pin output provides information on
current starting from a varying input voltage load disconnection condition.
source. The main target is to replace discrete
components

Table 1. Device summary

Order code Package Packaging

STCS2ASPR PowerSO-10 600 parts per reel

July 2008 Rev 3 1/18

www.st.com 18
Contents STCS2A

Contents

1 Application diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Pin configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5 Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

6 Typical performance characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

7 Detail description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1 Current setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.2 Enable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.3 PWM dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.4 Diagnostic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

8 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1 Reverse polarity protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.2 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

9 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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STCS2A Application diagram

1 Application diagram

Figure 1. Typical application diagram for 2 A LED current

VIN
4.5V up to 40V
BAT46ZFILM

RIN 100 ohm

CBYP
0.1µF VCC DRAIN

ON PWM
OFF STCS2A CDRAIN
0.47µF
ON EN
OFF SLOPE GND FB SOURCE
DISC

Load disconnection RFB

CSLOPE
(Open Drain output) 0.05 ohm
10nF

3/18
Pin configuration STCS2A

2 Pin configuration

Figure 2. Pin connections (top view)

Table 2. Pin description

Pin n° Symbol Note

1 VCC Supply voltage

2 PWM PWM dimming input
3 EN Shutdown
4 NC No connect
5 DRAIN Internal N-MOSFET drain
Internal N-MOSFET source. Reference voltage is 100 mV. An external resistor between
6 SOURCE
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
7 FB 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
8 GND Ground
9 SLOPE Capacitor for slope control
10 DISC Load disconnection flag (open drain)
exp-pad Internally connected to ground

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STCS2A Maximum ratings

3 Maximum ratings

Table 3. Absolute maximum ratings

Symbol Parameter Value Unit

VCC DC supply voltage -0.3 to +45

V
DRAIN Drain pin -0.3 to +45
SOURCE Source pin -0.3 to + 3.3 V
PWM, EN, DISC Logic pins -0.3 to + VCC + 3.3 V
SLOPE, FB Configuration pins -0.3 to + 3.3 V
ESD Human body model (all pins) ±2 kV
TJ (1) Junction temperature -40 to 150 °C
TSTG Storage temperature range -55 to 150 °C
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.

Table 4. Thermal data

Symbol Parameter PowerSO-10 Unit

RthJC Thermal resistance junction-case 2 °C/W

(1)
RthJA Thermal resistance junction-ambient 50 °C/W
RthJA Thermal resistance junction-ambient (2) 35 °C/W
RthJA Thermal resistance junction-ambient (3) 12 °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 Electrical characteristics

Table 5. Electrical characteristics (VCC = 12 V; IO = 100 mA; TJ = -40 °C to 125 °C; VDRAIN = 1 V;
CDRAIN = 1 µF; CBYP = 100 nF typical values are at TA = 25°C, unless otherwise specified)
Symbol Parameter Test conditions Min. Typ. Max. Unit

VCC Supply voltage range 4.5 40 V

Output current range 1 2000 mA
IO Output current RFB = 50 mΩ 2 A
Regulation (percentage with VCC = 4.5 to 40 V,
-1 +1 %
respect to VCC = 12 V) IO = 100 mA; VDRAIN = 1 V
VFB Feedback voltage IO = 0 to 2A 90 100 110 mV
On Mode 450 750
Shutdown Mode;
Quiescent current (Measured on 1
ICC VCC = 5 to 12V µA
VCC pin)
Shutdown Mode;
3
VCC = 12 to 40V
IO = 100 mA 0.12 0.16
VDROP Dropout voltage (VDRAIN to GND) V
IO = 2 A 0.58 0.9
LEAKDRAIN Drain leakage current Shutdown; VDRAIN = 40 V 10 µA
Rise/Fall time of the current on CSLOPE = 10 nF,
TR/TF 800 µs
PWM transition TJ = -40 °C to 105 °C
VPWM rising, VCC = 12 V
3
CSLOPE = floating
TD Delay on PWM signal (Figure 1) µs
VPWM falling, VCC = 12 V
1.2
CSLOPE = floating
Low level voltage ISINK = 5 mA 0.2 0.5 V
Leakage current VDISC = 5 V 1 µA
DISC
Load disconnection threshold DISC Turn-ON 75
mV
(VDRAIN-GND) DISC Turn-OFF 110

Thermal Shutdown temperature 155

°C
Protection Hysteresis 25

Logic inputs (PWM and EN)

VL Input low level 0.4 V

VH Input high level 1.2 V
EN, PWM leakage current VEN = 5 V; VPWM = 5 V 2
EN input leakage current VEN = 40 V 60 µA
PWM input leakage current VPWM = 40 V 120

Note: All devices 100 % production tested at TA = 25 °C. Limits over the operating temperature
range are guaranteed by design.

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STCS2A Timing

5 Timing

Figure 3. PWM and output current timing

PWM

90%

10%
Current

TD Trise TD Tfall

Figure 4. Block diagram

High Voltage Low Voltage 3.3 V H.V.

45 V 45 V

Thermal
Preregulator Bandgap Shutdown
VCC
3.3 V 1.23 V DISC
Shutdown +
all blocks
75 mV

Enable
- Disc
EN Enable,
Input comp
PWM & DRAIN
Slope controll
PWM Logic
PWM Input

+
Logic
100 mV - Driver
Comp
GND Slope
Control SOURCE

SLOPE FB

7/18
Typical performance characteristics STCS2A

6 Typical performance characteristics

Figure 5. IDRAIN vs VCC, TA = 25°C Figure 6. IDRAIN vs RSET

1000

100

IDRAIN [mA]
10

1
0.1 1 10 100
RFB [Ω]

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 mΩ

7.2 Enable
When the enable pin is low the device completely off thus reducing current consumption to
less than 1 µA. 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 µs 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 µs, 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.

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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 Application information

8.1 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 µA 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 DRAIN

DISC
PWM

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 Application information

PD = (VDRAIN - VFB) x ILED + (VCC x ICC)

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
PDMAX = (TJMAX-TA)/RthJA RthJA [°C/W]
10.00
12
PDMAX [W]

8.00
35
6.00
4.00
2.00
0.00
25 35 45 55 65 75 85 95 105 115 125
[°C]

13/18
Package mechanical data STCS2A

9 Package mechanical data

In order to meet environmental requirements, ST offers these devices in ECOPACK®

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

mm. inch.
Dim.
Min. Typ. Max. Min. Typ. Max.
A 3.70 0.146
A1 0.10 0.004
A2 3.40 3.60 0.134 0.142
A3 1.25 1.35 0.049 0.053
b 0.40 0.53 0.016 0.021
c 0.35 0.55 0.014 0.022
D 9.40 9.60 0.370 0.378
D1 7.40 7.60 0.291 0.299
E 13.80 14.40 0.543 0.567
E1 9.30 9.50 0.366 0.374
E2 7.20 7.60 0.283 0.299
E3 5.90 6.10 0.232 0.240
e 1.27 0.050
L 0.95 1.65 0.037 0.065
α 0° 8° 0° 8°

0068039-E

15/18
Package mechanical data STCS2A

Tape & Reel PowerSO-10 Mechanical Data

mm. inch.
Dim.
Min. Typ. Max. Min. Typ. Max.

A 330 12.992

C 12.8 13.2 0.504 0.519

D 20.2 0.795

N 60 2.362

T 30.4 1.197

Ao 14.9 15.1 0.587 0.594

Bo 9.9 10.1 0.390 0.398

Ko 4.15 4.35 0.163 0.171

Po 3.9 4.1 0.153 0.161

P 23.9 24.1 0.941 0.949

W 23.7 24.3 0.933 0.957

16/18
STCS2A Revision history

10 Revision history

Table 6. Document revision history

Date Revision Changes

19-Feb-2008 1 Initial release.

28-May-2008 2 Modified: Table 3 on page 5.
02-Jul-2008 3 Modified: Table 5 on page 6.

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 STCS2A

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