STMicroelectronics STCS2A Datasheet
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STCS2A
2 A max constant current LED driver
Features
■
■
■
■
■
■
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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
Applications
■
■
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LED constant current supplying for varying input voltages
Low voltage lighting
Small appliances LED lighting
Car LED lights
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
10
1
PowerSO-10 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.
Table 1.
Device summary
Order code
STCS2ASPR
July 2008
Package
PowerSO-10
Rev 3
Packaging
600 parts per reel
1/18 www.st.com
18
Contents
Contents
STCS2A
Typical performance characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2/18
STCS2A
Figure 1.
Typical application diagram for 2 A LED current
Application diagram
3/18
Pin configuration
Figure 2.
Pin connections (top view)
STCS2A
Table 2.
Pin n°
1
2
3
4
5
6
7
8
9
10
Pin description
Symbol
V
CC
PWM
EN
NC
DRAIN
SOURCE
FB
GND
SLOPE
DISC exp-pad
Note
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
4/18
STCS2A
3 Maximum
Maximum ratings
Table 3.
Symbol
Absolute maximum ratings
Parameter Value Unit
V
CC
DRAIN
SOURCE
DC supply voltage
Drain pin
Source pin
-0.3 to +45
-0.3 to +45
-0.3 to + 3.3
V
V
PWM, EN, DISC
SLOPE, FB
Logic pins
Configuration pins
-0.3 to + V
CC
+ 3.3
-0.3 to + 3.3
V
V
ESD
T
J
(1)
T
STG
Human body model (all pins)
Junction temperature
Storage temperature range
±2
-40 to 150
-55 to 150
1.
T
J
is calculated from the ambient temperature T
A
T
J
= T
A
+ (P
D
x R thJA
and the power dissipation P
D
according the following formula:
). See
for details of max power dissipation for ambient temperatures higher than 25 °C.
kV
°C
°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.
Symbol
Thermal data
Parameter
R
R
R thJC thJA thJA
Thermal resistance junction-case
Thermal resistance junction-ambient
(1)
Thermal resistance junction-ambient
(2)
Thermal resistance junction-ambient
(3)
R thJA
1.
FR4 with using the recommended pad-layout
2.
FR4 with heat sink on board (6 cm
2
).
3.
FR4 with copper-filled through holes and external heat sink applied.
PowerSO-10
2
50
35
12
Unit
°C/W
°C/W
°C/W
°C/W
5/18
Electrical characteristics STCS2A
Table 5.
Electrical characteristics (V
CC
C
DRAIN
= 1 µF; C
BYP
= 12 V; I
O
= 100 mA; T
J
= -40 °C to 125 °C; V
DRAIN
= 1 V;
= 100 nF typical values are at T
A
= 25°C, unless otherwise specified)
Parameter Test conditions Min.
Typ.
Max.
Unit Symbol
V
CC
Supply voltage range
Output current range
V
I
O
FB
Output current
Regulation (percentage with respect to V
CC
= 12 V)
Feedback voltage
R
FB
= 50 m Ω
V
CC
= 4.5 to 40 V,
I
O
= 100 mA; V
DRAIN
= 1 V
I
O
= 0 to 2A
On Mode
V
I
CC
DROP
T
D
Quiescent current (Measured on
V
CC
pin)
Shutdown Mode;
V
CC
= 5 to 12V
Shutdown Mode;
V
CC
= 12 to 40V
Dropout voltage (V
DRAIN to GND)
I
O
= 100 mA
I
O
= 2 A
LEAK
DRAIN
Drain leakage current
T
R
/T
F
Rise/Fall time of the current on
PWM transition
Delay on PWM signal (
Low level voltage
)
Shutdown; V
C
SLOPE
T
J
DRAIN
= 10 nF,
= 40 V
= -40 °C to 105 °C
V
PWM
rising, V
CC
= 12 V
C
SLOPE
= floating
V
PWM
falling, V
CC
= 12 V
C
SLOPE
= floating
I
SINK
= 5 mA
DISC
Leakage current V
DISC
= 5 V
DISC Turn-ON
Load disconnection threshold
(V
DRAIN
-GND)
DISC Turn-OFF
Thermal
Protection
Shutdown temperature
Hysteresis
4.5
1
-1
90
2
100
450
0.12
0.58
800
3
1.2
0.2
75
110
155
25
40
2000
+1
110
750
1
3
0.16
0.9
10
0.5
1
V mA
A
% mV
µA
V
µA
µs
µs
V
µA mV
°C
Logic inputs (PWM and EN)
V
L
V
H
Input low level
Input high level
EN, PWM leakage current
EN input leakage current
PWM input leakage current
V
EN
= 5 V; V
PWM
= 5 V
V
EN
= 40 V
V
PWM
= 40 V
1.2
0.4
2
60
120
V
V
µA
Note:
6/18
All devices 100 % production tested at T
A
= 25 °C. Limits over the operating temperature range are guaranteed by design.
STCS2A
5 Timing
Figure 3.
PWM and output current timing
Figure 4.
Block diagram
Shutdown all blocks
Thermal
Shutdown
Timing
7/18
Typical performance characteristics
6 Typical performance characteristics
Figure 5.
I
DRAIN
vs V
CC
, T
A
= 25°C Figure 6.
I
DRAIN
vs R
SET
STCS2A
Figure 7.
I
DRAIN
vs temperature
[ Ω]
Figure 8.
V
DROP
(including V
FB
) vs temperature
Figure 9.
I
CC
vs temperature Figure 10.
I
CC
vs V
CC
8/18
STCS2A
Figure 11.
T rise
/T fall
vs C
SLOPE
Typical performance characteristics
Figure 12.
Turn-on time
Figure 13.
Dimming operation: Rise Figure 14.
Dimming operation: Fall
C
SLOPE
= 1 nF C
SLOPE
= 1 nF
9/18
Detail description STCS2A
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.
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, R
F the following equation:
should be selected according to
R
F
= V
FB
/ I
LEDs
= 100 mV / 700 mA = 142 m Ω
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.
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 C
SLOPE
=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 C
SLOPE
capacitor. The rise and fall time are linear dependent from the C
SLOPE
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 C
SLOPE
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 V
DRAIN
pin. If V
DRAIN
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
8.1 Reverse polarity protection
STCS2A must be protected from reverse connection of the supply voltage. Since the current sunk from V
CC
pin is in the range of 450 µA a small diode connected to V
CC
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 V
IN
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
12/18
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 cm
2
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:
STCS2A Application information
P
D
= (V
DRAIN
- V
FB
) x I
LED
+ (V
CC x I
CC
) basing on this and on the thermal resistance and ambient temperature, the junction temperature can be calculated as:
T
J
= R thJA
x P
D
+ T
A
A typical application could be:
– Input voltage: 12 V;
– 3 white LEDs with an typical V
F
= 3.6 V;
– LEDs current: 1000 mA;
– Package: PowerSO-10;
– T
A
= 50 °C;
In this case the drain voltage is given by:
V
DRAIN
= 12 - 3 x 3.6 = 1.2 V end the power dissipated in the IC is the following:
P
D
= (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:
T
J
= 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 T
A
for PowerSO-10
25 35 45 55 65 75 85 95 105 115 125
13/18
Package mechanical data
9 Package mechanical data
STCS2A
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
Dim.
E
E1
E2
E3 e
L
α b c
D
D1
A
A1
A2
A3
STCS2A Package mechanical data
Min.
3.40
1.25
0.40
0.35
9.40
7.40
13.80
9.30
7.20
5.90
0.95
0°
PowerSO-10 Mechanical Data
mm.
Typ.
Min.
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
1.27
1.65
8°
0.037
0° inch.
Typ.
0.050
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
0.065
8°
0068039-E
15/18
Package mechanical data
Tape & Reel PowerSO-10 Mechanical Data
Min.
mm.
Typ.
Min.
inch.
Typ.
Dim.
Ao
Bo
Ko
Po
P
W
D
N
A
C
T
12.8
20.2
60
14.9
9.9
4.15
3.9
23.9
23.7
Max.
330
13.2
30.4
15.1
10.1
4.35
4.1
24.1
24.3
0.504
0.795
2.362
0.587
0.390
0.163
0.153
0.941
0.933
STCS2A
Max.
12.992
0.519
1.197
0.594
0.398
0.171
0.161
0.949
0.957
16/18
STCS2A
10 Revision history
Table 6.
Date
Document revision history
Revision
19-Feb-2008
28-May-2008
02-Jul-2008
1
2
3
Initial release.
.
.
Changes
Revision history
17/18
STCS2A
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