STMicroelectronics AN2754 Buck high-brightness led driver based on the st1s10 Datasheet

AN2754
Application note
Buck high-brightness LED driver based on the ST1S10
step-down DC-DC converter voltage regulator
Introduction
High-brightness LEDs are becoming a prominent source of light because of their long life,
ruggedness, design flexibility, small size and energy efficiency. LEDs are now available in
higher and higher wattages per package (1 W, 3 W and 5 W) with currents up to 1.5 A. At
these current levels, the traditional means of limiting current with a resistor is not sufficiently
accurate nor efficient. Today, single-dice, white HBLEDs capable of delivering up to
90 lm/W of light are available. A typical 1 W white LED delivers an optical efficiency of
30 lm/W, whereas a typical 60 W light bulb delivers 15 lm/W.
It is known that the brightness of an LED is proportional to the forward current, so the best
way to supply LEDs is to control the forward current to get good matching of the output light.
LED manufacturers specify the characteristics (such as lumens, beam pattern) of their
devices at a specified forward current (IF), not at a specific forward voltage (VF).
This application note describes how to implement a constant current control to drive highbrightness LEDs by a step-down DC-DC converter voltage regulator. A switching regulator
is the best choice for driving HBLEDs when high efficiency and low power dissipation are
required.
The circuit uses the ST1S10 high-efficiency buck converter configured to drive a single
HBLED in constant current mode.
The ST1S10 is a step-down monolithic power switching regulator which needs few external
passive components and it is capable of delivering 3 A. An internal oscillator fixes the
switching oscillation at 900 kHz, and it is possible to synchronize the switching frequency
with an external clock from 400 kHz to 1.2 MHz.
This application note includes a schematic diagram, bill of material (BOM), and test data.
August 2008
Rev 1
1/20
www.st.com
Contents
AN2754
Contents
1
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
Buck topology switching power supply . . . . . . . . . . . . . . . . . . . . . . . . . 6
3
Design example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4
3.1
Design parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2
Power stage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3
Current sense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4
Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.5
Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.6
Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Description of the board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1
Input/output connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5
Schematic and bill of material (1 A LED current) . . . . . . . . . . . . . . . . . 14
6
Board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7
Typical application waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8
2/20
7.1
Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.2
Switching waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3
PWM dimming using the enable function . . . . . . . . . . . . . . . . . . . . . . . . . 18
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
AN2754
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Constant voltage control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Constant current control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Constant current control with VSENSE amplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Buck topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Buck converter circuit while the switch is in position 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Inductor current waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Buck converter circuit while the switch is in position 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Output impedence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Schematic - LED current 1 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Assembly layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Steady-state operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
PWM dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3/20
Background
1
AN2754
Background
When designing a power supply for a white high-brightness LED, the main requirements are
efficiency, size and cost of the complete solution.
A standard buck converter is the best choice for providing a constant current because only
the buck converter among the switching topologies has an average inductor current that is
equal to the average load current. For this reason, the conversion of a constant voltage into
constant current is much easier.
LEDs are current-driven devices whose brightness is proportional to their forward current.
Forward current can be controlled in two ways: voltage mode and current mode. The first
method uses the LED V-I curve to determine what voltage has to be applied to the LED in
order to generate the desired forward current. This is typically accomplished by applying a
voltage source and using a ballast resistor as shown in Figure 1. This method has two
serious drawbacks. The first is that every change in LED forward voltage creates a change
in LED current. The second problem is the power lost across the ballast resistor which
reduces the efficiency.
Figure 1.
Constant voltage control
Equation 1
⎛ R ⎞
VOUT = VFB ⎜⎜1 + H ⎟⎟ = n × VF _ MAX + IF × RFB
⎝ RL ⎠
LEDs are PN junction devices with a steep I - V curve. For this reason, driving an LED with
a voltage source can lead to large swings of forward current in response to even a small
change in forward voltage. In general, to meet the needs of a driver for an HBLED, the
current output must be in the ±5% to ±20% range.
The best way to drive the LEDs is to control the forward current so that it eliminates changes
in current due to variations in forward voltage, which translates into a constant LED
brightness. Figure 2 illustrates the configuration of a typical buck converter driver circuit.
The value of current-sense resistor (RSENSE) depends on the desired LED current and the
feedback voltage that the buck converter requires. Multiple LEDs should be connected in a
series configuration to keep an identical current flowing in each LED.
4/20
AN2754
Background
Figure 2.
Constant current control
VOUT
Voltage Regulator
FB
RSENSE
Equation 2
IF =
VFB
RFB
Accuracy and efficiency are the two main goals of the current sensing even if they are in
direct conflict. The higher the sense voltage is, the higher the signal-noise ratio, but the
higher the power dissipated on RSENSE.
To reduce the power dissipated in the series resistance, Figure 3 shows a simple method of
amplifying the current sense signal by using a single supply op-amp. This method allows the
user to select the current sense resistor RSENSE according to the desired power dissipation
while setting the average value of IF with the gain of the op-amp.
Figure 3.
Constant current control with VSENSE amplification
VOUT
Voltage Regulator
+
FB
OUT
-
RFB
RIN
RSENSE
Equation 3
IF =
VFB
⎛ R
R SENSE ⋅ ⎜⎜1 + FB
RIN
⎝
⎞
⎟⎟
⎠
5/20
Buck topology switching power supply
2
AN2754
Buck topology switching power supply
The buck topology switching power supply is an efficient voltage regulator which produces
an output voltage always less than or equal to the source voltage in the same polarity. The
first step of conversion is to generate a chopped version of input source. A single-pole
double-throw (SPDT) switch is connected as shown in Figure 4.
Figure 4.
Buck topology
The switch output voltage is equal to the converter input voltage when the switch is in
position 1 and equal to zero when the switch is position 2. The position is varied periodically
at a frequency of 1/T, where T represents the switching cycle period. The ratio of the on-time
to the period is referred to as the duty cycle D. So the switch output is a rectangular
waveform having amplitude equal to the source voltage, frequency equal to 1/T and duty
cycle equal to D. By inserting a low-pass filter between the (SPDT) switch and the load, a
basic buck topology is formed. The DC value of switch output voltage is simply the source
voltage multiplied by the duty cycle. The L-C filter cutoff is selected to pass the desired lowfrequency components of the switch output but also to attenuate the high-frequency
switching harmonics.
A power stage can operate in continuous or discontinuous inductor current mode.
Continuous inductor current mode is characterized by current flowing continuously in the
inductor during the entire switching cycle in steady-state operation. In discontinuous mode
the inductor current drops to zero for a portion of the switching cycle. In this section we will
derive the voltage conversion relationship for the continuous conduction mode buck power
stage. In continuous conduction mode, the power stage assumes two states per switching
cycle.
The ON state is when the high-side switch is ON and the low-side switch is OFF.
Figure 5.
Buck converter circuit while the switch is in position 1
During the ON state the voltage applied on the inductor is given by:
Equation 4
v L = Vs − Vout
6/20
AN2754
Buck topology switching power supply
We can find the inductor current by integrating the inductor voltage waveform.
Equation 5
iL (nT + t ) = i(nT ) +
Figure 6.
VS − Vout
⋅t
L
Inductor current waveform
iL
iL (nT + t ') = i(nT ) +
VS − VOUT
⋅ t'
L
iL (t ' ') = i(nT + DT ) −
VOUT
⋅ t' '
L
IOUT
nT
t’
nT+DT
t’’
(n+1)T
t
Equation 6
IL max = i(nT ) +
VS − Vout
⋅ DT
L
The inductor current increase during the ON state is given by:
Equation 7
∆iL =
VS − Vout
⋅ DT
L
The OFF state is when the high side is OFF and the low side ON.
Figure 7.
Buck converter circuit while the switch is in position 2
7/20
Buck topology switching power supply
AN2754
The inductor voltage during the OFF state is given by:
Equation 8
VL = −Vout
The inductor current during the OFF state is given by:
Equation 9
iL (t ) = i(nT + DT ) −
Vout
⋅t
L
The inductor current decrease during the OFF state is given by:
Equation 10
∆iL =
Vout
⋅ (1 − D)T
L
The volt-time product of each switch state must be equal in steady-state operation, so the
current increase during the ON state and the current decrease during the OFF state must be
equal.
Equation 11
VS − Vout
V
⋅ DT = out ⋅ (1 − D)T
L
L
From the above equation we obtain the continuous conduction mode buck voltage
conversion relationship.
Equation 12
Vout = D × Vs
To guarantee continuous mode, the following equation must be satisfied:
Equation 13
ILavg = ILOAD ≥
∆iL
2
The following relationship provides the minimum value of the inductance which is necessary
to guarantee the fixed ripple current in continuous mode:
Equation 14
L MIN =
[VinMAX − (RDSON + ESRL ) ⋅ ILavg − Vout] ⋅
∆IL ⋅ FSW
Vout
VinMAX
The function of the output capacitor is both filtering the AC current and providing the charge
that is necessary to supply the load during the transients. Constant current drivers are free
of load transient by design. For this reason, the capacitor is only needed to obtain a lower
current ripple amplitude across the LEDs. The value of the output capacitor is chosen to
8/20
AN2754
Buck topology switching power supply
reduce the ripple current on the LEDs branch. To calculate the ripple current that flows
through the LEDs it is necessary to estimate the impedances of the branches of both the
LED and output capacitor (Zo, Zc). In this procedure let us suppose that the triangular shape
of the ripple current on the inductor is approximately sinusoidal.
Figure 8.
Output impedence
Equation 15
Z O = rD + RFB
Equation 16
Z C = ESR +
1
2 ⋅ π ⋅ Fsw ⋅ Cout
The following equation can be used to estimate the impedance of the output capacitor which
guarantees the desired ripple current on the LEDs for a given inductor ripple current:
Equation 17
ZC =
∆iLED
× ZO
∆iL − ∆iLED
9/20
Design example
3
AN2754
Design example
This section outlines a step-by-step procedure for the design of a constant current control by
means of a switching step-down voltage regulator. The aim is to maintain both high
efficiency and good accuracy. The following design procedure is helpful in selecting the
component values of the application.
3.1
Design parameters
LED manufacturers generally recommend values for ∆IF ranging from ±5% to ±20% of IF .
The higher LED ripple allows the use of smaller inductors and smaller output capacitors.
The advantages of higher ripple current are reductions in the solution size and cost. Lower
ripple current requires more inductor output and more capacitor output. The advantages of a
lower ripple current are reductions in heating of the LED itself and a greater range of the
average LED current before the current limit is reached. The application is designed to
supply up to four HBLEDs. The LED used in the application is a Lumides LUXEON III
Emitter LXHL-PW09 with a typical forward voltage of 3.7 V at 700 mA.
Table 1 provides a summary of the specifications of a particular application.
Table 1.
Performance specification summary
Input source 4 AA batteries
Symbol
Value
Unit
Vin
6
V
White LED
LUXEON III Emitter LXHL-PW09
LED forward voltage
VF
3.7 at 700 mA
V
ILEDavg = Average inductor current
ILED
1
A
Ripple current on the inductor (%ILavg)
∆IL = % ILED
60
%
10
%
Ripple current on the LED branch (%∆ILED) ∆ILED = % ILED
3.2
Power stage selection
For the power stage we use the ST1S10 which is a general-purpose voltage regulator stepdown DC-DC converter which has been optimized for high-efficiency small-sized equipment.
A high switching frequency (900 kHz) allows the use of tiny surface-mount components.
The synchronous rectification is implemented in order to obtain efficiency higher than 90%.
The ST1S10 provides up to 3 A over an input voltage range from 2.5 V to 16 V.
The minimum input voltage to maintain regulation, depending on the load current and output
voltage, can be calculated as:
Equation 18
Vin min =
10/20
VOUT
+ ILED ⋅ (RL + RDS _ ON max )
DMAX
AN2754
Design example
where RDS (on) is the maximum PMOS switch-on resistance, RL is the DC resistance of the
inductor and VOUT is the nominal output voltage.
3.3
Current sense
Amplifying the sensed voltage is a way to reduce the power loss in the current sense
resistor. The operational amplifier selected for this application must be able to work with a
common mode input voltage close to zero. The selected device is the TS951, a rail-to-rail
BiCMOS operational amplifier. The value of the current sense resistor is determined by two
factors: power dissipation on RSENSE and the threshold level for amplifier input. Smaller
RSENSE reduces power dissipation but the detection of the feedback signal is more difficult.
In order to keep the power dissipation for the current sensing at a minimum value, a good
choice for the sense voltage with a forward current of 1 A is 100 mV.
The ratio between feedback voltage and sense voltage gives the value of the gain of the
amplification stage:
Equation 19
Gain =
VFB
ILED × R SENSE
Gain =
3.4
0. 8
=8
1× 0.1
Inductor selection
The buck power stage is designed to operate in continuous mode for load current greater
than 30% of full load. We choose an inductor value producing a maximum peak-peak ripple
current equal to sixty percent of the maximum load current. This limits the RMS current in
the output filter capacitor and, as a second order effect, keeps the core losses in the
inductor reasonable.
By using a single HBLED in conjunction with the chosen current sense resistor the output
voltage is given by the following equation:
Equation 20
VOUT = VF + VSENSE = 3.8 V + 0.1 V = 3.9 V
Let us set the value of the ripple current equal to 60% of the average current:
Equation 21
∆IL = 0.6 × ILEDAVG = 0.6 × 1 = 0.6 A
The minimum value of the output current to guarantee continuous mode is given by:
Equation 22
ILED min CCM = ∆IL / 2 = 0.6 / 2 = 300 mA
11/20
Design example
AN2754
The minimum value of inductance which guarantees a ripple current of 300 mA can be
calculated using Equation 14:
Equation 23
L MIN =
3.5
[VinMAX − (RDSON + ESRL ) ⋅ ILavg − Vout ] ⋅
∆IL ⋅ FSW
Vout
VinMAX
Output capacitor selection
The target tolerance for the LED ripple current is 10% of the forward current. In this
particular example the forward current is 1 A and the ripple current on LEDs branch is
100 mA. In current-mode converters, the load consists of the dynamic resistance of the
diodes, rD and the operating point resistance VO/ILED. Typical values for rD are provided by
LED manufacturers and for those do not, it must be determined by examining the slope of
the I-V curve that is provided in all LED datasheets.
The LUXEON III Emitter LXHL-PW09 datasheet gives a typical value for the dynamic
resistance rD of 0.8 Ω at 700 mA. Given the ripple current on the inductor with the equation
below, it is possible to calculate the ZC impedance to guarantee a ripple current on LEDs
branch equal to 10% of IF :
Equation 24
ZC =
ZC =
Z O ⋅ ∆iLED
∆iL − ∆iLED
(0.8 + 0.1) × 0.05 = 0.18 Ω
(0.3 − 0.05)
A ceramic capacitor is used and the required capacitance is selected based on the
impedance at 900 kHz.
Equation 25
C = 1/ [2 × π × (Z C − ESR ) × fSW ]
[
]
C = 1 / 2 × π × (0.18 − 0.01) × 900 ⋅ 10 3 = 1
3.6
Input capacitor selection
Because of the pulsating input current nature of the buck converter, a low ESR input
capacitor is required. A good input voltage filtering is important for minimizing the
interference with other circuits caused by high input voltage spikes. The following equation is
used to calculate the input ripple voltage due to capacitance and ESR:
Equation 26
∆Vinpp = Iin ⋅ RESR + Iin ⋅
D
Fsw ⋅ C
A 4.7 µF input capacitor is sufficient for effective input voltage filtering.
12/20
AN2754
4
Description of the board
Description of the board
The evaluation board is configured as constant current supply. Current regulation is
accomplished by regulating the voltage across a current sense resistor
4.1
Input/output connection
The following table describes the input/output connections.
Table 2.
Input/output connections
Reference
designator
Name
Description
J1
LED cathode
Output to cathode of LED
J2
LED anode
Output to anode of LED
Supply/sync
VIN_SW: Power input supply voltage to be tied to VIN_A. (VIN_SW max=18 V)
VIN_A: Analog input supply voltage to be tied to VIN_SW. (VIN_ A max=18 V)
SYNC: Synchronization and frequency select. Connect SYNC to GND for 900 kHz
switching frequency or connect to an external clock from 400 kHz to 1.2 MHz.
Enable
Use this connector to enable and disable DC-DC converter. Connect a jumper
between the ON pin and the center pin to enable the supply. Connect a jumper
between the OFF pin and the center pin to disable the supply. If this pin is left open,
the EVM does not operate correctly. This pin is also used for PWM dimming control
of the LED current.
J3
J4
13/20
Schematic and bill of material (1 A LED current)
Schematic and bill of material (1 A LED current)
Figure 9.
Schematic - LED current 1 A
CIN SW
4.7uF
CIN A
100nF
U1
Vin
Vin
1
2
3
4
5
1
6
8
4
SW
Vin_sw
ST1S10
PGND
AGND
Vf b
5
Supply
J2
L1 4.7uH
Vin_a
1
2
7
VIN
Rz1
330
Vin
J1
Dz
Cs
12V
100nF
1
2
J4
VIN
EN
GND
LED ANODE
COUT
2.2uF
3
Vin
Inh
Vin a
Vin sw
Pgnd
Agnd
SY N
Sync
J3
2
5
AN2754
LED CATHODE
U3
TS951ILT
1
2
3
+
R2 5.6k
DZ2
C3
22nF
2.4V
Rsense
0.1
RFB 6.8k
Table 3.
14/20
RIN
1k
Bill of material
Quantity
Reference
Part/value
PCB footprint
2
Cs,CIN A
100 nF
SM/C_0805
1
CIN SW
4.7 µF
SM/C_1210
1
COUT
2.2 µF
SM/C_1210
1
C3
22 nF
SM/C_0805
1
D3
12 V
SM/D_1406
1
J1
LED cathode
SIP/TM/L.200/2
1
J2
LED anode
SIP/TM/L.200/2
1
J3
Supply
SIP/TM/L.500/5
1
J4
Inh
SIP/TM/L.300/3
1
L1
4.7 µF
SM/L_2220
1
RFB
6.8 kΩ
SM/R_0805
1
RIN
1 kΩ
SM/R_0805
1
RSENSE
0.1
SM/R_0805
1
Rz
330
SM/R_0805
1
R2
5.6 kΩ
SM/R_0805
1
U1
ST1S10
VFDFPN 8L 4X4
1
U2
TS951ILT
SOT23-5
AN2754
6
Board layout
Board layout
Board layout is critical for all switched-mode power supplies. Figure 5, 6 and 7 show the
board layout for the ST1S10 constant current control evaluation board. It is essential to keep
the high switching current circulating paths as small as possible to reduce radiation and
resonance problems.
In general the following rules should be applied:
●
Traces should be as short as possible
●
L1, CIN, and COUT should be placed as close together as possible
●
Especially the connection from the IC pin SW and the inductor must be kept short
●
Ground areas should be as large as possible. If a 2-layer PCB is used, one layer should
be assigned as the ground layer and good connectivity between both layers should be
observed
●
CIN should be placed close to pin (VS) of the chip; CIN directly to pin 4
Figure 10. Assembly layer
15/20
Board layout
Figure 11. Top layer
Figure 12. Bottom layer
16/20
AN2754
AN2754
Typical application waveforms
7
Typical application waveforms
7.1
Startup
Figure 13. Startup
Conditions:
VIN: 6V; ILED: 1A; L=4.7uH; COUT=2.2uF.
Ch3: Inductor current
Ch4: LED current
7.2
Switching waveform
Figure 14. Steady-state operation
Conditions:
VIN: 6V; ILED: 1A; L=4.7uH; COUT=2.2uF.
CH1:SW
Ch3: Inductor current
Ch4: LED current
17/20
Typical application waveforms
7.3
AN2754
PWM dimming using the enable function
Figure 15. PWM dimming
Conditions:
VIN: 7 V;
IOUT: 1 A; L=4.7 µH;
COUT=2.2 µF, Freq=100 Hz;
duty cycle 50%.
CH1: SW; CH2: Enable Ch3: inductor current Ch4:LED
current;
18/20
AN2754
8
Revision history
Revision history
Table 4.
Document revision history
Date
Revision
28-Aug-2008
1
Changes
Initial release
19/20
AN2754
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