STLD20D
White LED power supply
General features
■
Typical guaranteed efficiency: 80%
■
Drives up to 4 LEDs in series from a 2.8V up to
4.2V supply voltage
■
Constant current regulation over the whole
operating voltage range
■
PWM control mode
■
Integrated load disconnect switch that opens
the LEDs path in shutdown mode
■
Integrated soft start peak inductor current
■
Programmable peak inductor current
(STLD20D-C8 only)
■
Shutdown pin allows digital dimming control up
to 10kHz
■
Over voltage and over temperature protection
with automatic restart
■
Low shutdown current (< 1µA)
■
Small external inductor (10µH)
■
Tiny external ceramic capacitor (1µF)
Application
■
White Led supply for LCD backlight
■
Mobile phone
■
PDA and organizers, MP3 players, Toys
QFN 3x3 8L
SOT23-8L
Description
The STLD20D is a constant switching frequency
boost regulator mainly dedicated to supply up to 4
white LEDs connected in series. A constant LED
current is achieved by sensing the LED current
through a sensing resistor RLED (see Figure 3.).
The device also includes a supply voltage
rejection circuit that prevents from any possible
flickering effect on the display that might happen
during input supply voltage variation. An
integrated Load Disconnect Switch open the LED
path to eliminate the current consumption in
shutdown mode. The maximum peak inductor
current can be programmed (STLD20D-C8 only).
Order code
Part number
Package
Marking
Packing
STLD20D-C8
SOT23-8L
L2D
Tape and reel
STLD20D-DEF
QFN 3x3 8L
L2D
Tape and reel
October 2006
Rev 4
1/31
www.st.com
31
Contents
STLD20D
Contents
1
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5
Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1
Boost converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2
Peak inductor current limitation and soft start function . . . . . . . . . . . . . . . . 9
5.3
Peak inductor current programmability (STLD20D-C8 only) . . . . . . . . . . . 9
5.4
Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.5
Brightness control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.6
Over temperature protection (OTP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.7
Over voltage protection (OVP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.8
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6
Typical performance characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7
Components selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1
L, Boost inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1.1
Calculation of the inductor value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1.2
Calculation of the saturation current I(sat) . . . . . . . . . . . . . . . . . . . . . . . 15
7.1.3
Choice of the RSET resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1.4
Reference selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.2
CIN and COUT capacitors selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3
1.3. D, Boost diode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3.1
8
2/31
Electrical characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.4
RLED feedback resistance selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.5
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
PWM dimming control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
STLD20D
9
Contents
Analog dimming control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1
Minimum dimming current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.2
Rd1 Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.3
Rdim calculation for dimming mode control . . . . . . . . . . . . . . . . . . . . . . . 22
10
Layout recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
11
Evaluation board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
12
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
13
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3/31
Pin description
STLD20D
Pin description
Figure 1.
Pin configuration (top view)
SW
GND
VIN
4/31
VOUT
SHDN
LDS
RSET
FB
1
2
GND
VIN
GND
3
4
SHDN RSET
SW
EXPOSED PAD
1
VIN
SHDN
N/C
VOUT
LDS
FB
5
6
7
8
1
2
3
4
5
6
7
8
FB
LDS
VOUT
SW
GND
VIN
SHDN
N/C
FB
LDS
VOUT
SW
STLD20D
Block diagram
2
Block diagram
Figure 2.
Block diagram
SW
OTP
OSCILLATOR
VIN
OVP
RAMP
OSCILLATOR
COMPENSATION
VOUT
POWER
FAULT
ENABLE
VIN
S
+
-
SW
Q
R
PWN
COMP.
SHDN
+
-
T°
VIN
DRAIN
CURRENT
REFERENCE
RSET (*)
LOAD DISCONNECT
LDS
VIN
SHDN
LDS
FB
+
LED CURRENT
REFERENCE
GND
(*) STLD20D-C8 only
5/31
Block diagram
STLD20D
Figure 3.
Basic connection
Table 1.
External components proposal
Value
Symbol
Parameter
Test conditions
Unit
Min.
RLED
CIN
COUT
LED current resistance
D
Note:
6/31
Max.
Ω
15
Input filtering capacitor
2.2
Ceramic type
µF
Output capacitance
1
Inductance
L
Typ.
Boost inductor (height < 2mm)
Boost diode
(STMicroelectronics STPS1L40M type)
10
Resistance at 500kHz
µH
1
Ω
ISAT (RSET = 100kΩ)
300
mA
VRRM
40
Vdc
IF (peak forward current)
1
VF @ IF = 1A Tj = 25°C
0.40
A
0.46
V
The external components proposal should be considered as a design reference guide.The
performances mentioned in the electrical characteristics table are not guaranteed for all the
possible electrical parameters of the components included in this list. On the other hand the
operation of STLD20D is not limited with the use of components included in this list.
STLD20D
Maximum ratings
3
Maximum ratings
Table 2.
Absolute maximum ratings
Value
Symbol
Parameter
Test conditions
Unit
Min.
VIN
Supply voltage range
Typ.
2.5
Max.
5
V
VESD
ESD ratings
TOP
Operating temperature
- 40
+ 85
°C
Tstg
Storage temperature
- 65
150
°C
HBM MIL STD 883C
BVDS
Breakdown voltage at pin SW and TSS and VOUT
SHDN
Maximum voltage applied on SHDN pin
Table 3.
2
kV
20
V
VIN
V
Thermal data
Value
Symbol
Parameter
Unit
Min.
RthJA
Mounted on epoxy board without
copper heatsink
Typ.
Max.
SOT23-8L
300
QFN
350
°C/W
7/31
Electrical characteristics
STLD20D
4
Electrical characteristics
Table 4.
Electrical characteristics (VIN = 2.8 to 4.2V and TJ = 25°C)
Value
Symbol
Parameter
Test conditions
Unit
Min.
VIN
Operating Input voltage range
ILED
Average regulated current ILED = 20mA
ISD
Stand-by current
SHDN = low
VIN = 4.2V
IQ
Quiescent current consumption
SHDN = high
VIN = 4.2V
TJ = 25°C
ISW = 250mA
VIN = 2.8V
0.48
SOT23-8L
VIN = 4.2V
0.38
TJ = 25°C
ISW = 250mA
VIN = 2.8V
0.57
VIN = 4.2V
0.42
VIN = 2.8V
5.0
VIN = 4.2V
4.2
VIN = 2.8V
5.1
VIN = 4.2V
4.3
SW
Boost switch
RDSON
QFN
SOT23-8L
LDS
Load Disconnect
Switch RDSON
QFN
FB
Line
Eff
DCMIN
RLED = 15Ω
TJ = 25°C
ILDS = 20mA
TJ = 25°C
ILDS = 20mA
19
20
0.43
0.285
0.302
Variation of the LED current versus the input voltage:
RLED = 15Ω
VIN = 2.8V
Efficiency with 4 LEDS, VOUT = 16V
OVP
Overvoltage protection
HystOV
Overvoltage hysteresis
V
21
mA
1
µA
0.6
mA
Ω
Ω
0.315
V
0.9
mA/V
%
400
500
600
20
L = 10µH
RSET = GND (STLD20D-C8)
17.5
18.5
640
mA
20
VDC
VDC
110
HystOT
Over temperature protection hysteresis
SHDN
Shutdown signal logic
°C
5
Disable Low
VIL
Enable high
VIH
kHz
%
0.7
Over temperature protection
1. Guaranteed by design.
4.2
85
Minimum duty cycle
Peak current boost switch (1)
Max.
80
VIN = 4.2V
Switching frequency
ILIM
OTP
2.8
Feedback voltage
fSW
8/31
Typ.
°C
0.3
V
1.2
STLD20D
Functional description
5
Functional description
5.1
Boost converter
The STLD20D is a PWM mode control boost converter operating at 500kHz.
An automatic compensation of the oscillation ramp allows rejection of the battery voltage
transient. The LED current regulation (see Figure 3.) is done by sensing the LED current
through the resistance RLED. The voltage across RLED is used by the feedback loop of the
controller (FB pin).
5.2
Peak inductor current limitation and soft start function
An integrated current sensor limits the switching current at 640 mA maximum.
Should the peak drain current exceed 640mA (if RSET = 0 for STLD20D-C8), the flip flop will
turn off the switch SW. During start up, this peak drain current limitation acts like a soft start
function.
5.3
Peak inductor current programmability (STLD20D-C8 only)
The converter peak current must be always below the inductor saturation current. For
flexibility reasons, the maximum peak inductor current can be programmed by connecting a
resistor at the pin RSET.
The Figure 12. gives the value of the resistance RSET versus the peak inductor current limit
ILMAX.
5.4
Shutdown
The SHDN pin is a low logic input signal and allows turning off the controller without cutting
the input voltage from the boost regulator circuit.
An integrated Load Disconnect Switch LDS disconnects the LEDs branch in shutdown
mode.This arrangement allows eliminating the DC current path that normally exists with
traditional boost regulator in shutdown mode.
5.5
Brightness control
The brightness of the LED is adjusted by pulsing the shutdown pin with a PWM signal as
high as 10kHz.
By using such a PWM signal the controller is alternatively ON and OFF and the LED current
changes from full current to zero.
The duty cycle allows regulating the average LED current. This scheme ensures that when
the LEDs are ON, they are driven at the full current without risk of color change.
9/31
Functional description
5.6
STLD20D
Over temperature protection (OTP)
An integrated temperature sensor senses the temperature of the junction of the controller.
As soon as this temperature exceeds 110°C min fixed internally, the controller is
automatically turned OFF. When the temperature is reduced of 5°C the operation of the
device automatically recovers.
5.7
Over voltage protection (OVP)
In case of failure and if the LED branch is cut, then there is no signal at the feedback pin FB
(Figure 3.), the PWM controller will then switches with a maximum duty cycle. This will
generate a voltage at the pin SW and VOUT that can exceed the maximum rating of the
device.
The overvoltage protection block senses the output voltage at the pin VOUT (Figure 3.). If the
voltage exceeds 18.5VDC typical the controller is automatically turned OFF.
When the voltage is reduced of 0.7V, the operation of the device automatically recovers.
5.8
Efficiency
(Figure 4. & Figure 9.)
The efficiency takes into account these following losses:
10/31
●
RLED ohmic losses
●
Boost switch SW losses
●
Load Disconnect Switch LDS
●
Boost inductor losses
●
Boost diode losses
STLD20D
Typical performance characteristics
6
Typical performance characteristics
Figure 4.
4 LEDs efficiency measurement
Figure 5.
Efficiency (%)
LED current vs input voltage
ILED(mA)
90.0
Ta = 25°C
21.00
ILED = 20mA, n.VLED= 16V, DC = 100%
Ta = 25°C
20.80
20.60
LQH32CN100K33
20.40
20.20
LPO04815-103
80.0
20.00
Shielded TDK
VLF3010AT-100MR49
19.80
19.60
CRDH2D14-100
19.40
19.20
70.0
19.00
2.5
3.0
3.5
4.0
4.5
2.5
3
3.5
VIN(V)
Figure 6.
4
4.5
5
VIN(V)
Feedback voltage
Figure 7.
Boost switch resistance (STLD20DC8)
RDSon(Ω)
VFB(mV)
310.0
0.8
0.7
305.0
Ta = 85°C
Ta = 25°C
300.0
0.6
0.5
Ta = -40°C
Ta = 85°C
0.4
Ta = 25°C
295.0
Ta = -40°C
0.3
290.0
0.2
2.5
3.0
3.5
4.0
4.5
2.5
3.0
3.5
VIN(V)
Figure 8.
4.0
4.5
VIN(V)
Boost switch resistance (STLD20D- Figure 9.
DEF)
RDSon(Ω)
Efficiency vs input voltage
(ILED=20mA; TA=25°C)
Efficiency (%)
0.8
88
87
0.7
86
85
0.6
Ta = 85°C
0.5
84
83
Ta = 25°C
82
0.4
81
Ta = -40°C
0.3
80
79
0.2
2.5
3.0
3.5
VIN(V)
4.0
4.5
78
2.5
3
3.5
4
4.5
5
Input voltage (VDC)
11/31
Typical performance characteristics
STLD20D
Figure 10. Load disconnect switch resistance Figure 11. Quiescent Current consumption
RDSon(Ω)
IQ(µA)
500.0
7.0
Ta = 85°C
450.0
6.0
Ta = 25°C
Ta = -40°C
400.0
5.0
Ta = 85°C
350.0
Ta = 25°C
300.0
Ta = -40°C
250.0
4.0
3.0
200.0
2.0
2.5
3.0
3.5
4.0
2.5
4.5
3.0
3.5
4.0
4.5
VIN(V)
VIN(V)
Figure 12. Max peak inductor current IL versus Figure 13. Max peak inductor current IL versus
L and RSET
RSET
ILIM(mA)
ILIM(mA)
600
600.0
ILIM = f(RSET)
(L = 10µH)
550
550.0
RSET = GND
500.0
500
450.0
450
400.0
400
ILIM = 450 mA
RSET = 56 kΩ
350
350.0
RSET = 100K
300.0
300
250
250.0
200
200.0
4.0
8.0
12.0
16.0
20.0
24.0
30
40
50
60
70
80
90
100
RSET(kΩ)
L(µH)
Figure 14. ILED versus duty cycle
Figure 15. Typical waveform
ILED(mA)
20
18
ILED = F(Duty), Ta= 25°C
VOUT
16
14
12
VSW
10
ILED
8
6
Theoretical
4
Real Values 300Hz
2
Real Values 10kHz
IL
0
0
10
20
30
40
50
60
Duty(%)
12/31
70
80
90
100
STLD20D
Figure 16.
Typical performance characteristics
Supply voltage rejection
Figure 17. Overvoltage protection
ILED
VIN
VSW
VOUT
IL
13/31
Components selection
STLD20D
7
Components selection
7.1
L, Boost inductor selection
To get a good trade-off thickness/efficiency, an attention must be given on the inductor
choice.
7.1.1
●
The inductance value must be selected to remain in the discontinuous conduction mode.
●
Its saturation current (Isat) must be equal or higher than the programmed current (ILIM).
●
An attention must be taken on the dynamic inductor parameters. Actually, some power
losses can occur in the boost inductor when it works at several hundred KHz and can
reduce the efficiency.
Calculation of the inductor value
The inductor must be dimensioned so that the STLD20D stays running in discontinuous
conduction mode operation in the worst operation condition (VIN = VIN_min= 2.8V). The limit
between continuous and discontinuous mode is called critical mode and characterized by an
uninterrupted current through the inductor (see figure 18).
Figure 18.
3 different conduction modes
IL
Continuous
Discontinuous
Critical
t
The formula [1] gives the maximum typical value of the inductor for a discontinuous mode
operation in the worst case condition (critical mode). Figure 19 shows the typical L value
versus the voltage across the LED branch N.VLED. Note that this curve includes the
STLD20D and inductance dispersions (20%).
N.VLED = 4x4V = 16V and ILOAD = 20mA
2
η⋅ V in ( min ) ( N ⋅ VLED – Vin ( min ) + V FB + ILED ⋅ R LDS )
L typ ≤-------------------------------------------------------------------------------------------------------------------------------------------------------------- [ 1 ]
2.4 ⋅ I LED ⋅ N ⋅ VLED ⋅ Fmax ⋅ ( N ⋅ V LED + I LED ⋅ R LDS )
14/31
STLD20D
Components selection
Where:
●
ηis the efficiency (80%)
●
N is the number of the white LEDs in series
●
VLED is the forward voltage of the LED for the ILED current (VLED = 4V in our example)
●
VIN(min) is the minimum input voltage (2.8V)
●
VFB is the error amplifier reference (0.3V)
●
RLDS is the internal resistance of the Load Disconnect Switch power MosFET (6Ω)
●
Fmax is the maximum frequency of the STLD20D (600kHz)
●
ILED_MAX is the current through the LED
For example, the case with 4 white LEDs can be considered in order to evaluate L value in
the worst case conduction.
Figure 19. Typical inductance value versus the white LED voltage for three IOUT
L typ (H)
Ltyp=f(nVLED)
2.1E-05
15mA
1.9E-05
20mA
1.7E-05
25mA
1.5E-05
1.3E-05
11µH
1.1E-05
9.0E-06
7.0E-06
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
N.VLED (V)
From figure 19, typical inductance must be lower than 11µH. By minimizing the inductance
to ensure the discontinuous mode operation, the standard coil value is equal to 10µH. Then:
L=10µH
7.1.2
Calculation of the saturation current I(sat)
The maximum peak current (Ip(max)) during steady state can be estimated by the formula [2]:
Ip ( max ) =
2 ⋅ I LED ⋅ N ⋅ VLED ⋅ ( N ⋅ VLED – V IN ( min ) + V FB + I LED ⋅ R LDS )
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- [2]
η⋅ Fmin ⋅ 0.8 ⋅ L typ ⋅ ( N ⋅ V LED + V FB + I LED ⋅ R LDS )
Where:
●
Ltyp is the typical inductance value
●
Fmin is the minimum frequency due to the STLD20D spread-off (400kHz)
15/31
Components selection
STLD20D
Figure 20. Maximum peak current (Ip(max)) versus the white LEDs voltages for 3 outputs current
- VIN > 2.8V
IP (max) (A)
0.5
10µH
0.45
0.4
0.35
0.3
15mA
20mA
0.25
25mA
0.2
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
N.VLED (V)
Figure 20 shows the maximum peak current Ip(max) through the coil for L=10µH versus the
voltage across the LED branch, N.VLED and the LED current ILED for VIN > 2.8V. As N.VLED
= 16V and ILED = 20mA, then Ip(max) = 0.45A. The curve below ends when the converter
reaches the critical mode operation.
Therefore, the saturation current (Isat) of the inductor must be higher than 0.45A. To
conclude:
I sat ≥ I p ( max )
Isat ≥ 0.4A
7.1.3
Choice of the RSET resistor
The resistor RSET fixes the maximum peak current flowing through the inductor whatever the
operating conditions. Thus, current saturation (Isat) is never reached. If the height constraint
is important, this function allows using low profile inductor with a small saturation current.
The Figure 12. on page 12 gives the corresponding typical value of the external resistor
RSET versus the ILIM value. This curve is slightly dependent of the temperature and the input
voltage.
To prevent the coil saturation RSET must be equal to 56kΩ, see Figure 12. Thus:
I sat ≥ I LIM ≥ I p ( max )
7.1.4
Reference selection
The table below gives some coil references suitable for the STLD20D versus L, DCR, Isat
value and sizing requirements.
16/31
STLD20D
Table 5.
Components selection
Reference selection
Name
Ref
Height (mm)
L typ (µH)
DCR (Ω)
ISAT (A)
LQH32CN4R7M33
2
4.7
0.15
0.65
LQH32CN100K33
2
10
0.3
0.45
LQH32CN4R7M53
1.55
4.7
0.15
0.65
LQH32CN100K53
1.55
10
0.3
0.45
LP04815-472MXC
1.5
4.7
0.15
0.77
LP04815-103MXC
1.5
10
0.23
0.55
744031100
1.65
10
0.205
0.74
744031150
1.65
15
0.285
0.62
744042100
1.8
10
0.15
1.3
1.855
10
0.294
0.7
CLS4D14
1.5
6.8
0.13
0.8
CLS4D14
1.5
10
0.18
0.65
VLF3010AT 100HR49
shielded
10
10
0.67
0.49
Murata
Coilcraft
Wurth Elektronik
(WE)
CDRH2014-100
Sumida
TDK
7.2
CIN and COUT capacitors selection
The capacitance values and its intrinsic resistance (ESR) must be selected in order to
reduce the output ripple.
The ceramic capacitor technology offers the best compromise between the space and the
performance (low ESR, value, voltage rating). Nevertheless, their values changes with the
time as well as with temperature, DC bias voltage and switching frequency. Thus it might be
necessary to use higher capacitor value if low ripple is an absolute need.
7.3
1.3. D, Boost diode selection
The diode selection is based upon two major criteria:
7.3.1
●
Low losses to get the best converter efficiency
●
Mechanical size
Electrical characteristic
VRRM (Repetitive peak reverse voltage) is the first parameter to consider in the selection of
the boost diode. Its value must be always higher than the reverse voltage (VR) occurring
during the steady state. Note that, some transient voltages occurs during the commutation
period due to the leakage inductance of the PCB. Generally, a power diode with a maximum
reverse voltage equal or just higher than 20V suits perfectly. Therefore a Schottky diode
technology can be used.
Schottky diode has a low forward voltage, nevertheless they have an additional reverse
current which provides additional losses at high ambient temperature.
17/31
Components selection
STLD20D
In fact, in boost backlighting converter, the conduction losses (Pcond) lead by the forward
characteristics can be negligible compared to the losses induced by the reverse current
(Prev), especially at high temperature.
7.4
RLED feedback resistance selection
The average output current is regulated by sensing a low external ohmic sensing resistor
RLED. Thus, a constant current value is fixed for each LED whatever the ambient
temperature conditions. RLED is given by:
VFB
0.3V
R LED = ----------- = ---------------- = 15Ω
I LED 20mA
7.5
[7]
Efficiency
Efficiency is a significant parameter for the application. The higher the efficiency, the longer
the life time of the battery. The efficiency is given by:
.
18/31
P output N ⋅ VLED ⋅ I LED
Efficiency = ------------------- = -------------------------------------------V IN ⋅ I input
P input
[8]
STLD20D
8
PWM dimming control
PWM dimming control
By applying a PWM signal on the shutdown pin SHDN, the average current and the
brightness of the LED can be adjusted. Figure 21. shows ILED current and the other typical
waveform during this dimming control mode.
Figure 21.
Typical waveform when the PWM dimming is used at 300Hz
Vshutdown
Vshutdown
δ = 0.38
ILED
ILED
VOUT
VOUT
ILIM
Ireg
IL
Note that the Load Disconnect Switch LDS turns ON/OFF at the same frequency and with
the same duty cycle as the PWM signal. Thus, the LED current is a perfect square wave
phased with the dimming signal. This leads to a good correlation between the real average
current of the LED and the theoretical current given by:
I LED – Theo = DC × I LED
Where:
●
ILED: is the nominal current programmed by the RLED resistance
●
DC: is the duty cycle of the dimming signal.
Figure 14. shows that the correlation between the real average current and the theoretical
value is given for a minimum duty cycle of 5% when the dimming frequency is 300Hz and
20% for a 10kHz dimming signal.
19/31
Analog dimming control
9
STLD20D
Analog dimming control
Some application are sensitive to low frequency dimming signal; in this case an analog
dimming control technic with a DC voltage Vdim to control the brightness of the LED can be
used with the circuit shown Figure 22.
The formula below gives the LED current versus the dimming voltage Vdim:
V FB ⋅ ( R dim + R d1 + R LED ) – Vdim ⋅ ( R LED + R d1 )
I LED = -------------------------------------------------------------------------------------------------------------------------------------Rdim ⋅ R LED
[ 19 ]
Where:
9.1
●
Vdim: Analog Dimming Voltage
●
Rdim, Rd1: Resistors of the dimming circuit (see figure 26)
Minimum dimming current
The PWM control of the STLD20D has a minimum duty cycle DCMIN that limits the dimming
current range. It exists a minimum dimming current ILEDC corresponding to the typical
DCMIN of the control loop.
Figure 22.
Analogical dimming schematics
LDS
C1
Vdim
C2
R2
Rdim
-
ILED
R1
Vfd
PWM
Ve
STLD20D
+
Vfd
Vref
Rd1
RLED
GND
This minimum dimming current depends on the maximum input voltage and the forward
voltage of the LED and can be estimated by:
2
( DC min ⋅ V IN ( max ) )
ILEDC ≈ -------------------------------------------------------------------------------------------------------------------------- [ 20 ]
2 ⋅ L typ ⋅ F typ ⋅ [ N ⋅ VLED + V FB – V IN ( max ) ]
20/31
STLD20D
Analog dimming control
Where:
●
VIN(max): is the maximum input voltage
●
DCMIN: is the typical minimum duty cycle of the STLD20D (18%)
●
Ltyp: is the typical vale of the inductance
●
Ftyp: is the typical switching frequency
Figure 23. gives the ILED versus the LED branch voltage N.VLED. This curve is calculated
with:
Ltyp = 10µH, Ftyp = 500kHz, DCMIN = 18% and VIN(max) = 4.2VDC.
Figure 23.
ILEDC current vs N.VLED corresponding at the DCMIN
ILEDC (A)
0.014
ILEDC = f (VLED)
0.012
0.010
0.008
0.006
0.004
0.002
0.000
9
10
11
12
13
14
15
16
17
18
19
20
N.VLED (V)
Higher the voltage across the branch LEDs, higher the range current control. After these
considerations, it is described here the basics rule to help the designer to choose the
external components such as Rd1, Rdim and RLED versus Vdim and brightness control
current ILED.
9.2
Rd1 Calculation
To avoid significant shifting of the cross over frequency and to keep enough high the
corrector network gain of the error amplifier, it is recommended to dimension the resistor
Rd1 below 10kΩ (10% of R1).
Dimension RLED for full brightness operating mode
RLED is dimensioned to get the nominal current ILED for the full brightness of the LED.
It is recommended to fix Vdim = VFB during the full brightness operating mode so that the
LED current correspond to the programmed value ILED. Thus:
V FB
R LED = ----------ILED
[ 21 ]
21/31
Analog dimming control
STLD20D
Where:
●
VFB is the feedback voltage
●
ILED is the LED current for full brightness
Note:
If Vdim is equal to 0 the LED current can be higher the programmed value.
9.3
Rdim calculation for dimming mode control
Rdim and Rd1 are dimensioned to get a current in the dimming circuit much smaller than the
LED current. From the formula 19, Rdim can be calculated by:
[ R d1 + R LED ] ⋅ [ VFB – V dim – max ]
R dim = ------------------------------------------------------------------------------------------I LEDmin RLED – V FB
[ 22 ]
Where:
22/31
●
Vdimmax is the maximum dimming voltage
●
ILEDmin is the expected minimum dimming current
STLD20D
10
Layout recommendation
Layout recommendation
The package connection of the STLD20D has been realized in order to facilitate the layout
of the PCB. The golden rule to obtain an optimized layout is to split the power and signal track
as shown on the Figure 24.
It is necessary to place the input capacitor as closed as possible between pin1 and pin2 of
the STLD20D package. If the CIN capacitor is not closed to the device, high frequency noise
due to gate driver dI/dt flows through the copper track of the board and can generate some
line voltage drop due to the line inductance.
For the same reason, in order to eliminate high frequency current loop, the connection of the
diode (D) and the output capacitor (COUT) must be as close as possible to the internal power
MosFET (SW) (close to pin 8 and 1).
Concerning the signal path, we recommend to create the PCB GND signal from the pin 1
("A" point in the Figure 24.). Thus all signal references such as feedback and the voltage
across Rset are not disturbed by the power stage.
Figure 24.
Layout suggested
D
L
8
STLD20D
A
RLED
RSET
COUT
1
2
GND
CIN
VIN
SHDN
23/31
Evaluation board
11
STLD20D
Evaluation board
Figure 25. shows the top view of the evaluation board that show all the application features
of the STLD20D.
Figure 25.
Evaluation board top view with its connections at the external equipment
Figure 26. Demo board layout top view
24/31
STLD20D
12
Package mechanical data
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
25/31
Package mechanical data
STLD20D
QFN8 (3x3) MECHANICAL DATA
DIM.
mm.
inch
MIN.
TYP
MAX.
MIN.
TYP.
MAX.
0.80
0.90
1.00
0.032
0.035
0.039
0.03
0.05
0.001
0.002
A2
0.65
0.70
0.75
0.026
0.028
0.030
A3
0.15
0.20
0.25
0.006
0.008
0.010
b
0.29
0.31
0.39
0.011
0.012
0.015
b1
0.17
0.30
0.007
A
A1
D
D2
3.00
1.92
E
E2
2.02
0.118
2.12
0.076
3.00
1.11
e
1.21
0.012
0.080
0.084
0.118
1.31
0.044
0.65
0.048
0.052
0.026
K
0.20
0.008
L
0.20
0.29
0.45
0.008
0.011
0.018
L1
0.16
0.24
0.40
0.006
0.009
0.016
L2
0.13
0.005
r
0.15
0.006
r1
0.15
0.006
7517789
26/31
STLD20D
Package mechanical data
SOT23-8L MECHANICAL DATA
mm.
mils
DIM.
MIN.
TYP
MAX.
MIN.
TYP.
MAX.
A
0.90
1.45
35.4
57.1
A1
0.00
0.15
0.0
5.9
A2
0.90
1.30
35.4
51.2
b
0.22
0.38
8.6
14.9
C
0.09
0.20
3.5
7.8
D
2.80
3.00
110.2
118.1
E
2.60
3.00
102.3
118.1
E1
1.50
1.75
59.0
68.8
e
0
e1
L
0.35
.65
25.6
1.95
76.7
0.55
13.7
21.6
27/31
Package mechanical data
STLD20D
Tape & Reel QFNxx/DFNxx (3x3) MECHANICAL DATA
mm.
inch
DIM.
MIN.
TYP
A
MIN.
TYP.
330
13.2
MAX.
12.992
C
12.8
D
20.2
0.795
N
60
2.362
T
28/31
MAX.
0.504
0.519
18.4
0.724
Ao
3.3
0.130
Bo
3.3
0.130
Ko
1.1
0.043
Po
4
0.157
P
8
0.315
STLD20D
Package mechanical data
Tape & Reel SOT23-xL MECHANICAL DATA
mm.
inch
DIM.
MIN.
TYP
A
MAX.
MIN.
TYP.
180
13.0
7.086
C
12.8
D
20.2
0.795
N
60
2.362
T
13.2
MAX.
0.504
0.512
14.4
0.519
0.567
Ao
3.13
3.23
3.33
0.123
0.127
0.131
Bo
3.07
3.17
3.27
0.120
0.124
0.128
Ko
1.27
1.37
1.47
0.050
0.054
0.0.58
Po
3.9
4.0
4.1
0.153
0.157
0.161
P
3.9
4.0
4.1
0.153
0.157
0.161
29/31
Revision history
STLD20D
13
Revision history
Table 6.
Revision history
Date
Revision
3-Aug-2004
1
Initial release.
12-Oct-2004
2
Table 4 on page 4 following parameters values updated:
. ILED (min), IQ (min), SW (QFN max), LDS (QFN max), ILIM, Hyst OT
. FB VAR symbol changed to Line and value changed from 0.7 to 0.9 mA/V
08-May-2006
3
Change figure 25, add figure 26 and new template.
23-Oct-2006
4
The SW, LDS and DCMIN values on table 4 have been updated, add note in
ILIM.
30/31
Changes
STLD20D
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31/31