STLD20D-C8 - STMicroelectronics

October 2006 Rev 4 1/31

31

Order code

Part number Package Marking Packing

STLD20D-C8 SOT23-8L L2D Tape and reel

STLD20D-DEF QFN 3x3 8L L2D Tape and reel

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

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).

SOT23-8LQFN 3x3 8L

www.st.com

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Contents STLD20D

2/31

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 Electrical characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

7.4 RLED feedback resistance selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7.5 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

8 PWM dimming control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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STLD20D Contents

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9 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

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Pin description STLD20D

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1 Pin description

Figure 1. Pin configuration (top view)

SW

V

OUT

LDS

FBFB

GND

V

IN

SHDN

R

SET

12345678

GND V

IN SHDN RSET FB LDS VOUT SW

GND

EXPOSED PAD

SW

VIN VOUT

SHDN LDS

N/C FB

12345678

GND VIN SHDN N/C FB LDS VOUT SW

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STLD20D Block diagram

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2 Block diagram

Figure 2. Block diagram

GND

SHDN

FB

VIN

LDS

VOUT

RSET (*)

OSCILLATOR

OTP

OVP

RAMP

OSCILLATOR

COMPENSATION

PWN

COMP.

S

Q

R

+

-

POWER

FAULT

VIN

ENABLE

DRAIN

CURRENT

REFERENCE

LOAD DISCONNECT

LED CURRENT

REFERENCE

SHDN

VIN

SW

LDS

SW

(*) STLD20D-C8 only

T°

VIN

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Block diagram STLD20D

6/31

Note: 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.

Figure 3. Basic connection

Table 1. External components proposal

Symbol Parameter Test conditions

Value

Unit

Min. Typ. Max.

RLED LED current resistance 15 Ω

CIN Input filtering capacitor Ceramic type 2.2 µF

COUT Output capacitance 1

L Boost inductor (height < 2mm)

Inductance 10 µH

Resistance at 500kHz 1 Ω

ISAT (RSET = 100kΩ) 300 mA

DBoost diode

(STMicroelectronics STPS1L40M type)

VRRM 40 Vdc

IF (peak forward current) 1 A

VF @ IF = 1A Tj = 25°C 0.40 0.46 V

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STLD20D Maximum ratings

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3 Maximum ratings

Table 2. Absolute maximum ratings

Symbol Parameter Test conditions

Value

Unit

Min. Typ. Max.

VIN Supply voltage range 2.5 5 V

VESD ESD ratings HBM MIL STD 883C 2 kV

TOP Operating temperature - 40 + 85 °C

Tstg Storage temperature - 65 150 °C

BVDS Breakdown voltage at pin SW and TSS and VOUT 20 V

SHDN Maximum voltage applied on SHDN pin VIN V

Table 3. Thermal data

Symbol Parameter

Value

Unit

Min. Typ. Max.

RthJA Mounted on epoxy board without

copper heatsink

SOT23-8L 300 °C/W

QFN 350

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Electrical characteristics STLD20D

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4 Electrical characteristics

Table 4. Electrical characteristics (VIN = 2.8 to 4.2V and TJ = 25°C)

Symbol Parameter Test conditions

Value

Unit

Min. Typ. Max.

VIN Operating Input voltage range 2.8 4.2 V

ILED Average regulated current ILED = 20mA RLED = 15Ω19 20 21 mA

ISD Stand-by current SHDN = low VIN = 4.2V 1 µA

IQQuiescent current consumption SHDN = high VIN = 4.2V 0.43 0.6 mA

SW Boost switch

RDSON

SOT23-8L TJ = 25°C

ISW = 250mA

VIN = 2.8V 0.48

Ω

VIN = 4.2V 0.38

QFN TJ = 25°C

ISW = 250mA

VIN = 2.8V 0.57

VIN = 4.2V 0.42

LDS Load Disconnect

Switch RDSON

SOT23-8L TJ = 25°C

ILDS = 20mA

VIN = 2.8V 5.0

Ω

VIN = 4.2V 4.2

QFN TJ = 25°C

ILDS = 20mA

VIN = 2.8V 5.1

VIN = 4.2V 4.3

FB Feedback voltage 0.285 0.302 0.315 V

Line Variation of the LED current versus the input voltage:

RLED = 15Ω0.9 mA/V

Eff Efficiency with 4 LEDS, VOUT = 16V VIN = 2.8V 80 %

VIN = 4.2V 85

fSW Switching frequency 400 500 600 kHz

DCMIN Minimum duty cycle 20 %

ILIM Peak current boost switch (1) L = 10µH

RSET = GND (STLD20D-C8) 640 mA

OVP Overvoltage protection 17.5 18.5 20 VDC

HystOV Overvoltage hysteresis 0.7 VDC

OTP Over temperature protection 110 °C

HystOT Over temperature protection hysteresis 5 °C

SHDN Shutdown signal logic Disable Low VIL 0.3 V

Enable high VIH 1.2

1. Guaranteed by design.

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STLD20D Functional description

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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.

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Functional description STLD20D

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5.6 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:

●RLED ohmic losses

●Boost switch SW losses

●Load Disconnect Switch LDS

●Boost inductor losses

●Boost diode losses

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STLD20D Typical performance characteristics

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6 Typical performance characteristics

Figure 4. 4 LEDs efficiency measurement Figure 5. LED current vs input voltage

Figure 6. Feedback voltage Figure 7. Boost switch resistance (STLD20D-

C8)

Figure 8. Boost switch resistance (STLD20D-

DEF)

Figure 9. Efficiency vs input voltage

(ILED=20mA; TA=25°C)

70.0

80.0

90.0

2.5 3.0 3.5 4.0 4.5

LQH32CN100K33

LPO04815-103

CRDH2D14-100

Shielded TDK

VLF3010AT-100MR49

V (V)

IN

Efficiency (%)

I = 20mA,

LED n.V = 16V, DC = 100%

LED

T = 25°C

a

V (V)

IN

I (mA)

LED

19.00

19.20

19.40

19.60

19.80

20.00

20.20

20.40

20.60

20.80

21.00

2.5 3 3.5 4 4.5 5

T = 25°C

a

290.0

295.0

300.0

305.0

310.0

2.5 3.0 3.5 4.0 4.5

T = 85°C

a

T = 25°C

a

T = -40°C

a

V (V)

IN

V (mV)

FB

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2.5 3.0 3.5 4.0 4.5

T = 85°C

a

T = 25°C

a

T = -40°C

a

V (V)

IN

RDSon( )Ω

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2.5 3.0 3.5 4.0 4.5

T = 85°C

a

T = 25°C

a

T = -40°C

a

V (V)

IN

RDSon( )Ω

78

79

80

81

82

83

84

85

86

87

88

2.5 3 3.5 4 4.5 5

Input voltage (V )

DC

Efficiency (%)

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Typical performance characteristics STLD20D

12/31

Figure 10. Load disconnect switch resistance Figure 11. Quiescent Current consumption

Figure 12. Max peak inductor current IL versus

L and RSET

Figure 13. Max peak inductor current IL versus

RSET

Figure 14. ILED versus duty cycle Figure 15. Typical waveform

2.0

3.0

4.0

5.0

6.0

7.0

2.5 3.0 3.5 4.0 4.5

T = 85°C

a

T = 25°C

a

T = -40°C

a

V (V)

IN

RDSon( )Ω

200.0

250.0

300.0

350.0

400.0

450.0

500.0

2.5 3.0 3.5 4.0 4.5

T = 85°C

a

T = 25°C

a

T = -40°C

a

V(V)

IN

I (µA)

Q

200.0

250.0

300.0

350.0

400.0

450.0

500.0

550.0

600.0

4.0 8.0 12.0 16.0 20.0 24.0

RSET = GND

RSET = 100K

L(µH)

I (mA)

LIM

300

200

250

350

400

450

500

550

600

30 40 50 60 70 80 90 100

R(k)

SET Ω

I (mA)

LIM

I = f(R )

(L = 10µH)

LIM SET

I = 450 mA

R

LIM

SET = 56 kΩ

0

2

4

6

8

10

12

14

16

18

20

0 102030405060708090100

Theoretical

Real Values 300Hz

Real Values 10kHz

Duty(%)

I (mA)

LED

I = F(Duty), T = 25°C

LED a

VOUT

VSW

ILED

IL

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STLD20D Typical performance characteristics

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Figure 16. Supply voltage rejection Figure 17. Overvoltage protection

V

IN

V

SW

I

LED

V

OUT

I

L

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Components selection STLD20D

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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.

●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.

7.1.1 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).

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

Figure 18. 3 different conduction modes

I

L

Continuous

Discontinuous

Critical

t

Ltyp

ηV

2

in min()

NV

LED Vin min()

–VFB ILED RLDS

⋅++⋅()⋅

2.4 ILED NV

LED Fmax NV

LED ILED RLDS

⋅+⋅()⋅⋅⋅ ⋅ ⋅

-------------------------------------------------------------------------------------------------------------------------------------------------------------- 1[]≤

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STLD20D Components selection

15/31

Where:

●η is 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.

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]:

Where:

●Ltyp is the typical inductance value

●Fmin is the minimum frequency due to the STLD20D spread-off (400kHz)

Figure 19. Typical inductance value versus the white LED voltage for three IOUT

Ltyp=f(nV )

LED

7.0E-06

9.0E-06

1.1E-05

1.3E-05

1.5E-05

1.7E-05

1.9E-05

2.1E-05

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

15mA

20mA

25mA

11µH

N.V (V)

LED

L typ (H)

Ipmax()

2I

LED NV

LED VIN min()

–VFB ILED RLDS

⋅++⋅()⋅⋅⋅ ⋅

ηFmin 0.8 Ltyp NV

LED VFB ILED RLDS

⋅++⋅()⋅⋅⋅⋅

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 2[]=

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Components selection STLD20D

16/31

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:

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:

7.1.4 Reference selection

The table below gives some coil references suitable for the STLD20D versus L, DCR, Isat

value and sizing requirements.

Figure 20. Maximum peak current (Ip(max)) versus the white LEDs voltages for 3 outputs current

- VIN > 2.8V

10µH

0.2

0.25

0.3

0.35

0.4

0.45

0.5

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

15mA

20mA

25mA

N.V (V)

LED

I (max) (A)

P

Isat Ipmax()

≥

Isat 0.4A≥

Isat ILIM Ipmax()

≥≥

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STLD20D Components selection

17/31

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:

●Low losses to get the best converter efficiency

●Mechanical size

7.3.1 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.

Table 5. Reference selection

Name Ref Height (mm) L typ (µH) DCR (Ω)I

SAT (A)

Murata

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

Coilcraft LP04815-472MXC 1.5 4.7 0.15 0.77

LP04815-103MXC 1.5 10 0.23 0.55

Wurth Elektronik

(WE)

744031100 1.65 10 0.205 0.74

744031150 1.65 15 0.285 0.62

744042100 1.8 10 0.15 1.3

Sumida

CDRH2014-100 1.855 10 0.294 0.7

CLS4D14 1.5 6.8 0.13 0.8

CLS4D14 1.5 10 0.18 0.65

TDK VLF3010AT 100HR49

shielded 10 10 0.67 0.49

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Components selection STLD20D

18/31

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:

7.5 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:

.

RLED

V

FB

ILED

----------- 0.3V

20mA

----------------15Ω 7[]===

Efficiency

P

output

Pinput

-------------------

NV

LED

I

LED

⋅⋅

VIN Iinput

⋅

-------------------------------------------- 8[]==

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STLD20D PWM dimming control

19/31

8 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.

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:

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.

Figure 21. Typical waveform when the PWM dimming is used at 300Hz

δ= 0.38

Vshutdown

ILED

VOUT

IL

ILIM Ireg

VOUT

ILED

Vshutdown

ILED Theo–DC ILED

×=

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Analog dimming control STLD20D

20/31

9 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:

Where:

●Vdim: Analog Dimming Voltage

●Rdim, Rd1: Resistors of the dimming circuit (see figure 26)

9.1 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.

This minimum dimming current depends on the maximum input voltage and the forward

voltage of the LED and can be estimated by:

ILED

V

FB

R

dim

R

d1

R

LED

++

()V

dim

R

LED

R

d1

+

()

⋅–⋅

Rdim RLED

⋅

-------------------------------------------------------------------------------------------------------------------------------------- 19[]=

Figure 22. Analogical dimming schematics

+

-

PWM

R1

C1

STLD20D

C2

R2 Rdim

ILED

Rd1

Vfd

Vref

Ve

Vfd

LDS

GND

RLED

Vdim

ILEDC

DCmin VIN max()

⋅()

2

2L

typ Ftyp NV

LED

⋅VFB VIN max()

–+[]⋅⋅⋅

-------------------------------------------------------------------------------------------------------------------------- 20[

]

≈

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STLD20D Analog dimming control

21/31

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.

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:

Figure 23. ILEDC current vs N.VLED corresponding at the DCMIN

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

91011121314151617181920

N.V (V)

LED

I (A)

LEDC

I = f (

LEDC

V)

LED

RLED

V

FB

ILED

----------- 21[]=

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Analog dimming control STLD20D

22/31

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:

Where:

●Vdimmax is the maximum dimming voltage

●ILEDmin is the expected minimum dimming current

Rdim

R

d1

R

LED

+

[]V

FB

V

dim max–

–

[]

⋅

ILEDmin RLED VFB

–

------------------------------------------------------------------------------------------- 22[]=

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STLD20D Layout recommendation

23/31

10 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

A

GND

VIN

COUT 12

8

D

LSTLD20D

CIN

RSET

SHDN

RLED

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Evaluation board STLD20D

24/31

11 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

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STLD20D Package mechanical data

25/31

12 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

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Package mechanical data STLD20D

26/31

DIM. mm. inch

MIN. TYP MAX. MIN. TYP. MAX.

A 0.80 0.90 1.00 0.032 0.035 0.039

A1 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 0.012

D 3.00 0.118

D2 1.92 2.02 2.12 0.076 0.080 0.084

E 3.00 0.118

E2 1.11 1.21 1.31 0.044 0.048 0.052

e 0.65 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

QFN8 (3x3) MECHANICAL DATA

7517789

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STLD20D Package mechanical data

27/31

DIM.

mm. mils

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

e0.65 25.6

e1 1.95 76.7

L 0.35 0.55 13.7 21.6

SOT23-8L MECHANICAL DATA

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Package mechanical data STLD20D

28/31

DIM.

mm. inch

MIN. TYP MAX. MIN. TYP. MAX.

A 330 12.992

C 12.8 13.2 0.504 0.519

D20.2 0.795

N60 2.362

T 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

Tape & Reel QFNxx/DFNxx (3x3) MECHANICAL DATA

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STLD20D Package mechanical data

29/31

DIM.

mm. inch

MIN. TYP MAX. MIN. TYP. MAX.

A 180 7.086

C 12.8 13.0 13.2 0.504 0.512 0.519

D 20.2 0.795

N 60 2.362

T 14.4 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

Tape & Reel SOT23-xL MECHANICAL DATA

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Revision history STLD20D

30/31

13 Revision history

Table 6. Revision history

Date Revision Changes

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.

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STLD20D

31/31

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