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PDF STLD20D Data sheet ( Hoja de datos )
| Número de pieza | STLD20D | |
| Descripción | WHITE LED POWER SUPPLY | |
| Fabricantes | ST Microelectronics | |
| Logotipo |  |
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® STLD20D
ASD™
FEATURES
■ High efficiency above 80%
■ Can drive up to 4 LEDs in series from 2.8V
supply
■ Constant LED current regulation
■ Integrated LED disconnect switch that cuts the
LEDs branch in shutdown mode
■ Constant switching frequency
■ Stable current regulation across the total input
voltage range
■ Supply voltage rejection
■ Inherent soft start by limiting the peak inductor
current
■ Peak inductor current adjustability (STLD20D-
C8 only)
■ Shutdown pin with possibility of PWM dimming
control
■ Over voltage and over temperature protection
with automatic restart
■ Low shutdown current < 1µA
■ Small external inductor (10µH, height < 2mm)
■ Tiny ceramic external capacitor
■ Can be supplied by a Li-ion battery - VIN range:
2.5Vdc to 5Vdc
WHITE LED POWER SUPPLY
PRELIMINARY DATA
APPLICATION
■ White Led supply for LCD backlight
■ Mobile phone
■ PDA and organizers
■ Any handsets powered from 2.8V to 4.2V
DESCRIPTION
The STLD20D is a constant switching frequency
boost regulator with specific features to supply up
to 4 white LEDs in series. A stable LED current
regulation, from 2.8V to 4.2V, is achieved by sens-
ing the LED current through a low ohmic shunt re-
sistor RLED (see figure 1). The device also
includes a supply rejection circuit that prevent any
kind of flickering effect on the display during dy-
namic supply voltage variation. A LED disconnect
switch cut the LED branch to reduce the current
consumption in shutdown mode. The maximum
peak inductor current can be programmed. The
STLD20D includes often numerous features and in-
novative design circuit that allows getting an efficien-
cy above 80% across the total supply voltage range.
Figure 1: Basic connection
VBATT
L = 10µH
D
Cin = 2.2µF
Cout = 1µF
SW
VOUT
RSET (*)
RSET
VIN
STLD20D
LDS
ILED = 20mA
SHDN
(*) STLD20D-C8 only
GND
FB
D: STM BAT20J
RLED
L: Murata LQH32CN100K33
Table 1: Order Codes
Part Number
STLD20D-C8
STLD20D-DEF
Marking
L2D
L2D
Package
SOT23-8L
QFN 3x3 8L
Figure 2: Efficiency versus input voltage
(ILED=20mA; Tamb=25°C)
Efficiency (%)
88
87
86
85
84
83
82
81
80
79
78
2.5 3 3.5 4
Input voltage (Vdc)
4.5
5
October 2004
REV. 2
1/8
This is preliminary information on a new product now in development or undergoing evaluation. Details are subject to change without notice.
1 page  
STLD20D
FUNCTIONAL DESCRIPTION
Figure 6: LED current versus input voltage
1. BOOST CONTROLLER:
The STLD20D is a PWM mode control boost con-
verter operating at 500kHz in discontinuous mode.
An automatic compensation of the oscillation ramp
allows rejection of the battery voltage transient. The
LED constant regulation (referred figure 4 ) is done
by sensing the LED current through the resistance
RLED (figure 1). The voltage across RLED is used by
the feedback loop of the controller (pin FB).
ILED(mA)
21.00
20.80
20.60
20.40
20.20
20.00
19.80
19.60
19.40
19.20
19.00
2.5
3 3.5
4 4.5
5
2. PEAK INDUCTOR CURRENT LIMITATION
VIN(V)
AND SOFT START FUNCTION:
An integrated current sensor senses the peak drain current of the switch SW in order to keep the inductor
current below its saturation level. Since the peak drain current exceeds 590mA (if RSET = GND for
STLD20D-C8), the RS flip flop turns off the switch SW. During start up, this peak drain current limitation
acts inherently like a soft start function .
3. PEAK INDUCTOR CURRENT ADJUSTABILITY
(STLD20D-C8 ONLY)
The peak current of the boost inductor should always
be below the saturation current. In order to provide
flexibility in the selection of the inductor, the maxi-
mum peak inductor current can be adjusted by con-
necting a resistor at the pin RSET. The figure 5 gives
the value of the resistance RSET versus the peak in-
ductor current limit ILMAX at 25°C. If a low ripple is es-
pected on the battery voltage bus, then the
maximum peak inductor current should be reduced.
Figure 7: RSET (kΩ) versus ILMAX (A)
RSET(kΩ)
RSET = f(ILmax)
120
VIN=3.2V
100 VIN=4.2V
80
60
40
20
0
0.3
0.4 0.5 0.6
ILmax(A)
0.7
4. SHUTDOWN:
The SHDN pin is a low logic input signal and allows turning off the controller without cutting the input volt-
age from the boost regulator circuit. An integrated LED 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. BRIGHTNESS CONTROL:
The brightness of the Led is adjusted by pulsing the shutdown pin with a low frequency PWM signal.
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 to regulate 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.
6. OTP:
An integrated temperature sensor senses the temperature of the junction of the controller. when this tem-
perature exceeds 110°C min fixed internally the controller is automatically turned OFF . When the tem-
perature is reduced the operation of the device automatically recovers.
5/8
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| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet STLD20D.PDF ] | |
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