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PDF ISL97631 Data sheet ( Hoja de datos )
| Número de pieza | ISL97631 | |
| Descripción | White LED Boost Regulator | |
| Fabricantes | Intersil Corporation | |
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Data Sheet
December 21, 2005
ISL97631
FN7370.1
White LED Boost Regulator With
Integrated Schottky Diode
The ISL97631 represents a high efficiency, constant
frequency PWM regulator with integrated boost diode and
FET. Designed for use in white LED driving applications, the
ISL97631 features efficiencies up to 85%. It operates at
1.35MHz switching frequency and operates from an input
voltage of between 2.7V and 5.5V. The maximum output
voltage of 27V enables the ISL97631 to drive up to 6 LEDs in
series. It is also possible to use the ISL97631 to drive LEDs
in series/parallel combination for applications requiring up to
15 LEDs.
Available in the 6 Ld TSOT package, the ISL97631 offers
high efficiency, constant frequency operation. It is specified
for operation over the -40°C to +85°C ambient temperature
range.
Pinout
ISL97631
(6 LD TSOT)
TOP VIEW
VOUT 1
GND 2
LX 3
6 VIN
5 FB
4 ENAB
Features
• Up to 6 LEDs in series
• 27V maximum output
• 2.7V to 5.5V input
• Up to 85% efficient
• 1.35MHz constant frequency
• Enable/PWM dimming control
• Pb-free plus anneal available (RoHS compliant)
Applications
• LED backlighting
• Cell phones
• PDAs
• Handheld games
• MP3 players
• GPS
• Other handheld devices
Ordering Information
PART NUMBER
PART
MARKING
PCS.
TAPE &
REEL
PACKAGE
(TAPE AND REEL)
PKG. DWG. #
ISL97631IHTZ-T7 (See Note)
631Z
3,000
7” 6 Ld TSOT (Pb-free)
MDP0049
ISL97631IHTZ-T7A (See Note)
631Z
270 7” 6 Ld TSOT (Pb-free)
MDP0049
NOTE: Intersil Pb-free plus anneal products employ special Pb-free material sets; molding compounds/die attach materials and 100% matte tin plate
termination finish, which are RoHS compliant and compatible with both SnPb and Pb-free soldering operations. Intersil Pb-free products are MSL
classified at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J STD-020.
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2005. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
1 page  
ISL97631
The magnitude of the PWM signal should be higher than the
minimum ENAB voltage high. The bench PWM dimming test
results are shown in Figure 8. In the test, two PWM
frequencies 400Hz and 1kHz are chosen to compare the
linear dimming range. It is clear that there is a wider linear
dimming range for the lower PWM frequency than for the
higher one, due to the self discharge of the output capacitor
through the LEDs during the low ENAB periods. To achieve
a better linearity with high frequencies an NMOS FET can be
placed between the FB pin and the LED stack, with its gate
driven by the same signal as ENAB. This acts to prevent self
discharge of the output capacitor during the off periods. In
the PWM dimming test, the output capacitor is 0.22µF.
20
18
16
14
12
10
8
6
4
2
0
0
1kHz
400Hz
20 40 60 80
DUTY-CYCLE (%)
100
FIGURE 8. PWM DIMMING LINEAR RANGE (FOR 400Hz AND
1kHz PWM FREQUENCIES CONDITION,
COUT = 0.22µF)
ANALOG DIMMING
The second dimming method applies a variable DC voltage
(VDim) at FB pin (see Figure 9) to adjust the LED current. As
the DC dimming signal voltage increases above VFB, the
voltages drop on R1 and R2 increase and the voltage drop
on RSET decreases. Thus, the LED current decreases.
ILED = V-----F---B-----⋅---(---R----1-R----+-2----R⋅---R-2----S)---–E----T-V----D----i--m-----⋅---R-----1-
(EQ. 3)
The DC dimming signal voltage can be a variable DC voltage
or a DC voltage generated by filtering a high frequency PWM
control signal.
As brightness is directly proportional to LED currents, VDim
may be calculated for any desired “relative brightness” (F)
using Equation 4.
VDim
=
RR-----21-
⋅
VFB
⋅
1
+
RR-----12-
–
F
(EQ. 4)
Where F = ILED (dimmed)/ILED (undimmed).
These equations are valid for values of R1 and R2 such that
both R1>>RSET and R2>>RSET.
VIN
2.7V~5.5V
C1
1µF
OFF/ON
L1
22µH
VIN LX
VOUT
ISL97631
ENAB FB
GND
R2
LEDs
R1
3.3k
C2
0.22µF
RSET
4.75Ω
VDim
FIGURE 9. ANALOG DIMMING CONTROL APPLICATION
CIRCUIT
The analog dimming circuit can be tailored to a desired
relative brightness for different VDim ranges using
Equation 5.
R2 = -[--(--V-----D[--V--i-m-F----B_---m--•--a--(-x--1---–--–--V--F--F--m--B---i-)n---•-)--]--R----1----]
(EQ. 5)
Where VDim_max is the maximum VDim voltage and Fmin is
the minimum relative brightness (i.e., the brightness with
VDim_max applied).
i.e., VDim_max = 5V, Fmin = 10% (i.e., 0.1), R2 = 189k
i.e., VDim_max = 1V, Fmin = 10% (i.e., 0.1), R2 = 35k
Open-Voltage Protection
In some applications, it is possible that the output is
opened, e.g. when the LEDs are disconnected from the
circuit or the LEDs fail. In this case the feedback voltage
will be zero. The ISL97631 will then switch to a high duty
cycle resulting in a high output voltage, which may cause
the LX pin voltage to exceed its maximum 27V rating. To
implement overvoltage protection, a zener diode Dz and a
resistor R1 can be used at the output and FB pin to limit the
voltage on the LX pin as shown in Figure 10. It is clear that
as the zener is turned on, due to the overvoltage, the zener
diode’s current will set up a voltage on R1 and RSET and this
voltage is applied on FB pin as the feedback node. This
feedback will prevent the output from reaching the
overvoltage condition. In the overvoltage protection circuit
design, the zener voltage should be larger than the
maximum forward voltage of the LED string.
5 FN7370.1
December 21, 2005
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| PDF Descargar | [ Datasheet ISL97631.PDF ] | |
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