intersil ISL97631 DATA SHEET

®

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ISL97631

Data Sheet December 21, 2005

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

6

VIN

FN7370.1

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

GND

LX

2

3

5FB

ENAB

4

Ordering Information

PAR T

PAR T N UMBE R

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.

MARKING PCS.

TAPE &

REEL

PACKAGE

(TAPE AND REEL) PKG. DWG. #

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.

All other trademarks mentioned are the property of their respective owners.

Copyright Intersil Americas Inc. 2005. All Rights Reserved

ISL97631

www.BDTIC.com/Intersil

Absolute Maximum Ratings (T

Input Voltage (VIN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6V

LX, Vout Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-0.3V to +27V

FB Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6V

ENAB Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6V

Operating Temperature . . . . . . . . . . . . . . . . . . . . . . .-40°C to +85°C

= 25°C) Thermal Information

A

Thermal Resistance (Typical, Note 1) θJA (°C/W)

6 Ld TSOT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

Maximum Junction Temperature (Plastic Package . . . . . . . . . 150°C

Maximum Storage Temperature Range. . . . . . . . . .-65°C to +150°C

Maximum Lead Temperature (Soldering, 10s). . . . . . . . . . . . +300°C

(TSOT - Lead Tips Only)

CAUTION: Stresses above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the
device at these or any other conditions above those indicated in the operational sections of this specification is not implied.

IMPORTANT NOTE: All parameters having Min/Max specifications are guaranteed over the specified temperature range. All parameters are based on pulsed tests,
therefore: T

= TC = T

J

A

NOTE:

1. θ

is measured with the component mounted on a high effective thermal conductivity test board in free air. See Tech Brief TB379 for details.

JA

Electrical Specifications V

= 3V, V

IN

= 3V, TA = -40°C to 85°C unless otherwise specified.

ENAB

PARAMETER DESCRIPTION CONDITION MIN TYP MAX UNIT

V

IN-MIN

V

IN-MAX

V

I

FB

I

FB

IN

Minimum Operating Voltage V

Maximum Operating Voltage V

Feedback Voltage 80 100 120 mV

FB Pin Bias Current 100 nA

Supply Current ENAB = 3V, output not switching 0.6 1.0 mA

OUT

OUT

= 16V, I

= 25V, I

= 20mA 2.7 V

LED

= 20mA 5.5 V

LED

ENAB = 0V 1 µA

F

D

OSC

MAX

Switching Frequency 0.8 1.35 1.8 MHz

Maximum Duty Cycle TA = 25°C 85 90 %

82 90 %

I

LIM

Switch Current Limit TA = 25°C 280 350 mA

250 350 mA

r

DS(ON)

I

LEAK(LX)

I

LEAK(VOUT)

V

DIODE

V

ENAB-HI

V

ENAB-LO

I

ENAB

/∆V

∆I

LED

Eff Efficiency I

Switch On Resistance ILX = 100mA 750 mΩ

Switch Leakage Current VLX = 27V, Vout = 27V 0.01 1 µA

Diode Leakage Current V

LX-V

Diode Forward Voltage I

OUT

= 27V 0.01 1 µA

OUT

= 100mA, TA = 25°C 0.75 0.9 V

DIODE

ENAB Voltage High 2.5 V

ENAB Voltage Low 0.6 V

ENAB Pin Bias Current 1µA

Line Regulation VIN = 2.7V to 5V 0.2 %/V

IN

= 20mA, 3 LEDs 85 %

LED

2

FN7370.1

December 21, 2005

Typical Application

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L1

22µH

V

IN

2.7V~5.5V

C1

1µF

OFF/ON

V

IN

ISL97631

ENAB

V

OUT

GND

LX

FB

R

4.75Ω

FIGURE 1. TYPICAL APPLICATION CIRCUIT FIGURE 2. EFFICIENCY vs LED CURRENT (VIN = 4V)

Typical Performance Curves

SET

LEDs

C2

0.22µF

ISL97631

90

22µH, V

85

80

75

70

65

EFFICIENCY (%)

60

55

50

0 5 10 15 20 25 30

IN

= 4V

I

LED

(mA)

700
600
500
400
300

Iq (µA)

200
100

0

024

VIN (V)

6

FIGURE 3. QUIESCENT CURRENT vs VIN (ENAB = hi)

18.33

18.32

18.31

18.30

18.29

18.28

(mA)

O

I

18.27

18.26

18.25

18.24

18.23

2.5 3.5 4.5 5.5
(V)

V

IN

FIGURE 5. LINE REGULATION FIGURE 6. SWITCHING FREQUENCY vs TEMPERATURE

18.244

18.242

18.240

18.238

18.236

(mA)

O

I

18.234

18.232

18.230

18.228
0 5 10 15 20

(V)

V

OUT

FIGURE 4. LOAD REGULATION (V

1.37

1.36

1.35

1.34

1.33

1.32

1.31

1.30

FREQUENCY (MHz)

1.29

1.28

1.27

-60 -40 -20 0 20 40 60 80 100
TEMPERATURE (°C)

IN

= 4V)

3

FN7370.1

December 21, 2005

Block Diagram

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Vin Enable LX

Vin Enable LX

1.35MHz

1.2MHz Oscillator and Ramp

1.2MHz Oscillator and Ramp
Generator

Generator

PWM

PWM

Comparator

Comparator

GM Amp

GM Amp
Compensation

Compensation

ISL97631

PWM Logic

PWM Logic

Controller

Controller

GM

GM

Amplifier

Amplifier

ISL97631

ISL97631

FET

FET

Driver

Driver

Current

Current

Sense

Sense

-

-

95mV

95mV

Bandgap

Bandgap

Reference

Reference
Generator

Generator

Vout

Vout

GND

GND

FB

FB

FIGURE 7. ISL97631 BLOCK DIAGRAM

Pin Descriptions

PIN

NUMBER

1 VOUT Output Pin. Connect to the anode of the top

2 GND Ground Pin. Connect to local ground.

3 LX Switching Pin. Connect to inductor.

4 ENAB Enable Pin. Connect to enable signal to turn-on

5 FB Feedback Pin. Connect to the cathode of

6 VIN Input Supply Pin. Connect to the input supply

PIN

NAME DESCRIPTION

LED and the output filter capacitor .

or off the device.

bottom LED and the sense resistor.

voltage, the inductor and the input supply
decoupling capacitor.

Detailed Description

The ISL97631 uses a constant frequency, current mode
control scheme to provide excellent line and load
regulation. It can drive up to 6 LEDs in series or 15 LEDs in
parallel/series configuration, with efficiencies of up 85%.
The ISL97631 operates from an input voltage of 2.7V to

5.5V and can boost up to 27V.

Steady-State Operation

The ISL97631 operates with constant frequency PWM. The
switching frequency is around 1.35MHz. Depending on the
input voltage, inductance, number of LEDs and the LED

current, the converter operates in either continuous
conduction mode or discontinuous conduction mode. Both
are normal. The forward current of the LED is set using the
R

resistor. In steady state mode, this current is given by

SET

the equation:

V

I

LED

---------------=

R

FB

SET

(EQ. 1)

Shut-Down

When taken low the ENAB pin places the ISL97631 into
power down mode. When in power down, the supply current
is reduced to less than 1µA.

Dimming Control

PWM DIMMING

The ENAB pin also doubles as a brightness control. There
are two different possible dimming control methods. The
first dimming method is controlled through the duty-cycle of
the ENAB input PWM waveform, which can operate at
frequencies up to 1kHz. For frequencies greater than 1kHz,
see Analog Dimming. The LEDs operate at either zero or
full current. This is the PWM dimming control method. The
relationship between the average LED current and the
duty-cycle (D) of the ENAB pin’s waveform is as follows:

V

FB

average I

LED

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

R

SET

D⋅=

(EQ. 2)

4

FN7370.1

December 21, 2005

ISL97631

www.BDTIC.com/Intersil

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

(mA)

O

I

8
6
4
2
0

020406080100

FIGURE 8. PWM DIMMING LINEAR RANGE (FOR 400Hz AND

1kHz PWM FREQUENCIES CONDITION,
C

OUT

1kHz

400Hz

DUTY-CYCLE (%)

= 0.22µF)

ANALOG DIMMING

The second dimming method applies a variable DC voltage
(V

) at FB pin (see Figure 9) to adjust the LED current. As

Dim

the DC dimming signal voltage increases above V
voltages drop on R
on R

I

LED

decreases. Thus, the LED current decreases.

SET

VFBR1R2+()V

------------------------------------------------------------ --------------= (EQ. 3)

and R2 increase and the voltage drop

1

⋅–⋅

R⋅

DimR1

SET

R

2

FB

, the

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, V

Dim

may be calculated for any desired “relative brightness” (F)
using Equation 4.

V

Dim

R

-------

R

2

⋅⋅=

V

FB

1

R



1

1

------- F–+



R



2

(EQ. 4)

L1

22µH

V

IN

2.7V~5.5V

C1

1µF

OFF/ON

FIGURE 9. ANALOG DIMMING CONTROL APPLICATION

CIRCUIT

V

LX

LX

IN

V

OUT

ISL97631

ENAB FB

GND

R2

R1

3.3k

LEDs

C2

0.22µF

R

SET

4.75Ω

V

Dim

The analog dimming circuit can be tailored to a desired
relative brightness for different V

ranges using

Dim

Equation 5.

R

-------------------------------------------------------------- -----=

2

Where V

V

Dim_maxVFB

V

Dim_max

–()R1•[]

1F

–()•[]

FB

min

is the maximum V

voltage and F

Dim

(EQ. 5)

is

min

the minimum relative brightness (i.e., the brightness with
V

Dim_max

i.e., V
i.e., V

applied).

Dim_max

Dim_max

= 5V, F
= 1V, F

= 10% (i.e., 0.1), R2 = 189k

min

= 10% (i.e., 0.1), R2 = 35k

min

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 R
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 R
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.

can be used at the output and FB pin to limit the

1

and R

1

SET

and this

Where F = I

(dimmed)/I

LED

(undimmed).

LED

These equations are valid for values of R1 and R2 such that
both R1>>RSET and R2>>RSET.

5

FN7370.1

December 21, 2005

ISL97631

www.BDTIC.com/Intersil

L1

V

IN

2.7V~5.5V

1µF

OFF/ON

C1

22µH

V

IN

ISL97631

ENAB

V

OUT

GND

LX

FB

Dz

R1

R

SET

4.75Ω

LEDs

C2

0.22µF

FIGURE 10. LED DRIVER WITH OVERVOLTAGE

PROTECTION CIRCUIT

Components Selection

The input capacitance is typically 0.22µF. The output
capacitor should be in the range of 0.22µF to 1µF. X5R or
X7R type of ceramic capacitors of the appropriate voltage
rating are recommended. The output capacitor value affects
PWM dimming performance. Lower output capacitor values
increase the range of PWM dimming. However, the ripple
voltage will be greater for lower values.

When choosing an inductor, make sure the average and
peak current ratings are adequate by using the following
formulas (80% efficiency assumed):

I

⋅

LEDVOUT

I

LAVG

I

LPKILAVG

∆

I

Where:

•

• L is the inductance in H.

•f

---------------------------------=

⋅

0.8 V

IN

1

-- -

∆⋅+=

I

L

2

V

INVOUTVIN

---------------------------------------------------=

L

LV

⋅⋅

∆I

is the peak-to-peak inductor current ripple in Amps

L

is the switching frequency, typically 1.35MHz

OSC

–()⋅

OUTfOSC

(EQ. 6)

(EQ. 7)

(EQ. 8)

90%

22µH, V

85%

80%

2LED

75%

70%

65%

EFFICIENCY (%)

60%

55%

50%

0 5 10 15 20 25 30

3LED

IN

= 4V

6LED

4LED

5LED

I

LED

(mA)

FIGURE 11. EFFICIENCY CURVE WITH 2, 3, 4, 5 AND 6 LEDs

LOAD

White LED Connections

One leg of LEDs connected in series will ensure brightness
uniformity. The 27V maximum output voltage specification
enables up to 6 LEDs to be placed in series.

In order to drive more LEDs, series/parallel connections are
used. A current mirror circuit (as shown in Figure 12) should
be used to balance LED currents.

Parallel strings of LEDs may draw significantly different
currents due to manufacturing and temperature differences.
For correct operation of the current mirror circuit, the total
voltage between FB pin and the top of the primary LED
string must be equal to or greater than the other strings. To
ensure this, a small value resistor may be inserted between
FB pin and the primary LED string.

D1

D1

L1

L1

V

IN

2.7V~5.5V

C1

OFF/ON

VDD

VIN

LX

LX

ISL97631

EL7630

VOUT

ENAB FB

ENAB FB

GND

GND

C2

LEDs

R

SET

The ISL97631 supports a wide range of inductance values
(22µH~82µH). For lower inductor values or lighter loads, the
boost inductor current may become discontinuous. For high
boost inductor values, the boost inductor current will be in

FIGURE 12. LEDs IN SERIES/PARALLEL WITH MIRROR

continuous mode.

The demo board efficiency bench test results are shown in
Figure 11. The input voltage is 4V and curves are shown for
2, 3, 4, 5 and 6 LEDs (boost inductor L = 22µH).

6

CURRENT BALANCE

FN7370.1

December 21, 2005

PCB Layout Considerations

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The layout is very important for the converter to function
properly. R
and GND pins. Longer traces to the LEDs are OK. Similarly,
the supply decoupling cap and the output filter cap should be
as close as possible to the V

The heat of the IC is mainly dissipated through the GND pin.
Maximizing the copper area connected to this pin is
preferable. In addition, a solid ground plane is always helpful
for the EMI performance.

The demo board is a good example of layout based on the
principle. Please refer to the ISL97631 Application Brief for
the layout.

must be located as close as possible to the FB

SET

and V

IN

OUT

pins.

ISL97631

7

FN7370.1

December 21, 2005

Package Outline

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ISL97631

All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems.

Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality

Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without
notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and
reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.

For information regarding Intersil Corporation and its products, see www.intersil.com

8

FN7370.1

December 21, 2005