Fairchild FAN5608 service manual

www.fairchildsemi.com

FAN5608

Serial/Parallel LED Driver with Current-Regulated,
Step-Up DC/DC Converter

Features

• Two Independent Channels Drive up to Six LEDs per
Channel

• Adaptive Output Voltage Drive to Maximize Efficiency

• PWM/PFM Mode of Operation of the Boost Circuit

• Up to 85% Efficiency

• Up to 2 × 20mA Output

• Two Built-in DACs for Independent (Digital) Brightness
Control for Both Channels

• LED’s Current Can be Duty-Cycle-Modulated between
0 to 20mA

• Digital, Analog, and PWM Brightness Control

• 2.7V to 5.5V Input Voltage Range

• 0.5MHz Operating Frequency (8MHz internal clock)

• Soft Start

• Low Shutdown Current: I

CC

< 1

A

µ

• LED Short Circuit Protection

• Minimal External Components Needed

• Available in space saving 8-lead and 12-lead MLP
Packages.

Applications

• Cell Phones

• Handheld Computers

• PDAs, DSCs, MP3 Players

• Keyboard Backlights

• LED Displays

Description

The FAN5608 LED driver generates regulated output cur­rents from a battery with input voltage varying between 2.7V
to 5V. An internal NMOS switch drives an external inductor,
and a Schottky diode delivers the inductor’s stored energy to
the load. Proprietary internal circuitry continuously monitors
the currrent on both strings and automatically adjusts the
generated output DC voltage to the lowest minimum value
required by the LEDs string with the highest summarized
forward voltage. This adaptive nature of the FAN5608
ensures operation at the highest possible efficiency. Soft start
circuitry prevents excessive current drawn from the supply
during power on. Any number of LEDs can be connected in
series as long as the summed forward voltages do not exceed
the specified operating output voltage range. Although it is
not required to have an equal number of LEDs connected in
series within each branch, the highest efficiency and best
current regulation is always achieved when an equal number
of LEDs are serially connected.

In the FAN5608 device, two internal two-bit D/A converters
provide independent programmability of each output channel
current. Analog programming of the output current is
also possible in the FAN5608. To do this, ground the "B"
pins and connect a resistor between the "A" pins and a
fixed supply voltage. The output current can then be
programmed to any desired value within its specified range.
The FAN5608DMPX/FAN5608MPX version uses a single

Typical Application

C

IN

2.7V to 5.5V

DAC Input For CH1

DAC Input For CH2

L = 4.7µH

IND

V

IN

A1

B1

A2

B2

4X4mm MLP-12 Package

with internal Schottky diode

Order Code: FAN5608DHMPX

FAN5608DHMPX

IND

V

GND

CH2

CH1

NC

OUT

Digital Brightness Control

C

IN

V

4.7µF

NN

OUT

2.7V to 5.5V

DAC Input For CH1

DAC Input For CH2

L = 4.7µH

IND

V

IN

A1

B1

A2

B2

4X4mm MLP-12 Package

with external Schottky diode

Order Code: FAN5608HMPX

IND

NC

GND

CH2

CH1

FAN5608HMPX

NC

4.7µF

REV. 1.0.4 1/7/05

V

OUT

PRODUCT SPECIFICATION FAN5608

external resistor to set the current, and to turn the device ON
and OFF. The FAN5608DMPX/FAN5608MPX is available
in an 8-lead MLP package with or without an internal Schot-

Typical Application

(Continued)

Analog Brightness Control

C

2.7V TO 5V

V

EXTERNAL

IN

L = 4.7µH

IND

IND

VIN

R

A1

R

A2

FAN5608DMPX

3X3mm MLP-8 Package

with internal Schottky diode

Order Code: FAN5608DMPX

V

OUT

GND

CH2

CH1

4.7µF

Definition of Terms

tky diode. The FAN5608DHMPX is available in a 12-lead
MLP package with an internal Schottky diode.

C

2.7V TO 5V

V

EXTERNAL

IN

IND

L = 4.7µH

IND

VIN

R

A1

R

A2

FAN5608MPX

3X3mm MLP-8 Package

with external Schottky diode

Order Code: FAN5608MPX

NC

GND

CH2

CH1

4.7µF

V

OUT

Output Current Accuracy: reflects the difference between the measured value of the output current (LED) and

programmed value of this current .

I

Output Current Accuracy (%)

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

OUT

measured I

I

OUT

programmed–()100×

OUT

programmed

Current Matching: refers to the absolute value of difference in current between the two LED branches.

Current Matching (%)

branch 1 I

LED

=

----------------------------------------------------------------------------------------------­I

branch 1 I

LED

branch 2–()100×

LED

LED

branch 2+()2⁄

I

Efficiency: is expressed as a ratio between the electrical power into the LEDs and the total power consumed from the input

power supply.

Efficiency (%)

LED

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

branch 1 I

branch 1× V

LED

V

INIIN

branch 1 I

LED

×

branch 1×+()100×

LED

V

Although this definition leads to a lower value than the boost converter efficiency, it more accurately reflects better system per­formance, from the user’s point-of-view.

2

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FAN5608 PRODUCT SPECIFICATION

Pin Assignments

TOP-VIEW

NC

V

1

IN

2

A2

3

CH2

IND

45

A1

8

7

CH1

GND

6

V

NC IND

V

OUT

456

A1 A2

V

IN

1012 11

9

IND

8

CH2

7

B2

V

1

IN

2

A2

3

CH2

OUT

IND

4

8

A1

7

CH1

6

GND

5

NC

GND

CH1

1

2

3

B1

GND

CH1

B1

NC IND

1

2

3

456

V

A1 A2

IN

1012 11

9

IND

8

CH2

7

B2

3x3mm 8-Lead MLP

(Internal Schottky Diode)

FAN5608DMPX/FAN5608MPX

3x3mm 8-Lead MLP

(External Schottky Diode)

4x4mm 12-Lead MLP(QUAD)

(Internal Schottky Diode)

FAN5608DHMPX/FAN5608HMPX

4x4mm 12-Lead MLP(QUAD)

(External Schottky Diode)

Pin Descriptions

Pin No.

FAN5608DHMPX FAN5608HMPX FAN5608DMPX FAN5608MPX

1 GND GND GND GND Ground

2 CH1 CH1 1st LED Cathode

3 B1 B1 DAC B1

4 A1 A1 DAC A1

5V

IN

6 A2 A2 DAC A2

7 B2 B2 DAC B2

8 CH2 CH2 2nd LED Cathode

9 IND IND SD Anode

10 IND IND Inductor

11 V

OUT

12 NC NC No Connection

Pin Name

V

IN

V

IN

Pin Function Description

Input Voltage

A2 A2 DAC A2

CH2 CH2 2nd LED Cathode

IND IND Inductor

V

IN

V

OUT

Input Voltage

Output LEDs Anode

NC No Connection

GND GND Ground

CH1 CH1 1st LED Cathode

A1 A1 DAC A1

Output LEDs Anode

NC No Connection

REV. 1.0.4 1/7/05

3

3

PRODUCT SPECIFICATION FAN5608

Absolute Maximum Ratings

Parameter Min Typ Max Unit

V

, A, B Voltage to GND -0.3 6.5 V

IN

V

to GND -0.3 24 V

OUT

CH1, CH2 Voltage to GND 8 V

Any LED Short Circuit Duration (Anode to Cathode) Indefinite

Lead Soldering Temperature (10 seconds) 300 °C

Thermal Resistance θ

jc

8 °C/W

Operating Junction Temperature Range 150 °C

Storage Temperature Range -55 150 °C

Electrostatic Discharge (ESD) Protection (Note 1, 2) HBM 4 kV

CDM 1

DC Electrical Characteristics

= 2.7V to 5.5V, T

(V

IN

operating temperature.)

= 25°C, unless otherwise noted. Boldface values indicate specifications over the ambient

A

Parameter Conditions Min. Typ. Max. Units

Output Current Accuracy A = HIGH,

0.9 × I

NOM

I

= 20 1.1 × I

NOM

B = HIGH

Channel to Channel Current Matching A = HIGH,

B = HIGH

Efficiency (AVG) V

> 3.0V 80 %

IN

Switching Frequency 0.5 MHz

Multiplication Ratio FAN5608DMPX/

900 1000 1100

FAN5608MPX

FAN5608DHMPX/

850 1000 1150

FAN5608HMPX

Supply Current in OFF mode V

Input A1, A2 Threshold Digital

Mode

= V

= 0V 0.1

A

B

High V

-0.7 V

IN

Low 0 0.6

Analog Mode 1.2

Input B Threshold Digital Mode High 0.6 × V

IN

Low 0 0.3 × V

Input A1, A2 Current Digital Mode V

= V

A

IN

V

= 0 0.1 µA

A

50 60 µA

Input B1, B2 Current Digital Mode 0.1 µA

mA

NOM

%

A

µ

IN

V

IN

V

V

IN

Recommended Operating Conditions

Parameter Min Typ Max Unit

Input Voltage Range 2.7 5.5 V

Operating Ambient Temperature Range -40 25 85 °C

Output Voltage Range V

Notes:

1. Using Mil Std. 883E, method 3015.7(Human Body Model) and EIA/JESD22C101-A (Charge Device Model).

2. Avoid positive polarity ESD stress at the cathode of the internal Schottky diode.

IN

4

18 V

REV. 1.0.4 1/7/05

FAN5608 PRODUCT SPECIFICATION

Block Diagram

V

IN

B1

A1

B2

A2

OSC

START-UP

DAC1

BG

DAC2

GND

Note: In the 8-pin version (analog version only), pins B1 and B2 are omitted.

REF

POWER GOOD

DBB

REF CH1

REF CH2

COIL DRIVER

LINEAR REGULATOR

LINEAR REGULATOR

W_OR

IND

V

OUT

IND

CH1

CH2

Circuit Description

When the input voltage is connected to V
turned on, the bandgap reference acquires its nominal volt­age and the soft-start cycle begins. Once “power good” is
achieved (0.5mA in the diodes), the soft-start cycle stops and
the boost voltage increases to generate the desired current
selected by the input control pins. If the second channel is
not selected, its output will go high to about V
diodes are turned off.

The FAN5608 DC/DC converter automatically adjusts its
internal duty cycle to achieve high efficiency. It provides
tightly regulated output currents for the LEDs. An internal
circuit determines which LED string requires the highest
voltage in order to sustain the pre-set current levels, and
adjusts the boost regulator accordingly.

pin, the system is

IN

, and the

IN

To maintain the regulated current at the selected value, the
difference in the number of LEDs between branches should
not exceed one. If only one branch is used, another branch
should be disabled, connecting the corresponding DAC
inputs to low. If the output external capacitor is shorted, the
Schottky diode can be damaged, therefore such a condition
should be avoided.

LED Brightness Control

The control inputs are A1, B1 for CH1 and A2, B2 for CH2.
B1 and B2 are digital inputs, thus they require LOW (GND)
and HIGH (V
are connected to an external stable voltage source via an
external resistor, and B1 and B2 inputs are connected to
ground. The current flowing through the resistor is scaled by
a factor of approximately 1000.

) control signals. In analog mode, A1 and A2

CC

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5

PRODUCT SPECIFICATION FAN5608

Digital Control

The FAN5608’s digital decoder allows selection of the fol­lowing modes of operation: OFF, 5mA, 10mA, 20mA per
branch.

A0101

B0011

I

LED

OFF 5mA 10mA 20mA

Analog Control

Inputs A1 and A2 are used to control the LED currents.
Inputs B1 and B2 should be connected to GND (logic level
“0”). An external resistor (R) is connected from A1 and/or
A2 to a stable voltage source (V
current, I

LED

. The I

can be determined using the formula

LED

) to control the LED

External

and the graph below:

Where V

25

20

15

(mA)

LED

10

I

5

I

LED

Ref

V



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



= 1.22V, V

R=10Kohm

–

ExternalVRef

R

< V

Ref

External

Multiplication Ratio×=

< (V

R=100kohm

– 0.7V)

IN

PWM Control in Analog Mode

The logic level HIGH, VH and logic level LOW, VL of the
PWM signal should be:

V

Ref

< VH < (V

– 0.7V) and 0 < VL < 0.6V

IN

The frequency of the PWM signal should be within 50Hz
to 1kHz range. The VH sets the maximum LED current
while the duty cycle sets the average current between 0 and
I

LEDmax

.

If the analog inputs A1 and/or A2 are driven in digital mode
by an open drain output, it is important to choose the appro­priate value of the pull-up resistor. Its resistance should be
low enough to ensure less than 0.7V dropout, hence V

>

A

(VIN – 0.7V) as required for HIGH logic level:

pull up–

< 11.66k Ω=

60µA

R

700mV

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

Open-Circuit Protection

A built-in over voltage protection circuit prevents the device
from being damaged when it is powered up with no load.
This circuit reduces the boost converter duty cycle, to a
minimum thus limiting the output voltage to a safe value
when no load condition is detected. If one of the two enable
branches is accidentally disconnected, the converter contin­ues the operation, however, the current in the remaining
branch is no longer regulated and the actual branch current
will be determined by the input voltage, the inductor value
and the switching frequency.

0

1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25 3.5

V

(V)

External

PWM Control in Digital Mode

The logic level HIGH, VH and logic level LOW, VL of the
PWM signal should be:

(V

– 0.7V) < VH < V

IN

and 0 < VL < 0.6V

IN

The frequency of the PWM signal should be within 50Hz to
1kHz range; it can go up to 30kHz at any input if the other
input is kept HIGH. In the case of FAN5608MPX and
FAN5608DMPX, the B1 and B2 inputs are internally con­nected to GND and the PWM signal can be applied to A1
and A2 inputs only. Consequently, the maximum LED
current, for 100% duty cycle, is 5mA on each channel.

However, the FAN5608 can be damaged when a full load
(more than six LEDs, driven by 20mA) is suddenly discon­nected from V

. To protect the FAN5608 against this

OUT

unlikely event, an external 24V Zener diode can be con­nected between V

and GND.

OUT

Shutdown Mode

Each branch can be independently disabled by applying
LOW logic level voltage to the A and B inputs. When both
branches are disabled, the FAN5608 enters Shutdown mode
and the supply current is reduced to less than 1µA.

6

REV. 1.0.4 1/7/05