Analog Devices ADM8845 Datasheet

Charge Pump Driver for LCD

FEATURES

Drives 6 LEDs from 2.6 V to 5.5 V (Li-Ion) input supply
1×/1.5×/2× fractional charge pump to maximize power

efficiency
1% max LED current matching
Up to 88% power efficiency over Li-Ion range
Powers main and sub display LEDs with individual shutdown
Package footprint only 9 mm
Package height only 0.9 mm
Low power shutdown mode
Shutdown function
Soft-start limiting in-rush current

APPLICATIONS

Cellular phones with main and sub displays
White LED backlighting
Camera flash/strobes and movie lights
Micro TFT color displays
DSC
PDAs

2

(3 mm × 3 mm)

C1

1µF

FUNCTIONAL BLOCK DIAGRAM

C2

1µF

White LED Backlights

ADM8845

GENERAL DESCRIPTION

The ADM8845 uses charge pump technology to provide the
power required to drive up to six LEDs. The LEDs are used for
backlighting a color LCD display, having regulated constant
current for uniform brightness intensity. The main display can
have up to four LEDs, and the sub display can have one or two
LEDs. The digital CTRL1 and CTRL2 input control pins control
the shutdown operation and the brightness of the main and sub
displays.

To maximize power efficiency, the charge pump can operate in
either a 1×, 1.5×, or 2× mode. The charge pump automatically
switches between 1×/1.5×/2× modes based, on the input voltage,
to maintain sufficient drive for the LED anodes at the highest
power efficiency.

Improved brightness matching of the LEDs is achieved by a
feedback pin to sense individual LED current with a maximum
matching accuracy of 1%.

V

ADM8845

CC

C4

4.7µF

CTRL1

CTRL2

R

SET

Rev. 0

Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.

I

SET

CONTROL

LOGIC

LED
CURRENT
CONTROL

CIRCUIT

CHARGE PUMP

1×/1.5×/2× MODE

OSC

V

REF

CURRENT

CURRENT

CURRENT

CONTROL 1

CONTROL 2

CURRENT CONTROLLED SINKS

GND

CONTROL 3

CURRENT

CONTROL 4

CURRENT

CONTROL 5

Figure 1.

V

OUT

CURRENT

CONTROL 6

FB1
FB2
FB3
FB4
FB5
FB6

C3

2.2µF

MAIN SUB

04867-0-001

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone]
Fax: [phone] © 2004 Analog Devices, Inc. All rights reserved.

www.analog.com

ADM8845

TABLE OF CONTENTS

Specifications......................................................................................3

Absolute Maximum Ratings.............................................................4

Thermal Characteristics .............................................................. 4

ESD Caution.................................................................................. 4

Pin Configuration and Function Description ...............................5

Typical Performance Characteristics ..............................................6

Theory of Operation ...................................................................... 10

Output Current Capability........................................................ 11

Automatic Gain Control ............................................................ 11

Current Matching ....................................................................... 11

Brightness Control with a Digital PWM Signal..................... 11

LED Brightness Control Using a PWM Signal Applied to

............................................................................................. 13

V

PWM

REVISION HISTORY

LED Brightness Control Using a DC Voltage Applied to

.......................................................................................... 13

V

BRIGHT

Applications......................................................................................14

Layout Considerations and Noise ............................................ 14

White LED Shorting .................................................................. 14

Driving Fewer than Six LEDs................................................... 14

Driving Flash LEDs.................................................................... 15

Driving Camera Light, Main, and Sub LEDs.......................... 15

Driving Four Backlight White LEDs and Flash LEDs........... 16

Power Efficiency......................................................................... 17

Outline Dimensions ........................................................................18

Ordering Guide .......................................................................... 18

10/04—Revision 0: Initial Version

Rev. 0 | Page 2 of 20

ADM8845

SPECIFICATIONS

VCC = 2.6 V to 5.5 V; T

= −40°C to +85°C, unless otherwise noted; C1,C2 = 1.0 µF; C3 = 2.2 µF; C4 = 4.7 µF

A

Table 1.

Parameter Min Typ Max Unit Test Conditions

INPUT VOLTAGE, VCC 2.6 5.5 V
SUPPLY CURRENT, ICC 2.6 5 mA

All six LEDs disabled, V

= 3.3 V, R

CC

= 7.08 kΩ

SET

CTRL1 = 1, CRTL2 = 1
SHUTDOWN CURRENT 5 µA
CHARGE PUMP FREQUENCY 1.5 MHz
CHARGE PUMP MODE THRESHOLDS

1.5× to 2× 3.33 V
Accuracy 4 %

2× to 1.5× 3.36 V

Accuracy 4 %
Hysteresis 40 mV

1× to 1.5× 4.77 V

Accuracy 4 %

1.5× to 1× 4.81 V
Accuracy 4 %
Hysteresis 40 mV

I

PIN

SET

LED : LED Matching −1 +1 % I
LED : I

I
I

Accuracy −1 +1 %

SET

Pin Voltage 1.18 V

SET

to I

LED

Ratio 120

SET

MIN COMPLIANCE ON FBx PIN 0.2 0.3 V I

= 20 mA, VFB =0.4 V

LED

= 20 mA, R

I

LED

T

= 25°C

A

= 20 mA

SET

SET

= 7.08 kΩ, VFB = 0.4 V, VCC = 3.6 V,

Charge Pump Output Resistance 1.2 1.8 Ω 1× mode

3.5 5.1 Ω 1.5× mode

8.0 14 Ω 2× mode

LED Current 30 mA See Note 1 and Figure 21
PWM 0.1 200 kHz
DIGITAL INPUTS

Input High 0.5 VCC V

Input Low 0.3 VCC V

Input Leakage Current 1 µA
CHARGE PUMP POWER EFFICIENCY 88 % CTRL1 = 1, CRTL2 = 1, VCC = 3.4 V, VFB = 0.2 V, IFB = 20 mA
V

RIPPLE 30 m V VCC = 3.6 V, I

OUT

= 20 mA, all six LEDs enabled

LED

_______________________

1

Guaranteed by design. Not 100% production tested.

Rev. 0 | Page 3 of 20

ADM8845

ABSOLUTE MAXIMUM RATINGS

TA = 25°C, unless otherwise noted.

Table 2.

Parameter Rating

Supply Voltage, V
I

–0.3 V to +2.0 V

SET

CC

–0.3 V to +6.0 V

CTRL1, CTRL2 –0.3 V to +6.0 V
V

Shorted1 Indefinite

OUT

Feedback pins FB1 to FB6 –0.3 V to +6.0 V
Operating Temperature Range

Six LEDs Enabled with 30 mA/LED2 –40°C to +65°C
Six LEDs Enabled with 20 mA/LED2 –40°C to +85°C

3

V

180 mA

OUT

Storage Temperature Range –65°C to +125°C
Power Dissipation 2 mW
ESD Class 1

1

Short through LED.

2

LED current should be derated above TA > 65°C, refer to . Figure 21

3

Based on long-term current density limitations.

Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.

THERMAL CHARACTERISTICS

16-Lead LFCSP Package:

θ

= 50°C/W

JA

ESD CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.

Rev. 0 | Page 4 of 20

ADM8845

V

PIN CONFIGURATION AND FUNCTION DESCRIPTION

1
L
R

+

C

–

1

1

T

C

V

C

C

C

4

3

5

6

1

1

1

1

PIN 1
INDICATOR

1

OUT

2C2+

ADM8845

3I
4FB1

TOP VIEW

(Not to Scale)

5

6

2

3

B

B

F

F

SET

Figure 2. Pin Configuration

Table 3. Pin Function Descriptions

Pin No. Mnemonic Function

1 V

OUT

Charge Pump Output. A 2.2 µF capacitor to ground is required on this pin. Connect V

the LEDs.
2 C2+ Flying Capacitor 2 Positive Connection.
3 I

4–9 FB1–FB6

SET

Bias Current Set Input. The current flowing through R

Connect R

to GND to set the bias current as V

SET

LED1–LED6 Cathode Connection and Charge Pump Feedback. The current flowing in these LEDs is 120 times

the current flowing through R

, I

. When using fewer than six LEDs, this pin can be left unconnected or

SET

SET

connected to GND.
10 GND Device Ground Pin.
11

C2−

Flying Capacitor 2 Negative Connection.
12 CTRL2 Digital Input. 3 V CMOS Logic. Used with CTRL1 to control the shutdown operation of the main and sub LEDs.
13 CTRL1 Digital Input. 3 V CMOS Logic. Used with CTRL2 to control the shutdown operation of the main and sub LEDs.
14 C1− Flying Capacitor 1 Negative Connection.
15 VCC Positive Supply Voltage Input. Connect this pin to a 2.6 V to 5.5 V supply with a 4.7 µF decoupling capacitor.
16 C1+ Flying Capacitor 1 Positive Connection.

- EP Expose Paddle. Connect the exposed paddle to GND.

12 CTRL2
11 C2–
10 GND
9 FB6

8

7

5

4

B

B

F

F

04867-0-003

to the anodes of all

OUT

, I

SET/RSET

, is gained up by 120 to give the I

SET

SET

. Note that V

= 1.18 V.

SET

current.

LED

Rev. 0 | Page 5 of 20

ADM8845

TYPICAL PERFORMANCE CHARACTERISTICS

35

0.4

30

25

20

15

LED CURRENT (mA)

10

5

4.75 6.75 12.75 14.758.75

Figure 3. I

10.75

R

(kΩ)

SET

(mA) Current vs. R

LED

20.35

20.30

20.25

20.20

20.15

LED CURRENT (mA)

20.10

20.05

20.00

2.6 3.1 4.6 5.63.6 5.1

Figure 4. I

SUPPLY VOLTAGE (kΩ)

(mA) vs. Temperature (°C), Six LEDs Enabled

LED

4.1

0.3

–40°C

25°C

85°C

0.3

0.2

0.1

0

–0.1

MATCHING ERROR (%)

–0.2

–0.3

–0.4

04867-0-004

2.6 4.2 5.03.4 4.6

SET

Figure 6. I

MAX POSITIVE MATCHING ERROR

MAX NEGATIVE MATCHING ERROR

3.0 5.4

(mA) Matching Error (%) vs. Supply Voltage (V),

LED

3.8

SUPPLY VOLTAGE (V)

T

= 25°C and I

A

= 20 mA

LED

04867-0-007

20.24

20.22

20.20

20.18

20.16

20.14

LED CURRENT (mA)

20.12

20.10

20.08

04867-0-005

–40 0 8040

Figure 7. I

(mA) Variation over Temperature (°C), VCC = 3.6 V

LED

TEMPERATURE (°C)

04867-0-008

35

0.2

0.1

0

% ERROR

–0.1

–0.2

–0.3

–40 –20 45 850 65

Figure 5. I

Matching (%) over Temperature (°C), VCC = 3.6 V, I

LED

TEMPERATURE (°C)

Six LEDs Enabled

30

25

20

15

LED CURRENT (mA)

10

25

= 20 mA,

LED

04867-0-006

5

2.6 3.0 4.6 5.43.4 5.04.2

Figure 8. I

3.8

SUPPLY VOLTAGE (V)

(mA) vs. Supply Voltage (V)

LED

04867-0-009

Rev. 0 | Page 6 of 20