Datasheet ADM8843 Datasheet (Analog Devices)

4 White LED Backlight Driver

FEATURES

Drives 4 LEDs from a 2.6 V to 5.5 V (Li-Ion) input supply 1×/1.5×/2× fractional charge pump to maximize

power efficiency

0.3% typical 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 light applications Micro TFT color displays DSC PDAs

2

(3 mm × 3 mm)

ADM8843

GENERAL DESCRIPTION

The ADM8843 uses charge pump technology to provide the power to drive up to four LEDs. The LEDs are used for backlighting a color LCD display, having regulated constant current for uniform brightness intensity. The main display can use up to three LEDs, and the sub display uses one LED. The CTRL1 and CTRL2 digital 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 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 that senses individual LED current with a typical matching accuracy of 0.3%.

4.7µF

CTRL1

CTRL2

R

C4

SET

V

I

CC

SET

ADM8843

CONTROL

LOGIC

LED CURRENT CONTROL

CIRCUIT

FUNCTIONAL BLOCK DIAGRAM

C1

C2

1µF

CHARGE PUMP

1×/1.5×/2× MODE

OSC

V

REF

CURRENT

CONTROL 1

CONTROL 2

CURRENT CONTROLLED SINKS

GND

CURRENT

CONTROL 3

CONTROL 4

Figure 1.

V

OUT

FB1 FB2 FB3 FB4

C3

2.2µF

MAIN SUB

05050-001

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.

www.analog.com

ADM8843

TABLE OF CONTENTS

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

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

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

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

Pin Configuration and Function Descriptions..............................5

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

Theory of Operation .........................................................................9

Automatic Gain Control ............................................................ 10

Brightness Control with a Digital PWM Signal..................... 10

LED Brightness Control Using a PWM Signal Applied to

............................................................................................. 12

V

PWM

REVISION HISTORY

10/04—Revision 0: Initial Version

LED Brightness Control Using a DC Voltage Applied to

.......................................................................................... 12

V

BRIGHT

Applications......................................................................................13

Layout Considerations and Noise ............................................ 13

White LED Shorting .................................................................. 13

Driving Four LEDs in the Main Display Only ....................... 13

Driving Fewer than Four LEDs ................................................ 13

Using Smaller Capacitor Values ............................................... 14

Power Efficiency......................................................................... 15

Outline Dimensions ........................................................................16

Ordering Guide .......................................................................... 16

Rev. 0 | Page 2 of 16

ADM8843

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 four 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 2× to 1.5× 3.36 V

Hysteresis 40 mV

1× to 1.5× 4.77 V

1.5× to 1× 4.81 V Hysteresis 40 mV

I

SET

LED : LED Matching 0.3 % I I

Pin Voltage 1.18 V

SET

I

to I

LED

I

LED

Ratio 120

SET

to I

Ratio Accuracy ±5%

SET

MIN COMPLIANCE ON FBx PIN 0.15 V I

= 20 mA, VFB = 0.4 V

LED

= 15 mA

SET

CHARGE PUMP OUTPUT RESISTANCE 1.2 Ω 1× mode

3.5 Ω 1.5× mode

8.0 Ω 2× mode

LED CURRENT 20 mA PWM 0.1 200 kHz Note 1 DIGITAL INPUTS

Input High 0.7 VCC V Input Low 0.3 VCC V

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

RIPPLE 30 mV VCC = 3.6 V, I

OUT

______________________

1

Guaranteed by design. Not 100% production tested.

= 20 mA, all four LEDs enabled

LED

Rev. 0 | Page 3 of 16

ADM8843

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

OUT

Shorted

1

Indefinite Feedback Pins FB1 to FB4 –0.3 V to +6.0 V Operating Temperature Range –40°C to +85°C

2

V

180 mA

OUT

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

1

Short through LED.

2

Based on long-term current density limitations.

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.

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:

= 50°C/W

θ

JA

Rev. 0 | Page 4 of 16

ADM8843

V

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

1 L R

+

C

–

1

T

C

V

C

4

3

5

6

1

PIN 1 INDICATOR

1

OUT

2C2+

ADM8843

3I 4GND

TOP VIEW

(Not to Scale)

5

6

1

2

B

F

SET

Figure 2. Pin Configuration

Table 3. Pin Function Descriptions

Pin No. Mnemonic Description

1 V

OUT

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

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

SET

Bias Current Set Input. The current flowing through the R

current. Connect a resistor R

I

LED

to GND to set the bias current as V

SET

4, 9, 10 GND Device Ground Pins. 5–8 FB1–FB4

LED1–LED4 Cathode Connection and Charge Pump Feedback. The current flowing in these LEDs is

120 times the current flowing through R

unconnected or connected to GND. 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 V

CC

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 GND

8

7

4

3

B

F

05050-003

OUT

resistor, I

SET

, I

. When using fewer than four LEDs, this pin can be left

SET

, is gained up by 120 to give the

SET

. Note that V

SET/RSET

to the anodes of

= 1.18 V.

SET

Rev. 0 | Page 5 of 16

ADM8843

TYPICAL PERFORMANCE CHARACTERISTICS

35

20.35

30

25

20

15

LED CURRENT (mA)

10

5

4.75 6.75 12.75 14.758.75

20.24

20.22

20.20

20.18

20.16

20.14

LED CURRENT (mA)

20.12

20.10

Figure 3. I

R

SET

(mA) vs. R

LED

(kΩ)

10.75

20.30

4.1

–40°C

+25°C

+85°C

05050-007

20.25

20.20

20.15

LED CURRENT (mA)

20.10

20.05

20.00

05050-004

SET

2.6 3.1 4.6 5.63.6 5.1

Figure 6. I

35

30

25

20

15

LED CURRENT (mA)

10

SUPPLY VOLTAGE (kΩ)

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

LED

20.08 –40 0 8040

Figure 4. I

20

16

12

8

LED CURRENT (mA)

4

0

0 20 60 1008040

Figure 5. I

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

LED

TEMPERATURE (°C)

DUTY CYCLE (%)

(mA) vs. PWM Dimming ( Varying Duty Cycle),

Four LEDs Enabled, Frequency = 1 kHz

05050-005

05050-006

5

2.6 3.0 4.6 5.43.4 5.04.2

Figure 7. I

95

90

85

80

75

EFFICIENCY (%)

70

65

60

10 20

04060807050

3.8

SUPPLY VOLTAGE (V)

(mA) vs. Supply Voltage (V)

LED

30

DUTY CYCLE (%)

90 100

Figure 8. LED Efficiency vs. Varying Duty Cycle of 1 kHz PWM Signal,

Four LEDs Enabled, 20 mA/LED

05050-008

05050-009

Rev. 0 | Page 6 of 16

ADM8843

C

180

160

140

–40°C

+25°C +85°C

CTRL1/2

1

120

100

SUPPLY CURRENT (mA)

80

60

2.6 3.0 3.8 4.64.23.4

5.0 5.4

SUPPLY VOLTAGE (V)

Figure 9. Supply Current vs. Supply Voltage over Temperature,

Four LEDs Enabled @ 20 mA/LED

V

2

CC

1

V

OUT

B

CH2 20.0mVCH1 20.0mV M 10.0µs CH1 –12.8mV

W

B

W

Figure 10. 1.5× Mode Operating Waveforms

05050-010

05050-011

URRENT

2

V

OUT

3

CH3 1.00V

B

CH2 160mACH1 2.00V M 10.0µs CH2 –2.44mV

W

B

W

Figure 12. Soft Start Showing the Initial In-Rush Current and V

Four LED s @ 20 mA /LED, V

= 3.6 V

CC

1

V

CC

2

V

OUT

B

CH2 20.0mVCH1 20.0mV M 400ns CH2 2.4mV

W

B

W

Figure 13. 2× Mode Operating Waveforms

Varia tion,

OUT

5050-013

05050-014

90

= 3.6V

V

85

80

1

V

CC

75

70

F

VF = 4.0V

V

= 4.3V

F

65

60

55

2

V

OUT

POWER EFFICIENCY

50

= 3.8V

V

F

VF = 3.2V

45

B

CH2 20.0mVCH1 20.0mV M 400ns CH2 2.4mV

W

B

W

Figure 11. 1× Mode Operating Waveforms

5050-012

40

2.8 3.0 3.4 3.83.63.2 4.0 4.22.9 3.1 3.3 3.5 3.7 3.9 4.1 V

CC

5050-015

Figure 14. Power Efficiency vs. Supply Voltage over Li-Ion Range,

Four LED s @ 20 mA /LED

Rev. 0 | Page 7 of 16

ADM8843

90

85

80

75

70

65

60

55

POWER EFFICIENCY

50

45

40

2.8 3.0 3.4 3.83.63.2 4.0 4.22.9 3.1 3.3 3.5 3.7 3.9 4.1

Figure 15. Power Efficiency vs. Supply Voltage over Li-Ion Range,

Four LED s @ 15 mA /LED

= 3.6V

V

F

VF = 3.2V

V

CC

= 3.8V

V

F

VF = 4.0V

V

= 4.3V

F

05050-016

CTRL1/2

V

OUT

∆: 44.0ms @: –44.4ms

1

2

CH2 2.00mVCH1 2.00V M 10.0ms CH2 4.36mV

C2 FALL 200µs LOW SIGNAL AMPLITUDE

05050-017

Figure 16. TPC Delay

Rev. 0 | Page 8 of 16

ADM8843

THEORY OF OPERATION

The ADM8843 charge pump driver for LCD white LED backlights implements a multiple-gain charge pump (1×, 1.5×, 2×) to maintain the correct voltage on the anodes of the LEDs over a 2.6 V to 5.5 V (Li-Ion) input supply voltage. The charge pump automatically switches between 1×/1.5×/2× modes, based on the input voltage, to maintain sufficient drive for the LED anodes, with V

input voltages as low as 2.6 V. It also includes

CC

regulation of the charge pump output voltage for supply voltages up to 5.5 V. The ADM8843’s four LEDs are arranged into two groups, main and sub. The main display can have up to three LEDs (FB1 to FB3), and the sub display has one LED (FB4) (see Figure 18). The CTRL1 and CTRL2 digital input control pins control the shutdown operation and the brightness of the main and sub displays (see Table 4).

Table 4. Shutdown Truth Table

CTRL1 CTRL2 LED Shutdown Operation

0 0 Sub Display Off / Main Display Off 0 1 Sub Display Off / Main Display On 1 0 Sub Display On / Main Display Off 1 1 Sub Display On / Main Display On

C1

C2

1µF

An external resistor, R GND. This resistor sets up a reference current, I

, is connected between the I

SET

, which is

SET

pin and

internally gained up by 120 within the ADM8843 to produce

currents of up to 20 mA/LED (I

I

LED

1.18 V/R

). The ADM8843 uses four individual current sinks

SET

LED

= I

SET

× 120 and I

SET

=

to individually sense each LED current with a typical matching performance of 0.3%. This current matching performance ensures uniform brightness across a color display.

The ADM8843 lets the user control the brightness of the white LEDs with a digital PWM signal applied to CTRL1 and/or CTRL2. The duty cycle of the applied PWM signal determines the brightness of the main and/or sub display backlight white LEDs. The ADM8843 also allows the brightness of the white LEDs to be controlled using a dc voltage (see Figure 17). Softstart circuitry limits the in-rush current flow at power-up. The ADM8843 is fabricated using CMOS technology for minimal power consumption, and is packaged in a 16-lead lead frame chip scale package.

ADM8843

I

SET

BRIGHT

R = 15kΩ

R

= 13.4kΩ

SET

05050-022

BRIGHT

V 0V–2.5V

Figure 17. PWM Brightness Control Using a DC Voltage Applied to V

V

ADM8843

4.7µF

CTRL1

CTRL2

R

C4

SET

I

SET

CC

CONTROL

LOGIC

LED CURRENT CONTROL

CIRCUIT

CHARGE PUMP

1×/1.5×/2× MODE

OSC

V

REF

CURRENT

CONTROL 1

CONTROL 2

CURRENT CONTROLLED SINKS

GND

CURRENT

CONTROL 3

CONTROL 4

V

OUT

FB1 FB2 FB3 FB4

C3

2.2µF

MAIN SUB

05050-001

Figure 18. Functional Block Diagram

Rev. 0 | Page 9 of 16

ADM8843

AUTOMATIC GAIN CONTROL

The automatic gain control block controls the operation of the charge pump by selecting the appropriate gain for the charge pump. Doing so maintains sufficient drive for the LED anodes at the highest power efficiency over a 2.6 V to 5.5 V input supply range. The charge pump switching thresholds are described in Table 5.

Table 5. Charge Pump Switching Thresholds

Gain Threshold

1.5× to 2× 3.33 V 2× to 1.5× 3.36 V 1× to 1.5× 4.77 V

1.5× to 1× 4.81 V

BRIGHTNESS CONTROL WITH A DIGITAL PWM SIGNAL

PWM brightness control provides the widest brightness control method by pulsing the white LEDs on and off using the digital input control pins, CTRL1 and/or CTRL2. PWM brightness control also removes any chromaticity shifts associated with changing the white LED current, because the LEDs operate at either zero current or full current (set by R

The digital PWM signal applied with a frequency of 100 Hz to 200 kHz turns the current control sinks on and off using CTRL1 and/or CTRL2. The average current through the LEDs changes with the PWM signal duty cycle. If the PWM frequency is much less than 100 Hz, flicker could be seen in the LEDs. For the ADM8843, zero duty cycle turns off the LEDs, and a 50% duty cycle results in an average LED current I grammed LED current. For example, if R 20 mA/LED, a 50% duty cycle results in an average I 10 mA/LED, I

3.4V

I

IN

PWM INPUT

OR

HIGH/LOW

PWM INPUT

OR

HIGH/LOW

R

SET

Figure 19. Digital PWM Brightness Control Application Diagram

being half the programmed LED current.

LED

C1

C2

1µF

V

FB1 FB2 FB3 FB4

OUT

V

CC

ADM8845

CTRL1

CTRL2

I

SET

).

SET

being half the pro-

LED

is set to program

SET

LED

of

C3

2.2µF

05050-018

By applying a digital PWM signal to the digital input control pins, CTRL1 and/or CTRL2 adjust the brightness of the sub and/or main displays. The ADM8843’s four white LEDs are organized into two groups, main display (FB1 to FB3) and sub display (FB4); refer to the Theory of Operation section.

The ADM8843’s main and sub display brightness can be controlled together or separately. It does this by applying a digital PWM signal to both the CTRL1 and CTRL2 pins. The duty cycle of the applied digital PWM signal determines the brightness of the main and sub displays together. Varying the duty cycle of the applied PWM signal varies the brightness of the main and sub displays from 0% to 100%.

By holding CTRL1 low and applying a digital PWM signal to CTRL2, the sub display is turned off and the main display is turned on. Then the brightness of the main display is determined by the duty cycle of the applied digital PWM signal.

By applying a digital PWM signal to CTRL1 and holding CTRL2 low, the sub display is turned on and the main display is turned off. Then the brightness of the sub display is determined by the duty cycle of the applied digital PWM signal.

By applying a digital PWM signal to CTRL1 and holding CTRL2 high, the sub display is turned on and the main display is turned on. Then the brightness of the sub display is determined by the duty cycle of the applied digital PWM signal. The brightness of the main display is set to the maximum (maximum is set

).

by R

SET

By holding CTRL1 high and applying a digital PWM signal to CTRL2, the sub display is turned on and the main display is turned on. Then the brightness of the main display is determined by the duty cycle of the applied digital PWM signal. The brightness of the sub display is set to the maximum (maximum is set

).

by R

SET

When CTRL1 and CTRL2 are low, the LED current control sinks shutdown. Shutdown of the charge pump is delayed by 15 ms. This timeout period, t

, allows the ADM8843 to

CP

determine if a digital PWM signal is present on CTRL1 and CTRL2, or if the user has selected a full chip shutdown (see Figure 20).

If digital PWM brightness control of the LEDs is not required, a constant Logic Level 1 (V

) or 0 (GND) must be applied.

CC

The four white LED in the ADM8843 are arranged into two groups, sub and main. It is possible to configure the four LEDs as in Table 6. Refer also to Figure 20.

Rev. 0 | Page 10 of 16

ADM8843

Table 6. Digital Inputs Truth Table

CTRL1 CTRL2 LED Operation

0 0 Sub Display Off / Main Display Off (Full Shutdown) 0 1 Sub Display Off / Main Display On 1 0 Sub Display On / Main Display Off

1, 3

1, 2

1 1 Sub Display On / Main Display On (Full On) 0 PWM Sub Display Off/ Digital PWM Brightness Control on Main Display PWM 0 Digital PWM Brightness Control on Sub Display / Main Display Off 1 PWM Sub Display On/ Digital PWM Brightness Control on Main Display PWM 1 Digital PWM Brightness Control on Sub Display / Main Display On PWM PWM Digital PWM Brightness Control on Sub and Main Display5

1

Sub display on means the display is on with the maximum brightness set by the R

2

Main display off means the main display only is off. CTRL2 = 0 means a constant logic level (GND) is applied to CTRL2.

3

Main display on means the display is on with the maximum brightness set by the R

4

Sub display off means the sub display LEDs only is off. CTRL1 = 0 means a constant logic level (GND) is applied to CTRL1.

5

PWM means a digital PWM signal is applied to the CTRL1 and/or the CTRL2 pin with a frequency from 100 Hz to 200 kHz.

resistor. CTRL1 = 1 means a constant logic level (VCC) is applied to CTRL1.

SET

resistor. CTRL2 = 1 means a constant logic level (VCC) is applied to CTRL2.

SET

1, 2

1, 3

4, 5

2, 4

1, 5

5

LED CONFIG.

CTRL1

CTRL2

V

(SUB)

(MAIN)

SUB DISPLAY

BRIGHTNESS

MAIN DISPLAY

BRIGHTNESS

OUT

I

LED

FULL ON

100%

SUB AND MAIN 50%

DUTY CYCLE

50%

Figure 20. Application Timing

MAIN AND SUB OFF

t

CP

37ms >

t

CP

> 15ms

SHDN

MAIN 80% DUTY CYCLE,

SUB OFF

80%

05050-020

Rev. 0 | Page 11 of 16

ADM8843

LED BRIGHTNESS CONTROL USING A PWM SIGNAL APPLIED TO V

Adding two external resistors and a capacitor, as shown in Figure 21, can also be used for PWM brightness control. This PWM brightness control method can be used instead of CTRL1 and/or CTRL2 digital PWM brightness control. With this configuration, CTRL1 and CTRL2 digital logic pins can control shutdown of the white LEDs, while V ness of all the white LEDs. This is done by applying a high frequency PWM signal (amplitude 0 V to 2.5 V) to drive an R-C-R filter on the I cycle corresponds to 20 mA/LED, while a 100% PWM duty cycle corresponds to a 0 mA/LED. At PWM frequencies above 5 kHz, C5 may be reduced (see Figure 21). The amplitude of the PWM signal must be 0 V and 2.5 V only in order to have 20 mA flowing in each LED.

LED

=

0V–2.5V

SET

V

I

pin of the ADM8843. A 0% PWM duty

SET

VoltageI

_

RR

×

2

SET

RR

+

2

SET

PWM

100% = I

0% = I

LED

R = 7.5kΩ

C5 = 1µF

= 0mA

= 20mA

Figure 21. PWM Brightness Control Using

Filter -PWM Signal

××

120

R = 7.5kΩ

PWM

can control the bright-

PWM

−

100

)1(

CycleDuty

ADM8843

I

SET

R

= 13.4kΩ

SET

05050-021

LED BRIGHTNESS CONTROL USING A DC VOLTAGE APPLIED TO V

By adding one resistor, as shown in Figure 17, this configuration can also be used for brightness control of the white LEDs by using a dc voltage applied to the V an application example of LED brightness control using a dc voltage with a amplitude of 0 V to 2.5 V applied to V

The equation for I

= [(1/R

I

SET

= 120 × I

I

LED

where: R = 15 kΩ

= voltage at I

V

SET

2.5V

V

BRIGHT

I

LED

0mA

Figure 22. PWM Brightness Control Application Diagram Using a

is

LED

+ 1/R)(V

SET

pin (1.18 V)

SET

DC Voltage Applied to V

BRIGHT

)] – [(1/R)(V

SET

1.6V

7.2mA

node. Figure 22 shows

BRIGHT

0.8V

13.6mA

BRIGHT

)]

BRIGHT

0V

20mA

.

05050-023

Rev. 0 | Page 12 of 16

ADM8843

APPLICATIONS

LAYOUT CONSIDERATIONS AND NOISE

Because of the ADM8843’s switching behavior, PCB trace layout is an important consideration. To ensure optimum performance, a ground plane should be used, and all capacitors (C1, C2, C3, C4) must be located with minimal track lengths to the pins of the ADM8843.

WHITE LED SHORTING

If an LED is shorted, the ADM8843 continues to drive the remaining LEDs with I

per LED (I

LED

is because the ADM8843 uses four internal currents sinks to produce the LED current. If an LED is shorted, the ADM8843 continues to sink (I

× 120

SET

LED

= I

× 120 mA). This

SET

DRIVING FOUR LEDS IN THE MAIN DISPLAY ONLY

The ADM8843 can be operated with four LEDs in the main display only (see Figure 23). With this configuration, CTRL1 and CTRL2 are used together to control the main display shutdown operation and brightness control.

DRIVING FEWER THAN FOUR LEDS

The ADM8843 can be operated with fewer than four LEDs in parallel by simply leaving the unused FBx pins floating or by connecting them to GND. For example, Figure 24 shows three LEDs being powered by the ADM8843.

2.6V–5.5V

CTRL1 CTRL2

R

SET

LCD

V

CC

V

OUT

I

SET

ADM8843

GND

FB1 FB2 FB3 FB4

05050-024

Figure 23. Driving Four White LEDs

MAIN DISPLAY

V

2.6V–5.5V

CTRL1 CTRL2

R

SET

CC

V

OUT

I

SET

ADM8843

GND

FB1 FB2 FB3 FB4

Figure 24. Driving Three White LEDs

MAIN DISPLAY

05050-025

SUB DISPLAY

V

CC

2.6V–5.5V

V

OUT

ADM8843

CTRL1 CTRL2

I

R

SET

GND

FB1 FB2 FB3 FB4

Figure 25. Typical Application Diagram

05050-002

Rev. 0 | Page 13 of 16

ADM8843

USING SMALLER CAPACITOR VALUES

The ADM8843 can be operated with the smaller capacitor values described here to reduce capacitor footprint sizes.

Option 1

Input and output ripple plots for 1× and 1.5× mode operation are shown with C1,C2 = 0.22 µF; C3 = 0.47 µF and C4 = 1 µF.

2

V

CC

3

V

OUT

CH2 10.0mV

CH3 10.0mV

B

W

Figure 26. 1× Mode Operation with Four LEDs with

20 mA/LED at V

2

V

CC

= 5.0 V, with a 1 µF VCC Decoupling Capacitor

CC

B

M 400ns CH3 –33mV

W

05050-026

Option 2

Input and output ripple plots for 1× and 1.5× mode operation are shown with C1,C2 = 0.22 µF; C3 = 0.47 µF and C4 = 4.7 µF.

2

V

CC

V

3

OUT

CH3 10.0mV

CH2 10.0mV

B

W

B

W

Figure 28. 1× Mode Operation with Four LEDs with

20 mA/LED at V

= 5.0 V, with a 4.7 µF VCC Decoupling Capacitor

CC

2

V

CC

M 1.00µs CH3 –33mV

05050-028

V

OUT

3

CH3 50.0mV

CH2 20.0mV M 400ns CH3 –33mV

B

W

B

W

Figure 27. 1.5× Mode Operation with Four LEDs with

20 mA/LED at V

= 3.6 V, with a 1 µF VCC Decoupling Capacitor

CC

05050-027

Rev. 0 | Page 14 of 16

3

V

OUT

CH3 50.0mV

CH2 20.0mV

B

W

B

W

Figure 29. 1.5× Mode Operation with Four LEDs with

20 mA/LED at V

= 3.6 V, with a 4.7 µF VCC Decoupling Capacitor

CC

M 400ns CH3 –33mV

05050-029

ADM8843

POWER EFFICIENCY

The ADM8843 power efficiency (η) equations are as follows:

η = P

OUT/PIN

PIN = ((VCC × I P

= 4 × (VF × I

OUT

× Gain) + (IQ × VCC))

LOAD

)

LED

where:

I

is the quiescent current of the ADM8843, 2.6 mA.

Q

is the LED forward voltage.

V

F

Gain is the charge pump mode (1×, 1.5×, 2×).

Example 1

The ADM8843 driving four white LED with 20 mA/LED at

= 3.4 V (1.5× mode), LED VF = 4.5 V.

V

CC

P

= ((VCC × I

IN

= ((3.4 × 80 mA × 1.5) + (3.4 × 2.6 mA))

P

IN

= ((0.408) + (0.00884))

P

IN

P

= 0.41684

IN

= 4(VF × I

P

OUT

= 4(4.5 V × 20 mA)

P

OUT

P

= 0.36

OUT

η = P

OUT/PIN

× Gain) + (VCC × IQ))

LOAD

)

LED

η = 0.36/0.41684 η = 87 %

V

CC

= 3.4V

I

R

SET

7.32kΩ

IN

V

CC

ADM8843

CTRL1 CTRL2

I

SET

GND

V

FB1 FB2 FB3 FB4

OUT

Figure 30. Charge Pump Power Efficiency Diagram, Example 1

I

LOAD

05050-019

Example 2

The ADM8843 driving four white LED with 20 mA/LED at

= 3.4 (1.5× mode), LED VF = 3.6 V.

V

CC

= ((VCC × I

P

IN

P

= ((3.4 × 80 mA × 1.5) + (3.4 × 2.6 mA))

IN

= ((0.408) + (0.00884))

P

IN

P

= 0.41684

IN

= 4(VF × I

P

OUT

= 4(3.6 V × 20 mA)

P

OUT

= 0.288

P

OUT

η = P

OUT/PIN

× Gain) + (VCC × IQ))

LOAD

)

LED

η = 0.288/0.41684 η = 70 %

Rev. 0 | Page 15 of 16

ADM8843

R

OUTLINE DIMENSIONS

PIN 1

INDICATO

1.00

0.85

0.80

SEATING

PLANE

12° MAX

3.00

BSC SQ

VIEW

0.30

0.23

0.18

TOP

*

2.75

BSC SQ

0.80 MAX

0.65 TYP

0.05 MAX

0.02 NOM

0.20 REF

COMPLIANT TO JEDEC STANDARDS MO-220-VEED-2 EXCEPT FOR EXPOSED PAD DIMENSION

0.45

0.50

BSC

1.50 REF

0.60 MAX

Figure 31. 16-Lead Lead Frame Chip Scale Package [LFCSP]

3 mm × 3 mm Body

(CP-16-3)

Dimensions shown in millimeters

13

12

EXPOSED

PA D

(BOTTOM VIEW)

9

8

0.50

0.40

0.30

16

1

4

5

PIN 1 INDICATOR

1.65 *

1.50 SQ

1.35

0.25 MIN

ORDERING GUIDE

Model Temperature Range Package Description Package Option Branding

ADM8843ACPZ-REEL71−40ºC to + 85ºC 16-Lead Lead Frame Chip Scale Package CP-16-3 M2U

1

Z = Pb-free part.

D05050–0–10/04(0)

Rev. 0 | Page 16 of 16

Datasheet ADM8843 Datasheet (Analog Devices)

Specifications and Main Features

Frequently Asked Questions

User Manual

FEATURES

APPLICATIONS

GENERAL DESCRIPTION

FUNCTIONAL BLOCK DIAGRAM

SPECIFICATIONS

ABSOLUTE MAXIMUM RATINGS

ESD CAUTION

THERMAL CHARACTERISTICS

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

TYPICAL PERFORMANCE CHARACTERISTICS

THEORY OF OPERATION

AUTOMATIC GAIN CONTROL

BRIGHTNESS CONTROL WITH A DIGITAL PWM SIGNAL

APPLICATIONS

LAYOUT CONSIDERATIONS AND NOISE

WHITE LED SHORTING

DRIVING FOUR LEDS IN THE MAIN DISPLAY ONLY

DRIVING FEWER THAN FOUR LEDS

USING SMALLER CAPACITOR VALUES

POWER EFFICIENCY

OUTLINE DIMENSIONS

ORDERING GUIDE