ZXSC310E5TA - Zetex Semiconductors

DEVICE DESCRIPTION

The ZXSC310 is a single or multi cell LED driver

designed for LCD backlighting applications. The input

voltage range of the device is between 0.8V and 8V.

This means the ZXSC310 is compatible with single

NiMH, NiCd or Alkaline cells, as well as multi-cell or

LiIon batteries.

The device features a shutdown control, resulting in a

standby current less than 5µA, and an output capable

of driving serial or parallel LED’s. The circuit generates

constant power output, which are ideal for driving

singleormultipleLED’soverawiderangeofoperating

voltages. These features make the device ideal for

drivingLED’sparticularlyinLCDbacklightapplications

for Digital Still cameras and PDA’s.

FEATURES

94% efficiency

Minimum operating input voltage 0.8V

Maximum operating input voltage 8V

Standby current less than 5µA

Programmable output current

Series or parallel LED configuration

Low saturation voltage switching transistor

SOT23-5 package

APPLICATIONS

LCD backlights:

Digital still camera

PDA

Mobile phone

LED flashlights and torches

White LED driving

Multiple LED driving

The ZXSC310 is a PFM DC-DC controller IC that drives

an external Zetex switching transistor with a very low

saturation resistance. These transistors are the best

switching devices available for this type of conversion

enabling high efficiency conversion with low input

voltages. The drive output of the ZXSC310 LED driver

generates a dynamic drive signal for the switching

transistor.

The circuit can start up under full load and operates

down to an input voltage of 0.8 volts. The solution

configurationensuresoptimumefficiencyover awider

range of load currents; several circuit configurations

are possible depending on battery life versus

brightness considerations.

TheZXSC310is offered in the SOT23-5 package which,

when combined with a SOT23 switching transistor,

generates a high efficiency small size circuit solution.

The IC and discrete combination offers the ultimate

cost Vs performance solution for LED backlight

applications.

ZXSC310

ISSUE 1 - SEPTEMBER 2002

LED DRIVER SOLUTION FOR LCD BACKLIGHTING

VIN

VCC

=3.3V/5V

DRIVE

ISENSE

GND

STDN

TYPICAL APPLICATIONS CIRCUIT

DEVICE REEL

SIZE TAPE

WIDTH QUANTITY

PER REEL

ZXSC310E5 180mm 8mm 3000

ORDERING INFORMATION

DEVICE MARKINGS

C310

ABSOLUTE MAXIMUM RATINGS:

Supply Voltage -0.3 to 10V

Maximum Voltage other pins -0.3 to VCC+0.3V

Power Dissipation 450mW

Operating Temperature -40 to 85 °C

Storage Temperature -55 to 125°C

ZXSC310

ISSUE 1 - SEPTEMBER 2002

Symbol Parameter Conditions Min Typ Max Units

␩Efficiency194 %

VCC Recommended supply

voltage range 0.8 8 V

VCC(min) Minimum startup and

operating voltage IDRIVE=-600µA,VDRIVE=0.7V

IDRIVE=-600µA, VDRIVE=0.7V,

TAMB=-10°C3

0.8

0.9 0.92 V

IQQuiescent current2VEN = VCC (enabled)

VEN = 0V (standby) 0.2

45mA

µA

IVDRIVE Base drive current VDRIVE = 0.7V, VISENSE = 0V 1.5 3.5 mA

ICC Supply current3VDRIVE = 0.7V, VISENSE =0V 2 4 mA

VVDRIVE(high) High level drive voltage VISENSE =0V,I

VDRIVE =-0.5mA VCC

-0.3 VCC V

VVDRIVE(low) Low level drive voltage VISENSE = 50mV, IVDRIVE = 5mA 0 0.2 V

VSTDN(high) Device enabled 0.7 V

VSTDN(low) Device in standby mode 0.15 V

ISTDN Enable input current -1 1 µA

VISENSE

(threshold)

Output current reference

voltage 14 19 24 mV

TCVISENSE ISENSE voltage temp co.20.4 %/°C

IISENSE ISENSE input current VISENSE = 0V 0 -30 -65 µA

ELECTRICAL CHARACTERISTICS:

Test conditions unless otherwise stated: VCC=1.5V, TAMB=25 C

Symbol Parameter Conditions Min Typ Max Units

TDRV Discharge Pulse Width 1.2 1.7 3.2 µs

FOSC Recommended operating

frequency4200 kHz

ELECTRICAL CHARACTERISTICS: AC PARAMETERS2

Test conditions unless otherwise stated: VCC=1.5V, TAMB=0 to 70 C

1 Application dependent, see reference designs

2 These parameters guaranteed by Design

3 Total supply current =IQ+I

VDRIVE, see typical characteristics

4 Operating frequency is application circuit dependent. See applications section.

FMMT618

For the circuits described in the applications section

ZetexFMMT618istherecommendedpasstransistor.

ZHCS1000

For the maximum brightness circuit described in the

applications section Zetex ZHCS1000 is the

recommended Schottky diode.

The following indicates outline data for the device,

more detailed information can be found in the Zetex

surface mount products data book or on Zetex Web

page: www.zetex.com

The following indicates outline data for the ZHCS,

more detailed information can be found on Zetex Web

page: www.zetex.com

ZXSC310

ISSUE 1 - SEPTEMBER 2002

Symbol Parameter Conditions Min Typ Max Units

VCE(sat) Collector-Emitter

Saturation Voltage5IC=0.1A, IB=10mA

IC=1A, IB=10mA

IC=2.5A, IB=50mA

130

150

200

V(BR)CEO Collector-Emitter

Breakdown Voltage5IC=10mA 20 27 V

ELECTRICAL CHARACTERISTICS:

Test conditions unless otherwise stated: TAMB=25 C

5Measured under pulse conditions. Pulse width=300µs. Duty cycle ⱕ2%

Symbol Parameter Conditions Min Typ Max Units

VFForward voltage IF= 500mA

IF=1A 400

500 mV

trr Reverse Recovery Time Switched from IF=500mA to

IR=500mA.

Measured at IR=50mA

12 ns

IRReverse Current VR= 30V 50 100 µA

ELECTRICAL CHARACTERISTICS:

Test conditions unless otherwise stated: TAMB=25 C

Part

Number VRIFIFSM VFat IRat Capacitance

at VR= 25V, f = 1MHz Package

Max.

VMax.

mA Max.

AMax.

mV IF

mA Max.

␮AVR

VTyp.

pF SOT23

BAT54 30 200 0.6 500 30 250 25 10 SOT23-6

ZHCS2000 40 2000 20 500 2000 1000 30 60 SOT23

ZHCS1000 40 1000 12 500 1000 100 30 25 SOT23

ZHCS750 40 750 12 540 750 100 30 25 SOT23

ZHCS500 40 500 6.75 550 500 40 30 20 SOT23

ZHCS400 40 400 6.75 500 400 40 30 20 SOT323

ZXSC310

ISSUE 1 - SEPTEMBER 2002

TYPICAL CHARACTERISTICS

DEVICE DESCRIPTION

The ZXSC310 is PFM, controller IC which, when

combinedwith a high performance external transistor,

enables the production of a high efficiency boost

converter for LED driving applications. A block

diagram is shown for the ZXSC310 below.

The on chip comparator forces the driver circuit and

therefore the external switching transistor off if the

voltage at ISENSE exceeds 19mV. An internal reference

circuit and divider set this threshold.

The voltage at ISENSE is taken from a current sense

resistor connected in series with the emitter of the

switching transistor. A monostable following the

outputofthecomparatorforcestheturn-offtimeofthe

output stage to be typically 1.7us. This ensures that

there is sufficient time to discharge the inductor coil

before the next on period.

With every on pulse the switching transistor is kept on

until the voltage across the current-sense resistor

exceeds the threshold of the ISENSE input. The on-pulse

length, and therefore the switching frequency, is

determinedbytheprogrammedpeakcurrent,theinput

voltageandtheinputtooutputvoltagedifferential.See

applications section for details.

The driver circuit supplies the external switching

transistor with a fixed drive current. To maximise

efficiency the external transistor switched quickly,

typically being forced off within 30ns.

ZXSC310

ISSUE 1 - SEPTEMBER 2002

DRIVE

ISENSE

VRE

VDRIVE

VCC

STDN

ZXSC310 Block Diagram

REFERENCE DESIGNS

Three typical LED driving applications are shown.

Firstly a typical LCD backlight circuit, then maximum

brightness LED driving solution and lastly an

optimised battery life LED driving solution.

This application shows the ZXSC310 in a typical LCD

backlight application for Digital Still Cameras and

PDA’s.Theinputvoltageforthese backlightcircuits are

usually fixed from the main system power, typically

3.3V or 5V. The LED’s are connected serially so that the

light is distributed uniformly in each LED. The current

provided to the LED’s can either be pulsed or DC. The

DC current is programmable via a sense resistor,

RSENSE, and is set to an optimum LED current of 20mA

for the reference designs. DC current is achieved by

adding a Schottky rectifying diode and an output

capacitor, as shown in the reference design below.

ZXSC310

ISSUE 1 - SEPTEMBER 2002

VIN

VCC

=3.3V/5V

DRIVE

ISENSE

GND

STDN

LCD backlight circuit

Ref Value Part Number Manufacture Comments

U1 N/A ZXSC310E5 Zetex Plc Single cell converter, SOT23-5

Q1 N/A FMMT618 Zetex Plc Low VCE(sat) NPN, SOT23

D1 N/A ZHCS1000 Zetex Plc 1A Schottky diode, SOT23

C1 2.2␮F Generic Various 0805 Size

R16150m⍀Generic Various 1206 Size

R17250m⍀Generic Various 1206 Size

L1868µH Surface mount inductor

Materials list

6Used for 3.3V input, ILED set to 20mA ⫾10%.

7Used for 5V input, ILED to 20mA ⫾10%.

8See Application section.

(Notes)

ZXSC310

ISSUE 1 - SEPTEMBER 2002

This circuit provides a continuous current output to the

LED by rectifying and buffering the DC-DC output. This

ensures maximum LED brightness.

Ref Value Part Number Manufacture Comments

U1 N/A ZXSC310E5 Zetex Plc Single cell converter, SOT23-5

Q1 N/A FMMT617 Zetex Plc Low VCE(sat) NPN, SOT23

D1 1A ZHCS1000 Zetex Plc 1A Shottky diode, SOT23

R1 100m⍀Generic Various 0805 Size

C1 2.2␮FGeneric Various Low ESR ceramic capacitor

L1 100µH8Surface mount inductor

Materials list

8See Application section.

BATT

Maximum brightness solution (Notes)

To ensure optimum efficiency, and therefore

maximum battery life, the LED is supplied with a

pulsed current. Maximum efficiency is ensured with

the removal of rectifier losses experienced in the

maximum brightness solution

ZXSC310

ISSUE 1 - SEPTEMBER 2002

Ref Value Part

Number Manufacture Comments

U1 N/A ZXSC310E5 Zetex Plc Single cell converter, SOT23-5

Q1 N/A FMMT617 Zetex Plc Low VCE(SAT) NPN, SOT23

R1 330m⍀Generic Various 0805 Size

L1 100␮H8Surface mount inductor

Materials list

8See Application section.

Maximum battery life solution

(Notes)

APPLICATIONS INFORMATION

The following section is a design guide for optimum

converter performance.

Switching transistor selection

The choice of switching transistor has a major impact

onthe converterefficiency.For optimumperformance,

a bipolar transistor with low VCE(SAT) and high gain is

required.

The Zetex FMMT618 is an ideal choice of transistor,

having a low saturation voltage. A data sheet for the

FMMT618 is available on Zetex web site or through

your local Zetex sales office. Outline information is

included in the characteristics section of this data

sheet.

Schottky diode selection

For the maximum battery life solution a Schottky

rectifier diode is not required. As with the switching

transistor the Schottky rectifier diode has a major

impact on the converter efficiency. A Schottky diode

with a low forward voltage and fast recovery time

should be used for this application.

The diode should be selected so that the maximum

forward current is greater or equal to the maximum

peakcurrent in theinductor,andthe maximumreverse

voltage is greater or equal to the output voltage.

The Zetex ZHCS1000 meets these needs. Datasheets

for the ZHCS Series are available on Zetex web site or

through your local Zetex sales office. Outline

information is included in the characteristics section of

this data sheet.

For the maximum brightness solution a pulsed current

is supplied to the LED and thus a Schottky rectifier

diode is required.

Inductor selection

The inductor value must be chosen to satisfy

performance, cost and size requirements of the overall

solution. For the LCD backlight reference design we

recommend an inductor value of 68uH with a core

saturation current rating greater than the converter

peak current value and low series resistance.

Inductor selection has a significant impact on the

converter performance. For applications where

efficiencyiscritical,aninductorwithaseriesresistance

of 500m⍀or less should be used.

A list of recommended inductors is shown in the table

below:

ZXSC310

ISSUE 1 - SEPTEMBER 2002

0.0

0.4

0.1

0.2

0.3

Part No. Manufacture L IPK RDC

DO1608C-683 Coilcraft 68␮H 0.4A 0.86⍀

CR54-680 Sumida 68␮H 0.61A 0.46⍀

P1174.683 Pulse 68␮H 0.4A 0.37⍀

SFOP5845-R61680 Samwha 68␮H 0.61A 0.46⍀

SIS43-680 Delta 68␮H 0.4 1.125⍀

Peak current definition

The peak current rating is a design parameter whose

value is dependent upon the overall application. For

the high brightness reference designs, a peak current

of was chosen to ensure that the converter could

provide the required output power to the LED.

In general, the IPK value must be chosen to ensure that

the switching transistor, Q1, is in full saturation with

maximum output power conditions, assuming

worse-case input voltage and transistor gain under all

operating temperature extremes.

Once IPK is decided the value of RSENSE can be

determined by:

A selection guide of sense resistor and inductor values

for given input voltages, output currents and number

of LED connected in series is provided in the table

below.

ZXSC310

ISSUE 1 - SEPTEMBER 2002

RVI

SENSE ISENSE

Input Voltage

(V)

LED current

(mA)

No. of LED’s RSENSE

(m )

Inductor

(H)

Efficiency

(%)

3.3V 10 3 510 68 80

3.3V 10 4 330 68 81

3.3V 10 6 150 68 79

3.3V 20 3 220 68 84

3.3V 20 4 150 68 93

3.3V 20 6 77 68 79

3.3V 30 3 170 68 84

3.3V 30 4 100 68 84

3.3V 30 6 47 68 77

5V 10 3 750 68 83

5V 10 4 510 68 84

5V 10 6 330 68 79

5V 20 3 440 68 85

5V 20 4 250 68 85

5V 20 6 150 68 82

5V 30 3 330 68 86

5V 30 4 170 68 85

5V 30 6 100 68 83

Output Power Calculation

By making the above assumptions for inductance and

peak current theoutputpowercanbedeterminedby:

Note:VOUT=output voltage + Schottky rectifier voltage

drop.

Where

TOFF ≅1.7us (internally set by ZXSC310)

and

Where

Operating frequency can be derived by:

Capacitor selection

For pulsed operation, as in the maximum battery life

solution,nocapacitors are required attheoutputtothe

LED. For rectified operation, as in the maximum

brightness solution, a small value ceramic capacitor is

required, typically 2.2uF.

Generallyaninputcapacitorisnotrequired,butasmall

ceramic capacitor may be added to aid EMC, typically

470nF to 1uF.

(notes)

ZXSC310

ISSUE 1 - SEPTEMBER 2002

()

IIVVT

MIN PK OUT IN OFF

=− −×

()

FTT

ON OFF

III

AV PK MIN

()

TTV V

ON OFF OUT IN

=−

PVVI T

OUT OUT IN AV OFF

ON OFF

=−××

()

Shutdown Control

The ZXSC310 offers a shutdown mode that produces a

standby current of less than 5uA when in operation.

When the voltage at the STDN pin is 0.7V or higher the

ZXSC310 is enabled, hence the driver is in normal

operation. When the voltage at the STDN pin is 0.1V or

lower the ZXSC310 is disabled, hence the driver is in

shutdown mode. If the STDN pin is open circuit the

ZXSC310 is also enabled.

Layout of LCD backlighting solution

Demonstration board

A demonstration board for the LCD backlighting

solution, is available upon request. These can be

obtained through your local Zetex office or through

Zetexwebpages. For all reference designs Gerber files

and bill of materials can be supplied.

ZXSC310

ISSUE 1 - SEPTEMBER 2002

Top Silk Drill File

Top Copper Bottom Copper

ZXSC310

ISSUE 1 - SEPTEMBER 2002

STDN

VCC

Gnd

ISENSE

VDRIVE

Top View

PINOUT DIAGRAM

Pin No. Name Description

CC Supply voltage, generally Alkaline, NiMH or NiCd single cell

nd Ground

TDN Shutdown

SENSE Inductor current sense input. Internal threshold voltage set to 19mV.

Connect external sense resistor

DRIVE Drive output for external switching transistor. Connect to base of

external switching transistor.

PIN DESCRIPTIONS

(NOTES)

ZXSC310

ISSUE 1 - SEPTEMBER 2002

(NOTES)

ZXSC310

ISSUE 1 - SEPTEMBER 2002

ZXSC310

ISSUE 1 - SEPTEMBER 2002

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DIM Millimetres Inches

MIN MAX MIN MAX

A 0.90 1.45 0.035 0.057

A1 0.00 0.15 0.00 0.006

A2 0.90 1.3 0.035 0.051

b 0.35 0.50 0.014 0.020

C 0.09 0.20 0.0035 0.008

D 2.80 3.00 0.110 0.118

E 2.60 3.00 0.102 0.118

E1 1.50 1.75 0.059 0.069

e 0.95 REF 0.037 REF

e1 1.90 REF 0.075 REF

L 0.10 0.60 0.004 0.024

a°010010

SOT23-5 PACKAGE DIMENSIONSSOT23-5 PACKAGE OUTLINE