Diodes ZXSC400 User Manual

LED DRIVER BOOST CONVERTER

DESCRIPTION

The ZXSC400 is a voltage mode boost converter in the SOT23-6 package. Its low feedback voltage allows the current in a chain of LEDs to be set and accurately monitored with asingle resistor giving minimal losses. Its excellent load and line regulation means that for the full supply range from lithium-ion cells, the LED currentwilltypicallychangebyless than 1%.Usinghigh efficiency Zetexswitching transistorswith ratings of 20V and higher allowmany LEDs to be chained inseries for the best LED current matching possible.

ZXSC400

FEATURES

1.8V to 8V supply range

•

Typical output regulation of ⫾1%

•

Over 80% typical efficiency

•

4.5␮A typical shutdown current

•

Series connection for ultimate LED current matching

•

APPLICATIONS

•

White LED backlighting for colour LCD panels

•

General LED backlighting

•

High performance white LED flashlights

•

General LED driving from batteries

ORDERING INFORMATION

DEVICE REEL

ZXSC400E6TA

SIZE

7“ 8mm 3000 units

TAPE

WIDTH

QUANTITY

PER REEL

DEVICE MARKING

•

C400

PINOUT

GND

STDN

Top View

DRIVE V

SENSE

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ZXSC400

ABSOLUTE MAXIMUM RATINGS

DRIVE -0.3V to V EOR -0.3V to V

STDN -0.3V to The lower of (+5.0V) or (V SENSE -0.3V to The lower of (+5.0V) or (V Operating Temp. -40°C to +85°C Storage Temp. -55°C to +125°C Power Dissipation 450mW

ELECTRICAL CHARACTERISTICS

Test Conditions V

Symbol Parameter Conditions Limits Units

Supply parameters

Iq I

STDN

Eff Acc

REF

TCO

REF

DRIVE

OSC

Input parameters

SENSE

Output parameters

OUT

DRIVE

dILD

Note

Excluding gate/base drive current.

IFBis typically half these values at 300mV.

System not device spec, including recommended transistors. 10.5V represents 3 white LEDs.

Change in LED current with changing supply voltage (LED VFx number of series LEDs).

-0.3V to +10V + 0.3V

+ 0.3V

= 3V, T= -40°C to 85°C unless otherwise stated.

CC CC

+ 0.3V) + 0.3V)

Min Typ Max

VCCRange 1.8 8 V Quiescent Current VCC= 8V 220 ␮A Shutdown Current 4.5 ␮A Efficiency 30mA > I

> 10mA 80 %

LED

Reference tolerance 1.8V < VCC< 8V -3.0 3.0 % Reference Temp Co 0.005 %/⬚C Discharge pulse width 1.8V < VCC<8V 1.7 ␮ s Operating Frequency 200 kHz

Sense voltage 22 28 34 mV Sense input current VFB=0V,V

= 0V -1 -7 -15 ␮A

SENSE

Feedback voltage 291 300 309 mV Feedback input current VFB=0V,V Shutdown high voltage 1.5 V

= 0V -1.2 -4.5 ␮A

SENSE

Shutdown low voltage 0 0.55 V Line voltage regulation 0.5 %/V

Output current Vin > 2V, Vout = 10.5V 50 mA Transistor drive current Vdrive = 0.7V 2 3.5 5 mA Transistor voltage drive 1.8V < VCC<8V 0 V

-0.4 V

Mosfet gate drive cpbty 300 pF Load current regulation 0.1 mA/V

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TYPICAL CHARACTERISTICS

ZXSC400

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ZXSC400

DESIGN INFORMATION IC operation description

Bandgap Reference

All threshold voltages and internal currents are derived from a temperature compensated bandgap reference circuit with a reference voltage of 1.22V nominal.

Dynamic Drive Output

Depending on the input signal, the output is either “LOW” or “HIGH”. In the high state a 2.5mA current source (max drive voltage = V or gate of the external transistor. In order to operate the external switching transistor at optimum efficiency, both output states are initiated with a short transientcurrent in order to quickly discharge the base or the gate of the switching transistor.

Switching Circuit

The switching circuit consists of two comparators, Comp1 and Comp2, a gate U1, a monostable and the driveoutput.Normally the DRIVE output is “HIGH”; the external switching transistor is turned on. Current ramps up in the inductor, the switching transistor and external current sensing resistor. This voltage is sensed by comparator, Comp2, at input I the current sense voltage across the sensing resistor exceeds 30mV, comparator Comp2 through gate U1 triggers a re-triggerable monostable and turns off the output drive stage for 2␮s. The inductor discharges to the load of the application. After 2␮s a new charge cycle begins, thus ramping the output voltage. When the output voltage reaches the nominal value and FB gets an input voltage of more than 300mV, the monostable is forced “on” from Comp1 through gate U1, until the feedback voltage falls below 300mV. The above action continues to maintain regulation.

-0.4V) drives the base

. Once

SENSE

Block diagram

2µs

Pin functions

Pin No. Name Description

1V 2 GND Ground 3 STDN Shutdown

4 SENSE

6 DRIVE

Supply voltage, 1.8V to 8V.

Inductor current sense input. Internal threshold voltage set to 30mV. Connect external sense resistor

Reference voltage. Internal threshold set to 300mV. Connect external resistor to program LED current

Drive output for external switching transistor. Connect to base or gate of external switching transistor.

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APPLICATIONS INFORMATION

ZXSC400

Switching transistor selection

The choice of switching transistor has a major impact ontheconverterefficiency.Foroptimumperformance, a bipolar transistor with low V required.TheV important parameter as this sees the full output

oftheswitchingtransistorisalsoan

CEO

and high gain is

CE(SAT)

voltage when the transistor is switched off. Zetex SuperSOT™ transistors are an ideal choice for this application.

Schottky diode selection

As with the switching transistor, the Schottky rectifier diodehasamajor 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 rating is greater or equal to the maximum peak current in the inductor, and the maximum reverse voltage is greater or equal to the output voltage. The Zetex ZHCS Series meet these needs.

Combination devices

To minimise the external component count Zetex recommends the ZX3CDBS1M832 combination of NPN transistor and Schottky diode in a 3mm x 2mm MLP package. This device is recommended for use in applications using 1 to 4 white LEDs.

Capacitor Selection

Asmall value, low ESR ceramic capacitor isrequired to filter the output, typically 1␮F to 4.7␮F.

Generallyan input capacitor is notrequired,buta small ceramic capacitor may be added to aid EMC, typically 1␮F to 4.7␮F.

Inductor Selection

The inductor value must be chosen to satisfy performance, cost and size requirements of the overall solution.

Inductor selection has a significant impact on the converter performance. For applications where efficiencyis critical, an inductor withaseriesresistance of 250m⍀ or less should be used.

A list of recommended inductors is listed in the table below:

The IC is also capable of driving MOSFETs. Zetex recommends the ZXMNS3BM832 combination of low threshold voltage N-Channel MOSFET and Schottky diode in a 3mm x 2mm MLP package. This device is recommendedfor use in applications using1to8 white LEDs.

Part No. Manufacture L I

CMD4D11-100MC Sumida 10␮H 0.5 0.457 CMD4D11-220MC Sumida 22␮H 0.4 0.676 LPO2506OB-103 Coilcraft 10␮H 1.0 0.24 ST2006103 Standex Electronics Inc 10␮H 0.6 0.1

(A) RDC( )

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