ht7939a-2 uvc led driver application...lamp panel current setting : current = 200mv/rfb ( max....

HT7939A-2 UVC LED Driver Application

AN0561EN V1.10 1 / 9 August 26, 2020


D/N: AN0561EN

Introduction

UVC refers to C-region ultraviolet light or germicidal ultraviolet light with a wavelength between

200nm and 280nm. Due to the features of short wavelength and high energy, UVC light can kill micro-

organisms such as bacteria, viruses and other pathogens in a short time by destroying the DNA or RNA

molecular structure which can block their reproduction. Therefore, UVC wavelength devices can be

widely used in products such as UVC disinfection boxes, UVC disinfection and sterilization sticks,

UVC disinfection torchlights and other home products equipped with sterilizing function.

For this application requirement, Holtek launches the application example. The UVC LED driver

using a boost constant current driving method is able to drive UVC LEDs which have a higher

forward voltage than input power supply. The device also provides multiple protection mechanisms

such as under voltage lockout, over-current protection, over-voltage protection and over-

temperature protection. Holtek UVC LED driver application example can drive a light board with

1~4 UVC LEDs. The driving current can be adjusted by users according to their lamp panel

requirements.

Power Source

Multiple cells dry battery

5V USB

Single cell lithium battery

Multiple cells lithium battery

Adapter AC-DC converter power supply

LDO

HT7150-1

*Optional

HT7939A-2 Constant Current Drive

(Max. 4S)

UVC LEDBoard

LED Power Supply

High Efficiency Boost Constant Current IC

MCUCharge

Management Touch Keys

Voice Man-machine

System Master

Enable/adjust light

Figure 1. Application and System Block Diagram


AN0561EN V1.10 2 / 9 August 26, 2020

Functional Description

UVC LED Driver

Input Power Supply

Single cell lithium battery: 3.0V~4.2V

USB Port 5V

Dual cells lithium battery: 6.0V~8.4V

Three cells lithium battery: 9.0V~12.6V

12V/24V adapter

Lamp panel current setting: Current = 200mV/Rfb (Max. current is shown in the next “Safe operation

ranges” item)

Rfb (Ω) Lamp Panel Current (mA) 8 25 5 40 2 100

1.7 120 0.8 250

0.77 260 0.66 300 0.5 400

Safe operation ranges: Introduced respectively based on the number of the UVC LEDs in series

on the lamp panel.

(LED: VF=6.6V at 25°C & 60mA; Manufacturer: Jason; Model No: J275S35C)

1 Series UVC LED

2 Series UVC LEDs


AN0561EN V1.10 3 / 9 August 26, 2020

3 Series UVC LEDs

On/Off Control

The driver on/off is controlled by an MCU provided signal

Controlled by the VIN voltage

Operating Temperature: -30°C ~ 60°C

Operating Principles

The HT7939A-2 chip is used to implement the UVC LED driver function. Its output current can be

set up by adjusting the external resistance, Rfb. As the UVC LED forward voltage, ranging from about

5V to 8V, is higher than input voltage, the boost structure of this device will be suitable for the UVC

LED driver applications. In addition, the constant current driving method make sure that the UVC

LED wattage is less affected by current errors, so that the luminance of each UVC LED will be uniform.

The driver input voltage can be directly powered by a 5V USB or a single-cell lithium battery.

However, if the driver input voltage is higher than 5V, a HT7150-1 LDO regulator should be placed

between the input voltage and the VIN pin of the HT7939A-2. This LDO outputs a 5V voltage to the

HT7939A-2. The HT7939A-2 output can drive 1~3 UVC LEDs in series on the lamp panel.

Hardware Description

Figure 2. UVC LED Driver Schematic – Using MCU to Control On/Off via EN Pin

Figure 3. UVC LED Driver Schematic – Using Input Voltage to Control On/Off


AN0561EN V1.10 4 / 9 August 26, 2020

Figure 4. UVC LED Driver Schematic – Input Voltage Is higher than 5V

The HT7939A-2 is a high efficiency UVC LED driver.

Input voltage range: 2.6V~5.5V

Operating temperature: -40°C ~ 85°C

EN pin used to control the lamp panel on/off.

Additionally, the HT7939A-2 provides the following protection functions.

Protection Trigger Conditions Output Voltage Internal Power MOSFET Release Conditions

UVLO Input voltage<2.1V =Input voltage Off Input voltage>2.1V *OCP Peak inductor current>1.2A Unstable Limited duty cycle Peak inductor current<1.2A OVP Output voltage>32V =Input voltage Off Repowered or restart EN signal OTP IC interior temperature>150°C =Input voltage Off IC interior temperature<125°C

Note: * The HT7939A-2 does not offer short-circuit protection function. If a short-circuit occurs,

the peripheral components and the chip may be damaged.

Hardware Considerations

The top view of the UVC LED driver layout is shown below.

There are four points should be noted during the PCB layout routing.

1. The input capacitor C1 should be placed as close as possible to the HT7939A-2.

2. The wire diameters of the inductor and the input power tracks should be greater than 0.6mm.

3. The GND pin to the input GND loop should be as short as possible.

4. The distance between the resistor Rfb and the inductor should be high than 3mm.

Figure 5. PCB Top View

Figure 6. PCB Bottom View


AN0561EN V1.10 5 / 9 August 26, 2020

PCB BOM List

1. VIN < 5.5V Reference Package Description Part Number Mfgr.

U1 SOT23-6 2.6~5.5VIN, LED Driver HT7939A-2 Holtek U2 SOT23-5 N.C. HT715 — L 5.2mm×5.2mm 10µH, 1.2A GS54-100K GangSong

D1 DO-214AC Schottky (1A 40V) SM5819 GW C1 SMD 0805 CAP 2.2µF /50V — — C2 SMD 0805 CAP 2.2µF/50V — — C3 SMD 0805 N.C. — — R1 SMD 0603 0Ω — — Rfb SMD 0603 1/10W, 2Ω (default) — —

2. VIN > 5.5V Reference Package Description Part Number Mfgr.

U1 SOT23-6 2.6~5.5VIN, LED Driver HT7939A-2 Holtek U2 SOT23-0035 30mA TinyPowerTM LDO HT7150-1 Holtek L 5.2mm×5.2mm 10µH, 1.2A GS54-100K GangSong

D1 DO-214AC Schottky (1A 40V) SM5819 GW C1 SMD 0805 CAP 10µF /50V — — C2 SMD 0805 CAP 2.2µF/50V — — C3 SMD 0805 CAP 10µF/50V — — R1 SMD 0603 N.C. — — Rfb SMD 0603 1/10W, 2Ω (default) — —

Test Data

Example 1: Take driving a lamp panel with 2 series and 2 parallel UVC LEDs as an example to

explain.

Input power supply: 3.0V~4.2V, single cell lithium battery input

UVC LEDs

Current vs. Voltage LED Current

Forward Voltage

2V 4V 5V 7V6V3V1V

0.04A

0.1A


Setup lamp panel current

Step1: Calculate the current

As shown in the UVC LED “Current vs. Voltage” Curve above, for the lamp panel with

2 series and 2 parallel LEDs, when the lamp panel current is set to 120mA, the voltage

is 13.3V, the wattage (total current × total voltage) is about 1.6W.

Step2: Check the HT7939A safe operation voltage

As shown in the figure in “Safe operation ranges >> 2 Series UVC LEDs”, for the lamp

panel with 2 series connected LEDs, operated under an input voltage of 3V~4.2V and at

the temperature between -30°C and 60°C, the maximum current is 120mA. Within the

lithium battery voltage range region, if the current is 120mA, an output power of 1.6W is

allowed for two series connected UVC LEDs.


AN0561EN V1.10 6 / 9 August 26, 2020

Step3: Setup the lamp panel current

Lamp panel current = 200mV/Rfb

120mA = 200mV/Rfb

Rfb = 1.7Ω

On/Off control: Connect EN pin to the VIN, the driver on/off is controlled by the battery voltage

Actual test data

UVC LED Lamp Panel Voltage vs. Battery Voltage

UVC LED Lamp Panel Current vs. Battery Voltage

UVC LED Lamp Panel Wattage vs. Battery Voltage

Efficiency vs. Battery Voltage


AN0561EN V1.10 7 / 9 August 26, 2020

Example 2: Take three cells lithium battery driving a lamp panel with 3 series and 2 parallel UVC

LEDs as an example to explain.

Input power supply: 9.0V~12.6V, three cells lithium battery input

Driver ICs: HT7150-1 + HT7939A-2

UVC LEDs

Current vs. Voltage

LED Current

Forward Voltage

2V 4V 5V 7V6V3V1V

0.04A

0.1A


Setup lamp panel current

Step1: Calculate the current

As shown in the UVC LED “Current vs. Voltage” Curve above, for the lamp panel with

3 series and 2 parallel LEDs, when the lamp panel current is set to 260mA, the voltage

is 19.8V, the wattage (total current × total voltage) is about 5.15W.

Step2: Check the HT7939A-2 safe operation voltage

As shown in the figure in “Safe operation ranges >> 3 Series UVC LEDs”, for the lamp

panel with 3 series connected LEDs, operated under an input voltage of 9V~12.6V and

at the temperature between -30°C and 60°C. Within the lithium battery voltage range

region, if the current is 260mA, an output power of 5.15W is allowed for three series

connected UVC LEDs.

Step3: Setup the lamp panel current

Lamp panel current = 200mV/Rfb

260mA = 200mV/Rfb

Rfb = 0.77Ω

On/Off control: Connect EN pin to the VIN, the driver on/off is controlled by the battery voltage

Actual test data

UVC LED Lamp Panel Voltage vs. Battery Voltage

1516171819202122232425

9 10 11 12 13

LED

V F (V

)

VIN (V)

Ta=60°C

Ta=-30°C


AN0561EN V1.10 8 / 9 August 26, 2020

UVC LED Lamp Panel Current vs. Battery Voltage

UVC LED Lamp Panel Wattage vs. Battery Voltage

Efficiency vs. Battery Voltage

Conclusion

This article provides two application examples about the Holtek UVC LED driver. It uses the

HT7939A-2 in boost constant current driving mode to drive UVC LEDs of forward voltage higher

than input voltage and can drive a maximum 5-Watts lamp panel.

Reference Material

Reference file: HT7939A datasheet.

For more information see Holtek website www.holtek.com.

240

245

250

255

260

265

270

275

280

9 10 11 12 13

LED

I OU

T(m

A)VIN (V)

Ta=60°C

Ta=-30°C

Ta=25°C

3

3.5

4

4.5

5

5.5

6

6.5

9 10 11 12 13

LED

WO

UT

(W)

VIN (V)

Ta=60°C

Ta=-30°C

Ta=25°C

85.00%

87.00%

89.00%

91.00%

93.00%

95.00%

97.00%

9 10 11 12 13

Effic

ienc

y

VIN (V)

Ta=60°C

Ta=-30°C

Ta=25°C

http://www.holtek.com/


AN0561EN V1.10 9 / 9 August 26, 2020

Version and Modification Information

Version Information

Data Author Issue Release 2020.07.16 邱執豪 V1.10

2020.05.06 游傑安 V1.00

Modification Information

V1.10: Add applications with input voltage VIN higher than 5V.

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ht7939a-2 uvc led driver application...lamp panel current setting : current = 200mv/rfb ( max....

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