HT7939A-2 UVC LED Driver Application
AN0561EN V1.10 1 / 9 August 26, 2020
HT7939A-2 UVC LED Driver Application
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
HT7939A-2 UVC LED Driver Application
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
HT7939A-2 UVC LED Driver Application
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
HT7939A-2 UVC LED Driver Application
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
HT7939A-2 UVC LED Driver Application
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
Operating temperature: -30°C ~ 60°C
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.
HT7939A-2 UVC LED Driver Application
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
HT7939A-2 UVC LED Driver Application
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
Operating temperature: -30°C ~ 60°C
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
HT7939A-2 UVC LED Driver Application
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
HT7939A-2 UVC LED Driver Application
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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