Data Sheet

HT7939
High Current and Performance White LED Driver
Features
· Input range from 2.6V~5.5V
· Under voltage lock-out protection
· Built-in Power MOSFET
· 1.2MHz fixed switching frequency
· Can drive up to 39 White LEDs with a 5V input
· High efficiency - up to 90%
· Low standby current: 0.1mA (typ.) with VEN low
· 6-pin SOT23-6 package
· Integrated Over-voltage, Over-temperature and
Over-current protection circuits
Applications
· Display Backlighting
-
· LED lighting
Automatic
DVD player
Digital photo frame
Handheld computer
General Description
The device includes over current protection, over temperature protection and under voltage protection preventing damage to the device when the output is
overloaded.
The HT7939 is a high efficiency boost converter for driving White LEDs using current mode operation. The device is designed to drive up to 39 White LEDs from a 5V
power supply. The White LED current is setup using an
external current setting resistor, which has a low feedback voltage of 0.2V to minimise power losses in the resistor which improves efficiency. The Over-voltage
function prevents damage to the IC by turning off the
converter when the LED load is open circuit.
Selection Table
Note:
Part No.
Package
Marking
HT7939
SOT23-6
7939#
7939+
Both lead free and green compound devices are available.
²#² stands for Lead-free devices.
²+² stands for green compound devices, which are Lead-free and Halogen-free.
Rev 1.50
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November 6, 2013
HT7939
Block Diagram
Pin Assignment
S O T 2 3 -6
V IN
6
O V P
5
E N
4
7 9 3 9
1
S W
2
G N D
T o p V ie w
3
F B
Pin Description
Pin No.
Pin Name
Description
1
SW
Switching pin. Internal power MOSFET drain. Connected to inductor and diode.
2
GND
Signal Ground.
3
FB
Feedback pin. Reference voltage. The HT7939 feedback voltage is 200mV. Connect the
sense resistor from FB to GND to set the LED current. Calculate resistor value according to
200mV
.
RFB =
ILED
4
EN
Shutdown & Dimming control input. Don¢t allow this pin to float.
5
OVP
Over voltage protection pin which is connected to the output.
6
VIN
Input supply pin. The input supply pin for the IC. Connect VIN to a supply voltage between
2.6V~5.5V.
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HT7939
Absolute Maximum Ratings
Input Voltage...........................................................6.0V
SW Voltage..............................................................38V
FB Voltage ..............................................................6.0V
EN ..........................................................................6.0V
OVP Voltage ............................................................38V
Operating Temperature Range .............-40°C to +85°C
Storage Temperature Range ..............-55°C to +150°C
Maximum Junction Temperature........................+150°C
Note: These are stress ratings only. Stresses exceeding the range specified under ²Absolute Maximum Ratings² may
cause substantial damage to the device. Functional operation of this device at other conditions beyond those listed
in the specification is not implied and prolonged exposure to extreme conditions may affect device reliability.
Electrical Characteristics
Symbol
VIN=5V, L=10mH, C1=C2=2.2mF, Ta=25°C (unless otherwise specified)
Parameter
Test Conditions
Min.
Typ.
Max.
Unit
VIN
Input Voltage
¾
2.6
¾
5.5
V
UVLO
Under Voltage Lockout
¾
1.8
2.1
2.4
V
IIN
Switching
¾
1.0
2.5
mA
Supply Current
VEN= 0V
¾
0.1
1.0
mA
190
200
210
mV
0.8
1.2
1.6
MHz
85
90
¾
%
Error Amplifier
VFB
Feedback Voltage
¾
Power Switch
fOSC
Switching Frequency
DC
Maximum Duty Cycle
RDS(ON)
SW On Resistance
¾
¾
0.5
¾
W
ISW(OFF)
Switch Leakage Current
¾
¾
0.1
1.0
mA
VIH
EN Voltage High
VIN=2.6V~5.5V
2.0
¾
¾
V
VIL
EN Voltage Low
VIN=2.6V~5.5V
¾
¾
0.8
V
29
32
35
V
Measurement at SW pin
EN Pin
OVP and OCP
VOVP
OVP Threshold
No load
IOCP
N-channel MOSFET Current Limit
¾
1000
1200
¾
mA
Thermal Shutdown Threshold
¾
¾
150
¾
°C
Thermal shutdown Hysteresis
¾
¾
25
¾
°C
Thermal Shutdown
TSHUT
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HT7939
Function Description
Choose an inductor that can handle the necessary
peak current without saturating, and ensure that the inductor has a low DCR to minimise power losses. A
10mH/22mH inductor should be a good choice for most
HT7939 applications. However, a more exact inductance value can be calculated. A good rule for choosing
an inductor value is to allow the peak-to-peak ripple
current to be approximately 30~50% of the maximum
input current. Calculate the required inductance value
using the following equation:
VIN Under-Voltage Lockout - UVLO
The device contains an Input Under Voltage Lockout
(UVLO) circuit. The purpose of the UVLO circuit is to ensure that the input voltage is high enough for reliable operation. When the input voltage falls below the under
voltage threshold, the internal FET switch is turned off. If
the input voltage rises by the under voltage lockout hysteresis, the device will restart. The UVLO threshold is
set below the minimum input voltage of 2.6V to avoid
any transient VIN drops under the UVLO threshold and
causing the converter to turn off.
L =
V
Current Limit Protection
I
The device has a cycle-by-cycle current limit to protect
the internal power MOSFET. If the inductor current
reaches the current limit threshold, the MOSFET will be
turned off. It is import to note that this current limit will not
protect the output from excessive current during an output short circuit. If an output short circuit has occurred,
excessive current can damage both the inductor and diode.
D I
I
L
´ (V
IN
O U T
´ F
V
O U T
=
= (3 0 %
L (P E A K )
- V
O U T
V
IN
´ D I
S W
´ I
´ h
+
IN (M A X )
L
O U T (M A X )
IN
~ 5 0 % ) ´ I
= I
)
1
2
IN (M A X )
D I L
In the equation above, IOUT(MAX) is the maximum load
current, DIL is the peak-to-peak inductor ripple current,
h is the converter efficiency, FSW is the switching frequency and IL(PEAK) is the peak inductor current.
Over-Voltage Protection - OVP
The device provides an over-voltage protection function. If the FB pin is shorted to ground or an LED is disconnected from the circuit, the FB pin voltage will fall to
zero and the internal power MOSFET will switch with its
full duty cycle. This may cause the output voltage to exceed its maximum voltage rating, possibly damaging the
IC and external components. Internal over-voltage protection circuitry turns off the power MOSFET and shuts
down the IC as soon as the output voltage exceeds the
VOVP threshold. As a result, the output voltage falls to
the level of the input supply voltage. The device remains
in shutdown mode until the power is recycled.
· Output Capacitor Selection
The output capacitor determines the steady state output voltage ripple. The voltage ripple is related to the
capacitor¢s capacitance and its ESR (Equivalent Series Resistance). A ceramic capacitor with a low ESR
value will provide the lowest voltage ripple and are
therefore recommended. Due to its low ESR, the capacitance value can be calculated by the equation:
C
Over-Temperature protection - OTP
o u t
=
V
(V
O
O U T
- V
´ F
IN
S W
) ´ IO
´ V
U T
r ip p le
In the equation above, Vripple =peak to peak output ripple, FSW is the switching frequency.
A 1mF~10mF ceramic capacitor is suitable for most application.
A thermal shutdown is implemented to prevent damages due to excessive heat and power dissipation.
Typically the thermal shutdown threshold is 150°C.
When the thermal shutdown is triggered the device
stops switching until the temperature falls below typically 125°C. Then the device starts switching again.
· Input Capacitor Selection
An input capacitor is required to supply the ripple current to the inductor, while limiting noise at the input
source. A low ESR ceramic capacitors is required to
keep the noise at the IC to a minimum.
A 4.7mF~10mF ceramic capacitor is suitable for most
application. This capacitor must be connected very
close to the VIN pin and inductor, with short traces for
good noise performance.
Application Information
· Inductor Selection
The selection of the inductor affects steady state operation as well as transient behavior and loop stability.
There are three important electrical parameters which
need to be considered when choosing an inductor: the
value of inductor, DCR (copper wire resistance) and
the saturation current.
Rev 1.50
IN (M A X )
V
4
November 6, 2013
HT7939
Layout Considerations
· Schottky Diode Selection
The output rectifier diode conducts during the internal
MOSFET is turn off. The average and peak current
rating must be greater than the maximum output current and peak inductor current. The reverse breakdown voltage must be greater than the maximum
output voltage. It is recommended to use a schottky
diode with low forward voltage to minimize the power
dissipation and therefore to maximize the efficiency of
the converter. A 1N5819 type diode is recommended
for HT7939 applications.
Circuit board layout is a very important consideration for
switching regulators if they are to function properly. Poor
circuit layout may result in related noise problems.
In order to minimize EMI and switching noise, please follow the guidelines below:
· All tracks should be as wide as possible.
· The input and output capacitors, C1 and C2, should
be placed close to the VIN, VO and GND pins.
· The Schottky diode, D1, and inductor, L, must be
placed close to the SW pin.
· LED Current Selection
· Feedback resistor, Rfb, must be placed close to the
The LED current is controlled by the current sense
feedback resistor Rfb, The current sense feedback reference voltage is 200mV. In order to have accurate
LED currents, precision resistors are the preferred
type with a 1% tolerance. The LED current can be calculated from the following formula.
I
L E D
=
V
R
F B
fb
=
FB and GND pins.
· A full ground plane is always helpful for better EMI
performance.
A recommended PCB layout with component locations
is shown below.
2 0 0 m V
R fb
Where ILED is the total output LED current, VFB=feedback voltage, Rfb=current sense resistor.
· Digital and Analog Dimming Control
The LED illumination level can be controlled using
both digital and analog methods.
The digital method uses a PWM signal applied to the
EN pin. This is shown in figure 13. The average LED
current increases proportionally with the PWM signal
duty cycle. A 0% duty cycle corresponds to zero LED
current. A 100% duty cycle corresponds to full LED
current. The PWM signal frequency should be set below 1kHz due to the delay time of device startup.
There are two methods of analog LED brightness control. The first method uses a DC voltage to control the
feedback voltage. If the DC voltage range is from 0V
to 3.3V, the selection of resistors control the LED current from 20mA to 0mA as shown in figure14. The
other way is to use a filtered PWM signal, as shown in
figure15. The filtered PWM signal application acts in
the same way as the DC voltage dimming control.
Top Layer
Bottom Layer
Rev 1.50
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November 6, 2013
HT7939
Typical Performance Characteristics
Fig.1 Efficiency VS Input Voltage
Fig.5 Feedback Voltage VS Input Voltage
Fig.2 Switching Frequency VS Input Voltage
Fig.6 Feedback Voltage VS Input Voltage
Fig.3 Supply Current VS Input Voltage
Fig.7 Enable Voltage VS Input Voltage
Fig.4 Supply Current VS Input Voltage
Fig.8 RDS(ON) VS Temperature
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HT7939
Rev 1.50
Fig.9 Switching Waveform
Fig.11 200Hz PWM Dimming Waveform
Fig.10 Open LED Protection
Fig. 12 1kHz PWM Dimming Waveform
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November 6, 2013
HT7939
Application Circuits
V
1 0 m H /2 2 m H
IN
4 .5 V ~ 5 .5 V
1 N 5 8 1 9
L
C 1
2 .2 m F
V IN
S W
E N
O V P
G N D
C 2
2 .2 m F
V
O U T
3 S 1 3 P
F B
R
F B
0 .7 7 W
L : G S 5 4 -1 0 0 K (G A N G S O N G )
C 1 : G R M 2 1 B R 6 1 E 4 7 5 K A 1 2 L (M U R A T A )
C 2 : G R M 2 1 B R 7 1 E 2 2 5 K A 7 3 L (M U R A T A )
Fig.12 Application Circuits for Driving 39 WLEDs
V
4 .5 V ~ 5 .5 V
P W M
1 0 m H /2 2 m H
IN
C 1
2 .2 m F
S ig n a l
1 N 5 8 1 9
L
V IN
S W
E N
O V P
G N D
C 2
2 .2 m F
V
O U T
3 S 1 3 P
F B
R
F B
0 .7 7 W
Fig.13 Application Circuit for Dimming Control Using a PWM Logic Signal
V IN
4 .5 V ~ 5 .5 V
1 0 m H /2 2 m H
C 1
2 .2 m F
1 N 5 8 1 9
L
V IN
S W
E N
O V P
G N D
C 2
2 .2 m F
V
O U T
3 S 1 3 P
1 0 k W
F B
1 5 0 k W
R
F B
0 .7 7 W
V D C D im m in g
0 V ~ 3 .3 V
Fig.14 Application Circuit for Dimming Control Using a DC Voltage
Rev 1.50
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November 6, 2013
HT7939
V
4 .5 V ~ 5 .5 V
1 N 5 8 1 9
1 0 m H /2 2 m H
IN
C 1
2 .2 m F
L
V IN
S W
E N
O V P
3 .3 V
0 V
O U T
1 0 k W
1 0 k W
P W M
V
3 S 1 3 P
F B
G N D
C 2
2 .2 m F
S ig n a l
1 5 0 k W
R
F B
0 .7 7 W
0 .1 m F
Fig.15 Application Circuit for Dimming Control Using a Filtered PWM Signal
V IN
5 .0 V
1 0 m H /2 2 m H
C 1
2 .2 m F
1 N 5 8 1 9
L
V IN
S W
E N
O V P
G N D
C 2
2 .2 m F
V
O U T
3 5 0 m A
F B
R
F B
0 .7 7 W
Fig.16 Application Circuit for Drive 3 High Brightness LEDs
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November 6, 2013
HT7939
Package Information
Note that the package information provided here is for consultation purposes only. As this information may be updated
at regular intervals users are reminded to consult the Holtek website for the latest version of the package information.
Additional supplementary information with regard to packaging is listed below. Click on the relevant section to be transferred to the relevant website page.
· Further Package Information (include Outline Dimensions, Product Tape and Reel Specifications)
· Packing Meterials Information
· Carton information
· PB FREE Products
· Green Packages Products
Rev 1.50
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November 6, 2013
HT7939
6-pin SOT23-6 Outline Dimensions
Symbol
A
Nom.
Max.
¾
¾
0.057
A1
¾
¾
0.006
A2
0.035
0.045
0.051
b
0.012
¾
0.020
C
0.003
¾
0.009
D
¾
0.114 BSC
¾
E
¾
0.063 BSC
¾
e
¾
0.037 BSC
¾
e1
¾
0.075 BSC
¾
H
¾
0.110 BSC
¾
L1
¾
0.024 BSC
¾
q
0°
¾
8°
Symbol
A
Rev 1.50
Dimensions in inch
Min.
Dimensions in mm
Min.
Nom.
Max.
¾
¾
1.45
A1
¾
¾
0.15
A2
0.90
1.15
1.30
b
0.30
¾
0.50
C
0.08
¾
0.22
D
¾
2.90 BSC
¾
E
¾
1.60 BSC
¾
e
¾
0.95 BSC
¾
e1
¾
1.90 BSC
¾
H
¾
2.80 BSC
¾
L1
¾
0.60 BSC
¾
q
0°
¾
8°
11
November 6, 2013
HT7939
Copyright Ó 2013 by HOLTEK SEMICONDUCTOR INC.
The information appearing in this Data Sheet is believed to be accurate at the time of publication.
However, Holtek assumes no responsibility arising from the use of the specifications described.
The applications mentioned herein are used solely for the purpose of illustration and Holtek
makes no warranty or representation that such applications will be suitable without further modification, nor recommends the use of its products for application that may present a risk to human
life due to malfunction or otherwise. Holtek¢s products are not authorized for use as critical components in life support devices or systems. Holtek reserves the right to alter its products without
prior notification. For the most up-to-date information, please visit our web site at
http://www.holtek.com.tw.
Rev 1.50
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November 6, 2013