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PDF HT7939 Data sheet ( Hoja de datos )
| Número de pieza | HT7939 | |
| Descripción | High Current and Performance White LED Driver | |
| Fabricantes | Holtek Semiconductor | |
| Logotipo |  |
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HT7939
High Current and Performance White LED Driver
Features
· Input range from 2.6V~5.5V
· Built-in Power MOSFET
· Can drive up to 39 White LEDs with a 5V input
· Low standby current: 0.1mA (typ.) with VEN low
· Integrated Over-voltage, Over-temperature and
Over-current protection circuits
· Under voltage lock-out protection
· 1.2MHz fixed switching frequency
· High efficiency - up to 90%
· 6-pin SOT23-6 package
Applications
· Display Backlighting
- Automatic
- DVD player
- Digital photo frame
- Handheld computer
· LED lighting
General Description
The HT7939 is a high efficiency boost converter for driv-
ing White LEDs using current mode operation. The de-
vice 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 feed-
back voltage of 0.2V to minimise power losses in the re-
sistor which improves efficiency. The Over-voltage
function prevents damage to the IC by turning off the
converter when the LED load is open circuit.
The device includes over current protection, over tem-
perature protection and under voltage protection pre-
venting damage to the device when the output is
overloaded.
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Rev 1.10
1 November 3, 2009
1 page  
HT7939
· 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 cur-
rent and peak inductor current. The reverse break-
down 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.
· LED Current Selection
The LED current is controlled by the current sense
feedback resistor Rfb, The current sense feedback ref-
erence 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 cal-
culated from the following formula.
ILE D =
V FB
R fb
=
200m V
R fb
Where ILED is the total output LED current, VFB=feed-
back 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 14. 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 be-
low 1kHz due to the delay time of device startup.
There are two methods of analog LED brightness con-
trol. The first method uses a DC voltage to control the
feedback voltage. If the DC voltage range is from 0V
www.DataSheet4U.ctoom3.3V, the selection of resistors control the LED cur-
rent from 20mA to 0mA as shown in figure15. The
other way is to use a filtered PWM signal, as shown in
figure16. The filtered PWM signal application acts in
the same way as the DC voltage dimming control.
Layout Considerations
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 fol-
low 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.
· Feedback resistor, Rfb, must be placed close to the
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.
Top Layer
Rev 1.10
Bottom Layer
5 November 3, 2009
5 Page 
Product Tape and Reel Specifications
Reel Dimensions
T2
D
HT7939
AB
C
T1
SOT23-6
Symbol
A
B
C
D
T1
T2
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Description
Reel Outer Diameter
Reel Inner Diameter
Spindle Hole Diameter
Key Slit Width
Space Between Flange
Reel Thickness
Dimensions in mm
178.0±1.0
62.0±1.0
13.0±0.2
2.50±0.25
8.4 +1.5/-0.0
11.4 +1.5/-0.0
Rev 1.10
11 November 3, 2009
11 Page | ||
| Páginas | Total 13 Páginas | |
| PDF Descargar | [ Datasheet HT7939.PDF ] | |
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