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PDF HT7938 Data sheet ( Hoja de datos )
| Número de pieza | HT7938 | |
| Descripción | High Current And Performance White LED Driver | |
| Fabricantes | Holtek Semiconductor | |
| Logotipo |  |
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HT7938
High Current and Performance White LED Driver
Features
· Efficiency up to 85% at VIN=3.6V, 9LEDs,
ILED=20mA
· 1.2MHz fixed switching frequency
· Low Standby Current: 0.1mA, VEN=0V (typ.)
· Matches LED current
· Tiny inductor and capacitors
· Up to 9 strings White-LED
· Tiny 6-lead SOT23-6 package
· Built in OVP, OCP, OTP, UVLO protection
Applications
· Cellular phones
· PDAs
· DSCs
· GPS
· Handheld devices
· White LED display backlighting
General Description
HT7938 is high efficiency boost converter with constant
current output to provide backlight in handheld devices.
Series connection of LEDs provides constant identical
LED currents resulting in uniform brightness. The con-
tinuous LED current is set with FB pin regulated voltage
across an external sense resistor (RFB) connected from
that pin to ground. The built-in open load protection pre-
vents the damage resulting from an open circuit condi-
tion. Low 200mV feedback voltage minimizes power
loss in the current setting resistor for better efficiency.
HT7938 switches at rates up to 1.2MHz to allow the use
of extremely small inductor and filter capacitor.
Block Diagram
36V
www.DataSheet4U.com
Rev 1.00
1 November 11, 2009
1 page  
HT7938
· 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 HT7938 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 7. 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
to 3.3V, the selection of resistors control the LED cur-
rent from 20mA to 0mA. The other way is to use a fil-
tered PWM signal, as shown in figure 9. The filtered
PWM signal application acts in the same way as the
DC voltage dimming control.
www.DataSheet4U.com
Rev 1.00
5 November 11, 2009
5 Page 
HT7938
Holtek Semiconductor Inc. (Headquarters)
No.3, Creation Rd. II, Science Park, Hsinchu, Taiwan
Tel: 886-3-563-1999
Fax: 886-3-563-1189
http://www.holtek.com.tw
Holtek Semiconductor Inc. (Taipei Sales Office)
4F-2, No. 3-2, YuanQu St., Nankang Software Park, Taipei 115, Taiwan
Tel: 886-2-2655-7070
Fax: 886-2-2655-7373
www.DataSheet4U.cFoamx: 886-2-2655-7383 (International sales hotline)
Holtek Semiconductor Inc. (Shenzhen Sales Office)
5F, Unit A, Productivity Building, No.5 Gaoxin M 2nd Road, Nanshan District, Shenzhen, China 518057
Tel: 86-755-8616-9908, 86-755-8616-9308
Fax: 86-755-8616-9722
Holtek Semiconductor (USA), Inc. (North America Sales Office)
46729 Fremont Blvd., Fremont, CA 94538, USA
Tel: 1-510-252-9880
Fax: 1-510-252-9885
http://www.holtek.com
Copyright Ó 2009 by HOLTEK SEMICONDUCTOR INC.
The information appearing in this Data Sheet is believed to be accurate at the time of publication. However, Holtek as-
sumes 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 1.00
11 November 11, 2009
11 Page | ||
| Páginas | Total 11 Páginas | |
| PDF Descargar | [ Datasheet HT7938.PDF ] | |
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