HOLTEK HT7938A

HT7938A
High Current and
Performance White LED Driver
Feature
General Description
• Efficiency up to 90% at VIN=4.0V, 5S2P,
ILED=20mA
The HT7938A is high efficiency boost converter with
constant current output for backlight applications in
handheld devices. Using a series connection of LEDs
provides constant identical LED currents resulting in
uniform brightness. The continuous LED current is
set using the device FB pin regulated voltage across
an external sense resistor, RFB, connected between this
pin and ground. The integrated open load protection
feature prevents device damage resulting from
open circuit load conditions. A low 200mV/300mV
feedback voltage minimises power losses in the
current setting resistor which provides enhanced
efficiency. The HT7938A has a dimming frequency of
up to 200kHz, which has excellent linear performance
over this dimming frequency range. The device
switches at rates of up to 1.2MHz to allow the use of
extremely small inductors and filter capacitors.
• 1.2MHz fixed switching frequency
• Low standby current: 0.1mA (typ.) at VEN=0V
• Matches LED current
• Tiny inductor and capacitors
• EN pin dimming frequency up to 200kHz
• Up to 10 strings White-LED (LED VF(Max.)=3.5V)
• Tiny 6-lead SOT23-6 package
• Built in OVP, OCP, OTP, UVLO protection
Applications
• Cellular phones
• PDAs
• DSCs
• Handheld devices
• White LED display backlighting
Selection Guide
Part No.
Package
Marking
HT7938A
SOT23-6
38A-2 (VFB=200mV)
38A-3 (VFB=300mV)
Note: Both lead free and green compound devices are available.
Rev. 1.00
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April 19, 2012
HT7938A
Block Diagram
   
 Pin Assignment
Pin Description
Pin Name
I/O
Desciption
SW
I
Switching pin. Internal power MOSFET drain. Connected to inductor and diode.
GND
―
FB
I
Feedback pin. Reference voltage. The HT7938A feedback voltage is
200mV/300mV. Connect the sense resistor from FB to GND to set the LED
current. Calculate resistor value according to RFB=200mV(300mV)/ILED.
EN
I
Shutdown & Dimming control input. Don't allow this pin to float.
When dimming control input high to low level period more than 20ms and have
no other dimming signal, the device is entered shutdown mode.
VOP
O
Over voltage protection pin which is connected to the output.
VIN
I
Input supply pin. The input supply pin for the IC. Connect VIN to a supply
voltage between 2.6V~5.5V.
Rev. 1.00
Signal Ground.
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April 19, 2012
HT7938A
Absolute Maximum Ratings
Input Voltage ........................................................ 6.0V
OVP Voltage ......................................................... 46V
SW Voltage ........................................................... 46V
Operating Temperature Range ........... -40°C to +85°C
FB Voltage ........................................................... 6.0V
Maximum Junction Temperature ...................... 150°C
EN ........................................................................ 6.0V
Note: These are stress ratings only. Stresses exceeding the range specified under Absolute Maximum Ratings 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
VIN= 3.6V, L=22mH, CIN=1mF, COUT=4.7mF, ILED=20mA, Ta=25°C, unless otherwise specified (Note 1)
Symbol
Parameter
Test Conditions
Min.
Typ.
Max.
Unit
Input Voltage
―
2.6
―
5.5
V
Under Voltage Lockout
―
1.8
2.1
2.4
V
Input Supply Voltage and Current
VIN
UVLO
IIN
Supply Current
Switching
―
1.0
5.0
mA
VEN=0
―
0.1
1.0
μA
For 38A-2 Marking
190
200
210
mV
For 38A-3 Marking
285
300
315
mV
―
1.0
―
%
0.8
1.2
1.6
MHz
Error Amplifier
VFB
Feedback Voltage
VLine
Line Regulation
VIN=3.0~4.3V, ILED=20mA
Power Switch
fOSC
Switching Frequency
DC
Maximum Duty Cycle
Measured at SW Pin
92
95
―
%
RDS(ON)
SW ON Resistance
―
―
0.7
―
Ω
ISW(OFF)
Switch Leakage Current
―
―
0.1
1.0
μA
VIH
EN Voltage High
―
2.0
―
―
V
VIL
EN Voltage Low
fEN
Dimming Clock Rate
Enable
―
―
0.8
V
Duty= 5%~100%
―
0.1
―
200
kHz
No Load
36
39
42
V
―
―
1000
―
mA
Thermal Shutdown Threshold
―
―
150
―
°C
Thermal Shutdown Hysteresis
―
―
25
―
°C
OVP and OCP
VOVP
OVP Threshold
IOCP
N-channel MOSFET Current limit
Thermal Shutdown
tSHUT
Note 1. Specifications are production tested at Ta=25 degree. Specifications over -40°C to 85°C degree operating
temperature range are assured by design, characterization.
Rev. 1.00
3
April 19, 2012
HT7938A
Function Description
parameters which need to be considered when
choosing an inductor: the value of inductor, DCR
(copper wire resistance) and the saturation current.
VIN Under-Voltage Lockout -- UVLO
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 HT7938A applications. However, a more exact
inductance value can be calculated. A good rule for
choosing an inductor value is to allow the peak-topeak ripple current to be approximately 30~50% of
the maximum input current. Calculate the required
inductance value using the following equation:
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.
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 important 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.
I
L
(V
IN
V
IN (M A X )
I
F
V
O U T
=
I
IN (M A X )
L
O U T (M A X )
IN
~ 5 0 % ) I
= I
L (P E A K )
)
IN
I
S W
V
= (3 0 %
V
O U T
O U T
+
1
2
IN (M A X )
I L
In the equation above, I OUT(MAX) is the maximum
load current, DIL is the peak-to-peak inductor ripple
current, η is the converter efficiency, F SW is the
converter 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 OVP 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 restarted.
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:
In the equation above, Vripple =peak to peak output
ripple, FSW is the switching frequency.
Over-Temperature Protection -- OTP
A 2.2mF~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 1mF~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
Rev. 1.00
V
L =
4
April 19, 2012
HT7938A
Schottky Diode Selection
Layout Considerations
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 HT7938A 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.
• Feedback resistor, Rfb, must be placed close to the
FB and GND pins.
• A full ground plane is always helpful for better
EMI performance.
LED Current Selection
The LED current is controlled by the current sense
feedback resistor R fb, The current sense feedback
reference voltage is 200mV/300mV. 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.
A recommended PCB layout with component
locations is shown below.
Where I LED 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 dimming method uses a PWM signal
applied to the EN pin. This is shown in figure 14.
When the PWM dimming frequency is lower than
200Hz, LED current is following by this PWM signal.
And now the device output current is under digital
mode dimming. It is under analog mode dimming,
when the PWM dimming frequency is higher than
200Hz, the PWM signal is converted to a DC voltage
by internal filter. And the LED current is a DC
current proportional to PWM signal. A 0% duty cycle
corresponds to zero LED current. A 100% duty cycle
corresponds to a full LED current. It provides high
dimming accuracy from duty 5% to 100%. To make
sure this switching process between on and off state
is invisible by human eyes; the switching frequency
must be greater than 100Hz. HT7938A can be applied
to the EN pin PWM dimming frequency up to
200kHz. As shown below, to adjust the analog mode
average output current value following the equation:
Top Layer
0 ≤ Duty cycle ≤ 1
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. The other way is to use
a filtered PWM signal, as shown in figure 16. The
filtered PWM signal application acts in the same way
as the DC voltage dimming control.
Rev. 1.00
Bottom Layer
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April 19, 2012
HT7938A
Typical Performance Characteristics
Fig.1 Efficiency VS Input Voltage (L=10μH)
Fig.5 LED Current VS PWM Dimming (3S3P LEDs)
Fig.6 Supply Current VS Input Voltage
Fig.2 Efficiency VS Input Voltage (L=22μH)
Fig.7 Feedback Voltage VS Input Voltage
Fig.3 LED Current VS PWM Dimming(10S1P LEDs)
Fig.8 Enable Voltage VS Input Voltage
Fig.4 LED Current VS PWM Di
Rev. 1.00
6
April 19, 2012
HT7938A
Fig.9 Switching Waveform(10S1P)
Fig.11 200Hz PWM Dimming Waveform
Fig.10 Open LED Protection
Rev. 1.00
Fig.12 1kHz PWM Dimming Waveform
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April 19, 2012
HT7938A
Application Circuits
1 0 H /2 2 H
V
1 N 5 8 1 9
V
O U T
IN
C 1
1 F
V IN
S W
E N
O V P
C 2
4 .7 F
1 0 S 1 P
F B
G N D
R F B
1 5 Fig.13 Application for Driving 10S1P WLEDs

Fig.14 Application for Dimming Control Using A PWM Signal
 
   Fig.15 Application for Dimming Control Using a DC Voltage
Rev. 1.00
8
April 19, 2012
HT7938A
  

Fig.16 Application for Dimming Control Using a Filtered PWM Signal
Note: As the above application circuits are unable to show the full drive capabilities and series/parallel drive
combinations of this device the following supplemental information is provided.
For the general full voltage range situation:
1. Vin Variable Voltage of 2.6V~5.5V
2. LED forward voltage of 3.5V max.
Maximum LED Drive Capability
Series
Parallel
Total
3
3
9
4
2
8
5
2
10
6
1
6
7
1
7
8
1
8
9
1
9
10
1
10
It is also important to note that for an input voltage supply which can be maintained at 5.0V or higher, the
device possesses even higher drive capabilities which can drive a higher number of parallel connected LEDs.
The following table depicts the maximum LED numbers possible for this higher input voltage range.
1. Vin Variable Voltage of 5.0V~5.5V
2. LED forward voltage of 3.5V max.
Maximum LED Drive Capability
Rev. 1.00
Series
Parallel
Total
3
13
39
3
10
40
3
9
27
4
8
32
5
4
20
6
4
24
7
3
21
8
2
16
9
2
18
10
2
20
9
April 19, 2012
HT7938A
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 (http://www.holtek.com.tw/english/
literature/package.pdf) for the latest version of the package information.
6-pin SOT23–6 Outline Dimensions
Symbol
Dimensions in inch
Min.
Nom.
Max.
A
0.039
―
0.051
A1
―
―
0.004
A2
0.028
―
0.035
b
0.014
―
0.020
C
0.004
―
0.010
D
0.106
―
0.122
E
0.055
―
0.071
e
―
0.075
―
H
0.015
―
0.118
L
0.015
―
―
θ
0°
―
9°
Symbol
Rev. 1.00
Dimensions in mm
Min.
Nom.
Max.
A
1.00
―
1.30
A1
―
―
0.10
A2
0.70
―
0.90
b
0.35
―
0.50
C
0.10
―
0.25
D
2.70
―
3.10
E
1.40
―
1.80
e
―
1.90
―
H
2.60
―
3.00
L
0.37
―
―
θ
0°
―
9°
10
April 19, 2012
HT7938A
Reel Dimensions
SOT23–6
Symbol
Rev. 1.00
Description
Dimensions in mm
A
Reel Outer Diameter
178.0+1.0
B
Reel Inner Diameter
62.0+1.0
C
Spindle Hole Diamete
13.0+0.2
D
Key Slit Width
2.50+0.25
T1
Space Between Flange
8.4 +1.5/-0.0
T2
Reel Thickness
11.4 +1.5/-0.0
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April 19, 2012
HT7938A
Carrier Tape Dimensions
SOT23–6
Symbol
Rev. 1.00
Description
W
Carrier Tape Width
Dimensions in mm
8.0+0.3
4.0+0.1
P
Cavity Pitch
E
Perforation Position
1.75+0.10
F
Cavity to Perforation (Width Direction)
0.50+0.05
D
Perforation Diameter
1.5 +0.1/-0.0
D1
Cavity Hole Diameter
1.5 +0.1/-0.0
P0
Perforation Pitch
P1
Cavity to Perforation (Length
Direction)
2.00+0.05
A0
Cavity Length
3.15+0.10
B0
Cavity Width
3.2+0.1
K0
Cavity Depth
4.0+0.1
1.4+0.1
t
Carrier Tape Thickness
C
Cover Tape Width
12
0.20+0.03
5.3+0.1
April 19, 2012
HT7938A
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
Fax: 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© 2012 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. 1.00
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April 19, 2012