HOLTEK HT7938

HT7938
High Current and Performance White LED Driver
Features
· Efficiency up to 85% at VIN=3.6V, 9LEDs,
· Tiny inductor and capacitors
ILED=20mA
· Up to 10 strings White-LED (LED VF(Max.)=3.5V)
· 1.2MHz fixed switching frequency
· Tiny 6-lead SOT23-6 package
· Low Standby Current: 0.1mA, VEN=0V (typ.)
· Built in OVP, OCP, OTP, UVLO protection
· Matches LED current
Applications
· Cellular phones
· GPS
· PDAs
· Handheld devices
· DSCs
· 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 continuous 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 prevents the damage resulting from an open circuit condition. 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
Rev 1.10
1
March 19, 2010
HT7938
Pin Assignment
V IN
6
O V P
5
E N
4
H T 7 9 3 8
S O T 2 3 -6 -A
1
S W
2
G N D
T o p V ie w
3
F B
Pin Description
Pin Name
EN
GND
I/O
I
¾
Description
Shutdown & Dimming control input. Don¢t allow this pin to float.
Signal Ground.
FB
I
Feedback pin. Reference voltage. The HT7938 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
SW
I
Switching pin. Internal power MOSFET drain. Connected to inductor and diode.
VIN
I
Input supply pin. The input supply pin for the IC. Connect VIN to a supply voltage between
2.6V~5.5V.
OVP
O
Over voltage protection pin which is connected to the output.
Absolute Maximum Ratings
Input Voltage...........................................................6.0V
SW Voltage..............................................................46V
FB Voltage ..............................................................6.0V
EN ..........................................................................6.0V
OVP Voltage ............................................................46V
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.
Rev 1.10
2
March 19, 2010
HT7938
Electrical Characteristics
Symbol
VIN=3.6V, L=10mH, CIN=1mF, CO=1mF, ILED=20mA, Ta=25°C
Parameter
Test Conditions
Min.
Typ.
Max.
Unit
VIN
Input Voltage
¾
2.6
¾
5.5
V
ULVO
Under Voltage Lockout
¾
¾
2.4
2.5
V
Switching, VFB=GND
¾
1.0
2.5
mA
VEN= 0V
¾
0.1
1.0
mA
Not switching, VFB=0.25V
¾
200
300
mA
190
200
210
mV
0.8
1.2
1.6
MHz
92
95
¾
%
IIN
Supply Current
Error Amplifier
VFB
Feedback Voltage
¾
Power Switch
fOSC
Switching Frequency
DC
Maximum Duty Cycle
RDS(ON)
SW On Resistance
¾
¾
0.7
¾
W
ISW(OFF)
Switch Leakage Current
¾
¾
0.1
1.0
mA
EN Voltage High
VIN=2.6V~5.5V
2.0
¾
¾
V
EN Voltage Low
VIN=2.6V~5.5V
¾
¾
0.8
V
Measurement at SW pin
EN Pin
VIH
VIL
OVP and OCP
VOVP
OVP Threshold
No load
36
39
42
V
IOCP
N-channel MOSFET Current Limit
¾
¾
650
¾
mA
Thermal Shutdown Threshold
¾
¾
150
¾
°C
Thermal Shutdown Hysteresis
¾
¾
15
¾
°C
Thermal Shutdown
TSHUT
Rev 1.10
3
March 19, 2010
HT7938
Function Description
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
HT7938 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:
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.
V
L =
Current Limit Protection
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.
I
D I
I
L
´ F
V
O U T
=
L (P E A K )
´ I
L
O U T (M A X )
´ h
IN
~ 5 0 % ) ´ I
= I
)
IN
´ D I
S W
V
= (3 0 %
- V
O U T
O U T
+
IN (M A X )
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
=
O
- V
IN
) ´ IO
U T
O U T
´ F
S W
´ V
r ip p le
(V
V
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 135°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 1.10
IN (M A X )
´ (V
IN
V
4
March 19, 2010
HT7938
· 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 HT7938 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
F B
R
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 8. 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. The other way is to use a filtered PWM signal, as shown in figure 10. The filtered
PWM signal application acts in the same way as the
DC voltage dimming control.
Top Layer
Bottom Layer
Rev 1.10
5
March 19, 2010
HT7938
Typical Performance Characteristics
Figure 5. LED Current vs PWM Dimming
(10S1P LEDs)
Figure 1. Efficiency vs Input Voltage
Figure 2. Enable Voltage vs. Input Voltage
Figure 6. LED Current vs PWM Dimming
(9S1P LEDs)
Figure 3. Operating current vs. Input Voltage
Figure 7. Switching Frequency (MHz) vs. Input
Voltage
Figure 4. Reference Voltage vs. Input Voltage
Rev 1.10
6
March 19, 2010
HT7938
Application Circuits
1 0 m H
V IN
2 .6 V ~ 5 .5 V
C IN
1 m F
1 N 5 8 1 9
V IN
S W
E N
O V P
C O
1 m F
1 0 L E D s
P W M
S ig n a l
F B
G N D
R
H T 7 9 3 8
F B
1 0 W
Figure 8. Application Circuits for Driving 10S1P WLEDs
1 0 m H
V IN
2 .6 V ~ 5 .5 V
C IN
1 m F
1 N 5 8 1 9
V IN
S W
E N
O V P
C O
1 m F
1 0 L E D s
1 0 k W
F B
G N D
R
F B
1 0 W
1 5 0 k W
H T 7 9 3 8
V D C
0 V ~ 3 .3 V
Figure 9. Application Circuit for Dimming Control Using a DC Voltage
1 0 m H
V IN
2 .6 V ~ 5 .5 V
C IN
1 m F
1 N 5 8 1 9
V IN
S W
E N
O V P
C O
1 m F
1 0 L E D s
1 0 k W
F B
G N D
R
1 5 0 k W
H T 7 9 3 8
3 .3 V
0 V
P W M
F B
1 0 W
1 0 k W
S ig n a l
0 .1 m F
Figure 10. Application Circuit for Dimming Control Using a Filtered PWM Signal
Rev 1.10
7
March 19, 2010
HT7938
Package Information
6-pin SOT23-6 Outline Dimensions
D
C
L
H
E
q
e
A
A 2
b
Symbol
Dimensions in inch
Min.
Nom.
Max.
0.039
¾
0.051
A1
¾
¾
0.004
A2
0.028
¾
0.035
A
b
0.014
¾
0.020
C
0.004
¾
0.010
D
0.106
¾
0.122
E
0.055
¾
0.071
e
¾
0.075
¾
H
0.102
¾
0.118
L
0.015
¾
¾
q
0°
¾
9°
Symbol
Rev 1.10
A 1
Dimensions in mm
Min.
Nom.
Max.
A
1.00
¾
1.30
A1
¾
¾
0.10
A2
0.70
¾
0.90
b
0.35
¾
0.50
C
0.10
¾
0.25
D
2.70
¾
3.10
E
1.40
¾
1.80
e
¾
1.90
¾
H
2.60
¾
3.00
L
0.37
¾
¾
q
0°
¾
9°
8
March 19, 2010
HT7938
Product Tape and Reel Specifications
Reel Dimensions
D
T 2
A
C
B
T 1
SOT23-6
Symbol
Description
Dimensions in mm
A
Reel Outer Diameter
178.0±1.0
B
Reel Inner Diameter
62.0±1.0
C
Spindle Hole Diameter
13.0±0.2
D
Key Slit Width
T1
Space Between Flange
8.4
T2
Reel Thickness
11.4
Rev 1.10
2.50±0.25
9
+1.5/-0.0
+1.5/-0.0
March 19, 2010
HT7938
Carrier Tape Dimensions
P 0
D
P 1
t
E
F
W
B 0
C
D 1
P
K 0
A 0
R e e l H o le
IC
p a c k a g e p in 1 a n d th e r e e l h o le s
a r e lo c a te d o n th e s a m e s id e .
SOT23-6
Symbol
Description
Dimensions in mm
W
Carrier Tape Width
8.0±0.3
P
Cavity Pitch
4.0±0.1
E
Perforation Position
1.75±0.1
F
Cavity to Perforation (Width Direction)
3.50±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
1.4±0.1
t
Carrier Tape Thickness
C
Cover Tape Width
Rev 1.10
4.0±0.1
0.20±0.03
5.3±0.1
10
March 19, 2010
HT7938
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Fax: 886-3-563-1189
http://www.holtek.com.tw
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Tel: 86-755-8616-9908, 86-755-8616-9308
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Tel: 1-510-252-9880
Fax: 1-510-252-9885
http://www.holtek.com
Copyright Ó 2010 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 1.10
11
March 19, 2010