HT7938 High Current and Performance White LED Driver

HT7938
High Current and Performance White LED Driver
Features
· Efficiency up to 85% at VIN=3.6V, 9LEDs,
· Tiny inductor and capacitors
ILED=20mA
· 1.2MHz fixed switching frequency
· Up to 10 strings White-LED (LED VF(Max.)=3.5V)
· Low Standby Current: 0.1mA (typ.) at VEN=0V
· Built in OVP, OCP, OTP, UVLO protection
· Tiny 6-lead SOT23-6 package
· 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.
Selection Table
Note:
Part No.
Package
Marking
HT7938
SOT23-6
7938#
7938+
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
1
November 22, 2011
HT7938
Block Diagram
UVLO=2.1V
Pin Assignment
V IN
6
O V P
5
E N
4
T o p V ie w
1
S W
2
G N D
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.
Rev 1.50
2
November 22, 2011
HT7938
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.
Electrical Characteristics
Symbol
VIN=3.6V, L=22mH, 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
UVLO
Under Voltage Lockout
¾
1.8
2.1
2.4
V
IIN
Switching, VFB=GND
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
92
95
¾
%
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
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
Measurement at SW pin
EN Pin
OVP and OCP
VOVP
OVP Threshold
No load
36
39
42
V
IOCP
N-channel MOSFET Current Limit
¾
¾
1000
¾
mA
Thermal Shutdown Threshold
¾
¾
150
¾
°C
Thermal Shutdown Hysteresis
¾
¾
25
¾
°C
Thermal Shutdown
TSHUT
Rev 1.50
3
November 22, 2011
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.
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 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 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 22, 2011
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
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 9. 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 11. The filtered
PWM signal application acts in the same way as the
DC voltage dimming control.
Top Layer
Bottom Layer
Rev 1.50
5
November 22, 2011
HT7938
Typical Performance Characteristics
Figure 1. Efficiency vs. Input Voltage
Figure 5. LED Current vs. PWM Duty Cycle (10S1P)
Figure 2. Efficiency vs. Input Voltage
(Boost Mode VOUT=10V)
Figure 6. LED Current vs. PWM Duty Cycle (9S1P)
Figure 3. Efficiency vs. Input Voltage
(Boost Mode VOUT=33V)
Figure 7. Switching Frequency vs. Input Voltage
Figure 4. Reference Voltage vs. Input Voltage (10S1P)
Figure 8. Supply Current vs. Input Voltage
(VFB=GND)
Rev 1.50
6
November 22, 2011
HT7938
Application Circuits
V
P W M
1 0 m H /2 2 m H
IN
C 1
1 m F
S ig n a l
V IN
S W
E N
O V P
1 N 5 8 1 9
C 2
1 m F
1 0 S 1 P
F B
G N D
R
H T 7 9 3 8
F B
1 0 W
Figure 9. Application Circuits for Driving 10S1P WLEDs
V
IN
1 0 m H /2 2 m H
C 1
1 m F
V IN
S W
E N
O V P
G N D
1 N 5 8 1 9
C 2
1 m F
1 0 S 1 P
1 0 k W
F B
H T 7 9 3 8
R
F B
1 0 W
1 5 0 k W
V D C D im m in g
0 V ~ 3 .3 V
Figure 10. Application Circuit for Dimming Control Using a DC Voltage
V
IN
1 0 m H /2 2 m H
C 1
1 m F
V IN
S W
E N
O V P
1 N 5 8 1 9
1 0 S 1 P
1 0 k W
H T 7 9 3 8
1 0 k W
P W M
O U T
C 2
1 m F
F B
G N D
V
S ig n a l
1 5 0 k W
R
F B
1 0 W
0 .1 m F
Figure 11. Application Circuit for Dimming Control Using a Filtered PWM Signal
Rev 1.50
7
November 22, 2011
HT7938
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 1.50
Series
Parallel
Total
3
9
27
4
8
32
5
4
20
6
4
24
7
3
21
8
2
16
9
2
18
10
2
20
8
November 22, 2011
HT7938
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
D
C
H
E
q
e
A
A 2
b
Symbol
A
A 1
Dimensions in inch
Min.
Nom.
Max.
0.039
¾
0.051
A1
¾
¾
0.004
A2
0.028
¾
0.035
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 1.50
L
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°
9
November 22, 2011
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 1.50
2.50±0.25
10
+1.5/-0.0
+1.5/-0.0
November 22, 2011
HT7938
Carrier Tape Dimensions
P 0
D
P 1
t
E
F
W
D 1
P
B 0
C
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 1.50
4.0±0.1
0.20±0.03
5.3±0.1
11
November 22, 2011
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
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 Ó 2011 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
12
November 22, 2011