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Datasheet: ZHCS750 (Zetex Semiconductors)

LED DRIVER SOLUTION FOR LCD BACKLIGHTING

 

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Zetex Semiconductors
DEVICE DESCRIPTION
The ZXSC310 is a single or multi cell LED driver
designed for LCD backlighting applications. The input
voltage range of the device is between 0.8V and 8V.
This means the ZXSC310 is compatible with single
NiMH, NiCd or Alkaline cells, as well as multi-cell or
LiIon batteries.
The device features a shutdown control, resulting in a
standby current less than 5A, and an output capable
of driving serial or parallel LED's. The circuit generates
constant power output, which are ideal for driving
single or multiple LED's over a wide range of operating
voltages. These features make the device ideal for
driving LED's particularly in LCD backlight applications
for Digital Still cameras and PDA's.
FEATURES
94% efficiency
Minimum operating input voltage 0.8V
Maximum operating input voltage 8V
Standby current less than 5A
Programmable output current
Series or parallel LED configuration
Low saturation voltage switching transistor
SOT23-5 package
APPLICATIONS
LCD backlights:
Digital still camera
PDA
Mobile phone
LED flashlights and torches
White LED driving
Multiple LED driving
The ZXSC310 is a PFM DC-DC controller IC that drives
an external Zetex switching transistor with a very low
saturation resistance. These transistors are the best
switching devices available for this type of conversion
enabling high efficiency conversion with low input
voltages. The drive output of the ZXSC310 LED driver
generates a dynamic drive signal for the switching
transistor.
The circuit can start up under full load and operates
down to an input voltage of 0.8 volts. The solution
configuration ensures optimum efficiency over a wider
range of load currents; several circuit configurations
are possible depending on battery life versus
brightness considerations.
The ZXSC310 is offered in the SOT23-5 package which,
when combined with a SOT23 switching transistor,
generates a high efficiency small size circuit solution.
The IC and discrete combination offers the ultimate
cost Vs performance solution for LED backlight
applications.
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
1
LED DRIVER SOLUTION FOR LCD BACKLIGHTING
V
IN
V
CC
V
= 3.3V / 5V
DRIVE
I
SENSE
Gnd
S
TDN
TYPICAL APPLICATIONS CIRCUIT
DEVICE
REEL
SIZE
TAPE
WIDTH
QUANTITY
PER REEL
ZXSC310E5
180mm
8mm
3000
ORDERING INFORMATION
DEVICE MARKINGS
C310
ABSOLUTE MAXIMUM RATINGS:
Supply Voltage
-0.3 to 10V
Maximum Voltage other pins
-0.3 to V
CC
+0.3V
Power Dissipation
450mW
Operating Temperature
-40 to 85 C
Storage Temperature
-55 to 125C
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
2
Symbol
Parameter
Conditions
Min
Typ
Max
Units
Efficiency
1
94
%
V
CC
Recommended supply
voltage range
0.8
8
V
V
CC(min)
Minimum startup and
operating voltage
I
DRIVE
=-600A
,
V
DRIVE
=0.7V
I
DRIVE
=-600A, V
DRIVE
=0.7V,
T
AMB
=-10C3
0.8
0.9
0.92
V
I
Q
Quiescent current
2
V
EN =
V
CC
(enabled)
V
EN =
0V (standby)
0.2
4
5
mA
A
I
VDRIVE
Base drive current
V
DRIVE
= 0.7V, V
ISENSE
= 0V
1.5
3.5
mA
I
CC
Supply current
3
V
DRIVE
= 0.7V, V
ISENSE
= 0V
2
4
mA
V
VDRIVE(high)
High level drive voltage
V
ISENSE
= 0V, I
VDRIVE
=-0.5mA
V
CC
-0.3
V
CC
V
V
VDRIVE(low)
Low level drive voltage
V
ISENSE
= 50mV, I
VDRIVE
= 5mA
0
0.2
V
V
EN(high)
Device enabled
0.7
V
V
EN(low)
Device in standby mode
0.15
V
I
EN
Enable input current
-1
1
A
V
ISENSE
(threshold)
Output current reference
voltage
14
19
24
mV
T
CVISENSE
I
SENSE
voltage temp co.
2
0.4
%/C
I
ISENSE
I
SENSE
input current
V
ISENSE
= 0V
0
-30
-65
A
ELECTRICAL CHARACTERISTICS:
Test conditions unless otherwise stated: V
CC
=1.5V, T
AMB
=25 C
Symbol
Parameter
Conditions
Min
Typ
Max
Units
T
DRV
Discharge Pulse Width
1.2
1.7
3.2
s
F
OSC
Recommended operating
frequency
4
200
kHz
ELECTRICAL CHARACTERISTICS: AC PARAMETERS
2
Test conditions unless otherwise stated: V
CC
=1.5V, T
AMB
=0 to 70 C
1 Application dependent, see reference designs
2 These parameters guaranteed by Design
3 Total supply current =I
Q
+ I
VDRIVE
, see typical characteristics
4 Operating frequency is application circuit dependent. See applications section.
FMMT618
For the circuits described in the applications section
Zetex FMMT618 is the recommended pass transistor.
ZHCS1000
For the maximum brightness circuit described in the
a p p l i c a t i o n s s e c t i o n Z e t e x Z H C S 1 0 0 0 i s t h e
recommended Schottky diode.
The following indicates outline data for the device,
more detailed information can be found in the Zetex
surface mount products data book or on Zetex Web
page: www.zetex.com
The following indicates outline data for the ZHCS,
more detailed information can be found on Zetex Web
page: www.zetex.com
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
3
Symbol
Parameter
Conditions
Min
Typ
Max
Units
V
CE(sat)
Collector-Emitter
Saturation Voltage
5
I
C
=0.1A, I
B
=10mA
I
C
=1A, I
B
=10mA
I
C
=2.5A, I
B
=50mA
8
70
130
15
150
200
mV
V
(BR)CEO
Collector-Emitter
Breakdown Voltage
5
I
C
=10mA
20
27
V
ELECTRICAL CHARACTERISTICS:
Test conditions unless otherwise stated: T
AMB
=25 C
5
Measured under pulse conditions. Pulse width=300s. Duty cycle
2%
Symbol
Parameter
Conditions
Min
Typ
Max
Units
V
F
Forward voltage
I
F
= 500mA
I
F
= 1A
400
500
mV
t
rr
Reverse Recovery Time
Switched from I
F
=500mA to
I
R
=500mA.
Measured at I
R
=50mA
12
ns
I
R
Reverse Current
V
R
= 30V
50
100
A
ELECTRICAL CHARACTERISTICS:
Test conditions unless otherwise stated: T
AMB
=25 C
Part
Number
V
R
I
F
I
FSM
V
F
at
I
R
at
Capacitance
at V
R
= 25V, f = 1MHz
Package
Max.
V
Max.
mA
Max.
A
Max.
mV
I
F
mA
Max.
A
V
R
V
Typ.
pF
SOT23
BAT54
30
200
0.6
500
30
250
25
10
SOT23-6
ZHCS2000
40
2000
20
500
2000
1000
30
60
SOT23
ZHCS1000
40
1000
12
500
1000
100
30
25
SOT23
ZHCS750
40
750
12
540
750
100
30
25
SOT23
ZHCS500
40
500
6.75
550
500
40
30
20
SOT23
ZHCS400
40
400
6.75
500
400
40
30
20
SOT323
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
4
TYPICAL CHARACTERISTICS
DEVICE DESCRIPTION
The ZXSC310 is PFM, controller IC which, when
combined with a high performance external transistor,
enables the production of a high efficiency boost
converter for LED driving applications. A block
diagram is shown for the ZXSC310 below.
The on chip comparator forces the driver circuit and
therefore the external switching transistor off if the
voltage at I
SENSE
exceeds 19mV. An internal reference
circuit and divider set this threshold.
The voltage at I
SENSE
is taken from a current sense
resistor connected in series with the emitter of the
switching transistor. A monostable following the
output of the comparator forces the turn-off time of the
output stage to be typically 1.7us. This ensures that
there is sufficient time to discharge the inductor coil
before the next on period.
With every on pulse the switching transistor is kept on
until the voltage across the current-sense resistor
exceeds the threshold of the I
SENSE
input. The on-pulse
length, and therefore the switching frequency, is
determined by the programmed peak current, the input
voltage and the input to output voltage differential. See
applications section for details.
The driver circuit supplies the external switching
transistor with a fixed drive current. To maximise
efficiency the external transistor switched quickly,
typically being forced off within 30ns.
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
5
I
2
Drive
I
SENSE
R2
V
RE
R1
V
DRIVE
G
N
D
I
V
CC
S
TDN
ZXSC310 Block Diagram
REFERENCE DESIGNS
Three typical LED driving applications are shown.
Firstly a typical LCD backlight circuit, then maximum
brightness LED driving solution and lastly an
optimised battery life LED driving solution.
This application shows the ZXSC310 in a typical LCD
backlight application for Digital Still Cameras and
PDA's. The input voltage for these backlight circuits are
usually fixed from the main system power, typically
3.3V or 5V. The LED's are connected serially so that the
light is distributed uniformly in each LED. The current
provided to the LED's can either be pulsed or DC. The
DC current is programmable via a sense resistor,
R
SENSE
, and is set to an optimum LED current of 20mA
for the reference designs. DC current is achieved by
adding a Schottky rectifying diode and an output
capacitor, as shown in the reference design below.
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
6
V
IN
V
CC
V
= 3.3V / 5V
DRIVE
I
SENSE
Gnd
S
TDN
LCD backlight circuit
Ref
Value
Part Number
Manufacture
Comments
U1
N/A
ZXSC310E5
Zetex Plc
Single cell converter, SOT23-5
Q1
N/A
FMMT618
Zetex Plc
Low V
CE(sat
) NPN, SOT23
D1
N/A
ZHCS1000
Zetex Plc
1A Schottky diode, SOT23
C1
2.2 F
Generic
Various
0805 Size
R1
6
150m
Generic
Various
1206 Size
R1
7
250m
Generic
Various
1206 Size
L1
8
68H
Surface mount inductor
Materials list
6
Used for 3.3V input, I
LED
set to 20mA
10%.
7
Used for 5V input, I
LED
to 20mA
10%.
8
See Application section.
(Notes)
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
7
This circuit provides a continuous current output to the
LED by rectifying and buffering the DC-DC output. This
ensures maximum LED brightness.
Ref
Value
Part Number
Manufacture
Comments
U1
N/A
ZXSC310E5
Zetex Plc
Single cell converter, SOT23-5
Q1
N/A
FMMT617
Zetex Plc
Low
VCE(sat)
NPN, SOT23
D1
1A
ZHCS1000
Zetex Plc
1A Shottky diode, SOT23
R1
100m
Generic
Various
0805 Size
C1
2.2 F
Generic
Various
Low ESR ceramic capacitor
L1
100H
8
Surface mount inductor
Materials list
8
See Application section.
Q1
FMMT617
R1
L1
V
D2
CC
G
I
V
S
U1
ZXSC310
D1
ZHCS1000
C1
BATT
V
DRIVE
TDN
SENSE
ND
Maximum brightness solution
(Notes)
To ensure optimum efficiency, and therefore
maximum battery life, the LED is supplied with a
pulsed current. Maximum efficiency is ensured with
the removal of rectifier losses experienced in the
maximum brightness solution
.
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
8
Ref
Value
Part
Number
Manufacture
Comments
U1
N/A
ZXSC310E5
Zetex Plc
Single cell converter, SOT23-5
Q1
N/A
FMMT617
Zetex Plc
Low V
CE(SAT)
NPN, SOT23
R1
330m
Generic
Various
0805 Size
L1
100 H
8
Surface mount inductor
Materials list
8
See Application section.
Maximum battery life solution
(Notes)
APPLICATIONS INFORMATION
The following section is a design guide for optimum
converter performance.
Switching transistor selection
The choice of switching transistor has a major impact
on the converter efficiency. For optimum performance,
a bipolar transistor with low V
CE(SAT)
and high gain is
required.
The Zetex FMMT618 is an ideal choice of transistor,
having a low saturation voltage. A data sheet for the
FMMT618 is available on Zetex web site or through
your local Zetex sales office. Outline information is
included in the characteristics section of this data
sheet.
Schottky diode selection
For the maximum battery life solution a Schottky
rectifier diode is required. As with the switching
transistor the Schottky rectifier diode has a major
impact on the converter efficiency. A Schottky diode
with a low forward voltage and fast recovery time
should be used for this application.
The diode should be selected so that the maximum
forward current is greater or equal to the maximum
peak current in the inductor, and the maximum reverse
voltage is greater or equal to the output voltage.
The Zetex ZHCS1000 meets these needs. Datasheets
for the ZHCS Series are available on Zetex web site or
through your local Zetex sales office. Outline
information is included in the characteristics section of
this data sheet.
For the maximum brightness solution a pulsed current
is supplied to the LED and thus a Schottky rectifier
diode is not required.
Inductor selection
The inductor value must be chosen to satisfy
performance, cost and size requirements of the overall
solution. For the LCD backlight reference design we
recommend an inductor value of 68uH with a core
saturation current rating greater than the converter
peak current value and low series resistance.
Inductor selection has a significant impact on the
converter performance. For applications where
efficiency is critical, an inductor with a series resistance
of 500m
or less should be used.
A list of recommended inductors is shown in the table
below:
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
9
/ = 50
C
100 C
50 C
10 C
10A
Part No.
Manufacture
L
I
PK
R
DC
DO1608C-683
Coilcraft
68 H
0.4A
0.86
CR54-680
Sumida
68 H
0.61A
0.46
P1174.683
Pulse
68 H
0.4A
0.37
SFOP5845-R61680
Samwha
68 H
0.61A
0.46
SIS43-680
Delta
68 H
0.4
1.125
Peak current definition
The peak current rating is a design parameter whose
value is dependent upon the overall application. For
the high brightness reference designs, a peak current
of was chosen to ensure that the converter could
provide the required output power to the LED.
In general, the I
PK
value must be chosen to ensure that
the switching transistor, Q1, is in full saturation with
maximum output power conditions, assuming
worse-case input voltage and transistor gain under all
operating temperature extremes.
Once I
PK
is decided the value of R
SENSE
can be
determined by:
A selection guide of sense resistor and inductor values
for given input voltages, output currents and number
of LED connected in series is provided in the table
below.
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
10
R
V
I
SENSE
ISENSE
PK
=
Input Voltage
(V)
LED current
(mA)
No. of LED's
R
SENSE
(m )
Inductor
( H)
Efficiency
(%)
3.3V
10
3
510
68
80
3.3V
10
4
330
68
81
3.3V
10
6
150
68
79
3.3V
20
3
220
68
84
3.3V
20
4
150
68
93
3.3V
20
6
77
68
79
3.3V
30
3
170
68
84
3.3V
30
4
100
68
84
3.3V
30
6
47
68
77
5V
10
3
750
68
83
5V
10
4
510
68
84
5V
10
6
330
68
79
5V
20
3
440
68
85
5V
20
4
250
68
85
5V
20
6
150
68
82
5V
30
3
330
68
86
5V
30
4
170
68
85
5V
30
6
100
68
83
Output Power Calculation
By making the above assumptions for inductance and
peak current the output power can be determined by:
Note:V
OUT
=output voltage + Schottky rectifier voltage
drop.
Where
T
OFF
1.7us (internally set by ZXSC310)
and
and
Where
Operating frequency can be derived by:
Capacitor selection
For pulsed operation, as in the maximum battery life
solution, no capacitors are required at the output to the
LED. For rectified operation, as in the maximum
brightness solution, a small value ceramic capacitor is
required, typically 2.2uF.
Generally an input capacitor is not required, but a small
ceramic capacitor may be added to aid EMC, typically
470nF to 1uF.
(notes)
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
11
(
)
I
I
V
V
T
L
MIN
PK
OUT
IN
OFF
=
-
-
(
)
F
T
T
ON
OFF
=
+
1
I
I
I
AV
PK
MIN
=
+
2
(
)
T
T
V
V
V
ON
OFF
OUT
IN
IN
=
-
P
V
V
I
T
T
T
OUT
OUT
IN
AV
OFF
ON
OFF
=
-
+
(
)
Shutdown Control
The ZXSC310 offers a shutdown mode that produces a
standby current of less than 5uA when in operation.
When the voltage at the S
TDN
pin is 0.7V or higher the
ZXSC310 is enabled, hence the driver is in normal
operation. When the voltage at the S
TDN
pin is 0.1V or
lower the ZXSC310 is disabled, hence the driver is in
shutdown mode. If the S
TDN
pin is open circuit the
ZXSC310 is also enabled.
Layout of LCD backlighting solution
Demonstration board
A demonstration board for the LCD backlighting
solution, is available upon request. These can be
obtained through your local Zetex office or through
Zetex web pages. For all reference designs Gerber files
and bill of materials can be supplied.
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
12
Top Silk
Drill File
Bottom Copper
Top Copper
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
13
STDN
VCC
Gnd
ISENSE
VDRIVE
1
2
3
4
5
Top View
PINOUT DIAGRAM
Pin No.
Name
Description
1
V
CC
Supply voltage, generally Alkaline, NiMH or NiCd single cell
2
G
nd
Ground
3
S
TDN
Shutdown
4
I
SENSE
Inductor current sense input. Internal threshold voltage set to 19mV.
Connect external sense resistor
5
V
DRIVE
Drive output for external switching transistor. Connect to base of
external switching transistor.
PIN DESCRIPTIONS
ZXSC310
PROVISIONAL ISSUE A - SEPTEMBER 2001
14
Zetex plc
Fields New Road
Chadderton
Oldham, OL9 8NP
United Kingdom
Telephone (44) 161 622 4422
Fax: (44) 161 622 4420
Zetex GmbH
Streitfeldstrae 19
D-81673 Mnchen
Germany
Telefon: (49) 89 45 49 49 0
Fax: (49) 89 45 49 49 49
Zetex Inc
Suite 315
700 Veterans Memorial Highway
Hauppauge NY11788
USA
Telephone: (631) 360 2222
Fax: (631) 360 8222
Zetex (Asia) Ltd
3701-04 Metroplaza, Tower 1
Hing Fong Road
Kwai Fong, Hong Kong
China
Telephone: (852) 26100 611
Fax: (852) 24250 494
These offices are supported by agents and distributors in major countries world-wide.
This publication is issued to provide outline information only which (unless agreed by the Company in writing) may not be used, applied or
reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the products or services
concerned. The Company reserves the right to alter without notice the specification, design, price or conditions of supply of any product or
service.
For the latest product information, log on to
www.zetex.com
Zetex plc 2001
SOT23-5
DIM
Millimetres
MIN
MAX
A
0.90
1.45
A1
0.00
0.15
A2
0.90
1.3
b
0.35
0.50
C
0.09
0.20
D
2.80
3.00
E
2.60
3.00
E1
1.50
1.75
e
0.95 REF
PACKAGE DIMENSIONS
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