DESCRIPTION
The ZXSC400 is a voltage mode boost converter in the SOT23-6 package. Its low
feedback voltage allows the current in a chain of LEDs to be set and accurately
monitored with a single resistor giving minimal losses. Its excellent load and line
regulation means that for the full supply range from lithium-ion cells, the LED
current will typically change by less than 1%. Using high efficiency Zetex switching
transistors with ratings of 20V and higher allow many LEDs to be chained in series
for the best LED current matching possible.
FEATURES
1.8V to 8V supply range
Typical output regulation of
1%
Over 80% typical efficiency
4.5 A typical shutdown current
Series connection for ultimate LED current matching
APPLICATIONS
White LED backlighting for colour LCD panels
General LED backlighting
High performance white LED flashlights
General LED driving from batteries
DEVICE MARKING
C400
ZXSC400
ISSUE 1 - JANUARY 2003
1
LED DRIVER BOOST CONVERTER
DEVICE
REEL
SIZE
TAPE
WIDTH
QUANTITY
PER REEL
ZXSC400E6TA
7"
8mm
3000 units
ORDERING INFORMATION
STDN
GND
V
CC
DRIVE
SENSE
V
FB
PINOUT
SOT23-6
Top View
ABSOLUTE MAXIMUM RATINGS
V
CC
-0.3V
to
+10V
DRIVE
-0.3V
to
V
CC
+ 0.3V
EOR
-0.3V
to
V
CC
+ 0.3V
STDN
-0.3V
to
The lower of (+5.0V) or (V
CC
+ 0.3V)
SENSE
-0.3V
to
The lower of (+5.0V) or (V
CC
+ 0.3V)
Operating Temp.
-40C to
+85C
Storage Temp.
-55C to
+125C
Power Dissipation
450mW
ZXSC400
ISSUE 1 - JANUARY 2003
2
Symbol
Parameter
Conditions
Limits
Units
Min
Typ
Max
Supply parameters
V
IN
V
CC
Range
1.8
8
V
Iq
1
Quiescent Current
V
CC
= 8V
220
A
I
STDN
Shutdown Current
4.5
A
Eff
1
Efficiency
30mA > I
LED
> 10mA
80
%
Acc
REF
Reference tolerance
1.8V < V
CC
< 8V
-3.0
3.0
%
TCO
REF
Reference Temp Co
0.005
%/ C
T
DRIVE
Discharge pulse width
1.8V < V
CC
< 8V
1.7
s
F
OSC
Operating Frequency
200
kHz
Input parameters
V
SENSE
Sense voltage
22
28
34
mV
I
SENSE
Sense input current
V
FB
= 0V, V
SENSE
= 0V
-1
-7
-15
A
V
FB
Feedback voltage
291
300
309
mV
I
FB
2
Feedback input current
V
FB
= 0V, V
SENSE
= 0V
-1.2
-4.5
A
V
IH
Shutdown high voltage
1.5
V
C C
V
V
IL
Shutdown low voltage
0
0.55
V
dV
LN
Line voltage regulation
0.5
%/V
Output parameters
I
OUT
3
Output current
Vin > 2V, Vout = 10.5V
50
mA
I
DRIVE
Transistor drive current
Vdrive = 0.7V
2
3.5
5
mA
V
DRIVE
Transistor voltage drive
1.8V < V
CC
< 8V
0
V
C C
-0.4
V
C
DRIVE
Mosfet gate drive cpbty
300
pF
dILD
4
Load current regulation
0.1
mA/V
ELECTRICAL CHARACTERISTICS
Test Conditions V
CC
= 3V, T= -40
C to 85
C unless otherwise stated.
Note
1
Excluding gate/base drive current.
2
I
FB
is typically half these values at 300mV.
3
System not device spec, including recommended transistors. 10.5V represents 3 white LEDs.
4
Change in LED current with changing supply voltage (LED V
F
x number of series LEDs).
TYPICAL CHARACTERISTICS
ZXSC400
ISSUE 1 - JANUARY 2003
3
DESIGN INFORMATION
IC operation description
Bandgap Reference
All threshold voltages and internal currents are
derived from a temperature compensated bandgap
reference circuit with a reference voltage of 1.22V
nominal.
Dynamic Drive Output
Depending on the input signal, the output is either
"LOW" or "HIGH". In the high state a 2.5mA current
source (max drive voltage = V
CC
-0.4V) drives the base
or gate of the external transistor. In order to operate
the external switching transistor at optimum
efficiency, both output states are initiated with a short
transient current in order to quickly discharge the base
or the gate of the switching transistor.
Switching Circuit
The switching circuit consists of two comparators,
Comp1 and Comp2, a gate U1, a monostable and the
drive output. Normally the DRIVE output is "HIGH"; the
external switching transistor is turned on. Current
ramps up in the inductor, the switching transistor and
external current sensing resistor. This voltage is
sensed by comparator, Comp2, at input I
SENSE
. Once
the current sense voltage across the sensing resistor
exceeds 30mV, comparator Comp2 through gate U1
triggers a re-triggerable monostable and turns off the
output drive stage for 2 s. The inductor discharges to
the load of the application. After 2 s a new charge
cycle begins, thus ramping the output voltage. When
the output voltage reaches the nominal value and FB
gets an input voltage of more than 300mV, the
monostable is forced "on" from Comp1 through gate
U1, until the feedback voltage falls below 300mV. The
above action continues to maintain regulation.
ZXSC400
ISSUE 1 - JANUARY 2003
4
Pin No.
Name
Description
1
V
CC
Supply voltage, 1.8V to 8V.
2
GND
Ground
3
STDN
Shutdown
4
SENSE
Inductor current sense input. Internal threshold voltage set to 30mV. Connect
external sense resistor
5
V
FB
Reference voltage. Internal threshold set to 300mV. Connect external resistor
to program LED current
6
DRIVE
Drive output for external switching transistor. Connect to base or gate of
external switching transistor.
Pin functions
2s
Block diagram
APPLICATIONS INFORMATION
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 V
CEO
of the switching transistor is also an
important parameter as this sees the full output
voltage when the transistor is switched off. Zetex
SuperSOTTM transistors are an ideal choice for this
application.
Schottky diode selection
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 rating 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 ZHCS Series meet these
needs.
Combination devices
To minimise the external component count Zetex
recommends the ZX3CDBS1M832 combination of
NPN transistor and Schottky diode in a 3mm x 2mm
MLP package. This device is recommended for use in
applications using 1 to 4 white LEDs.
The IC is also capable of driving MOSFETs. Zetex
recommends the ZXMNS3BM832 combination of low
threshold voltage N-Channel MOSFET and Schottky
diode in a 3mm x 2mm MLP package. This device is
recommended for use in applications using 1 to 8 white
LEDs.
Capacitor Selection
A small value, low ESR ceramic capacitor is required to
filter the output, typically 1 F to 4.7 F.
Generally an input capacitor is not required, but a small
ceramic capacitor may be added to aid EMC, typically
1 F to 4.7 F.
Inductor Selection
The inductor value must be chosen to satisfy
performance, cost and size requirements of the overall
solution.
Inductor selection has a significant impact on the
converter performance. For applications where
efficiency is critical, an inductor with a series resistance
of 250m
or less should be used.
A list of recommended inductors is listed in the table
below:
ZXSC400
ISSUE 1 - JANUARY 2003
5
Part No.
Manufacture
L
I
P K
(A)
R
D C
( )
CMD4D11-100MC
Sumida
10 H
0.5
0.457
CMD4D11-220MC
Sumida
22 H
0.4
0.676
LPO2506OB-103
Coilcraft
10 H
1.0
0.24
ST2006103
Standex Electronics Inc
10 H
0.6
0.1
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