ZXSC400
LED DRIVER BOOST CONVERTER
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
SOT23-6
APPLICATIONS
•
White LED backlighting for colour LCD panels
•
General LED backlighting
•
High performance white LED flashlights
•
General LED driving from batteries
PINOUT
ORDERING INFORMATION
DEVICE
ZXSC400E6TA
REEL
SIZE
7“
TAPE
WIDTH
8mm
QUANTITY
PER REEL
3000 units
V
CC
GND
STDN
Top View
DRIVE
V
FB
SENSE
DEVICE MARKING
•
C400
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ZXSC400
ABSOLUTE MAXIMUM RATINGS
V
CC
DRIVE
EOR
STDN
SENSE
Operating Temp.
Storage Temp.
Power Dissipation
-0.3V to
-0.3V to
-0.3V to
-0.3V to
-0.3V to
-40°C to
-55°C to
450mW
+10V
V
CC
+ 0.3V
V
CC
+ 0.3V
The lower of (+5.0V) or (V
CC
+ 0.3V)
The lower of (+5.0V) or (V
CC
+ 0.3V)
+85°C
+125°C
ELECTRICAL CHARACTERISTICS
Test Conditions V
CC
= 3V, T= -40°C to 85°C unless otherwise stated.
Symbol
Parameter
Conditions
Min
Supply parameters
V
IN
Iq
1
I
STDN
Eff
1
Acc
REF
TCO
REF
T
DRIVE
F
OSC
V
SENSE
I
SENSE
V
FB
I
FB 2
V
IH
V
IL
dV
LN
I
OUT 3
I
DRIVE
V
DRIVE
C
DRIVE
dILD
Note
1
2
3
4
Limits
Typ
Max
Units
V
CC
Range
Quiescent Current
Shutdown Current
Efficiency
Reference tolerance
Reference Temp Co
Discharge pulse width
Operating Frequency
1.8V < V
CC
< 8V
30mA > I
LED
> 10mA
1.8V < V
CC
< 8V
V
CC
= 8V
1.8
8
220
4.5
80
V
A
A
%
-3.0
0.005
1.7
3.0
%
%/ C
s
200
kHz
Input parameters
Sense voltage
Sense input current
Feedback voltage
Feedback input current
Shutdown high voltage
Shutdown low voltage
Line voltage regulation
V
FB
= 0V, V
SENSE
= 0V
V
FB
= 0V, V
SENSE
= 0V
22
-1
291
-1.2
1.5
0
0.5
28
-7
300
34
-15
309
-4.5
V
CC
0.55
V
V
%/V
mV
A
mV
A
Output parameters
Output current
Transistor drive current
Transistor voltage drive
Mosfet gate drive cpbty
Load current regulation
Vin > 2V, Vout = 10.5V
Vdrive = 0.7V
1.8V < V
CC
< 8V
50
2
0
300
0.1
3.5
5
V
CC
-0.4
mA
mA
V
pF
mA/V
4
Excluding gate/base drive current.
I
FB
is typically half these values at 300mV.
System not device spec, including recommended transistors. 10.5V represents 3 white LEDs.
Change in LED current with changing supply voltage (LED V
F
x number of series LEDs).
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ZXSC400
TYPICAL CHARACTERISTICS
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ZXSC400
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.
Block diagram
2µs
Pin functions
Pin No.
1
2
3
4
5
6
Name
V
CC
GND
STDN
SENSE
V
FB
DRIVE
Description
Supply voltage, 1.8V to 8V.
Ground
Shutdown
Inductor current sense input. Internal threshold voltage set to 30mV. Connect
external sense resistor
Reference voltage. Internal threshold set to 300mV. Connect external resistor
to program LED current
Drive output for external switching transistor. Connect to base or gate of
external switching transistor.
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ZXSC400
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
SuperSOT™ 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:
Part No.
CMD4D11-100MC
CMD4D11-220MC
LPO2506OB-103
ST2006103
Manufacture
Sumida
Sumida
Coilcraft
Standex Electronics Inc
L
10 H
22 H
10 H
10 H
I
PK
(A)
0.5
0.4
1.0
0.6
R
DC
( )
0.457
0.676
0.24
0.1
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