- 1 -
NJU6050
Ver.2004-02-25
High Efficiency White LED Driver
GENERAL DESCRIPTION
The NJU6050 is a high-efficiency white LED driver. It
contains a high-efficiency step-up DC/DC converter and an
output driver. The IC ensures a 28V maximum capacity
which allows series connection of 8 white LEDs with a
constant current so that the LED currents are identical for
uniform brightness.
The high frequency of the step-up converter permits the
use of small, low-profile inductors and capacitors to
minimize the footprint in space-conscious applications. And
additional features are a low operating voltage of 2.5V and
the small package SOT-23 (MTP-5).
All of these benefits make the NJU6050 suitable for the
battery-powered portable applications such as a cellular
phone, a camcorder, PDA, etc.
FEATURES
Drives Up to 8 White LEDs in Series
V
SW
= 28V, I
OUT
= 20mA
Uses Small Inductor and Capacitors
2.5V to 6.5V Operating Voltage for Step-up Circuits (V
DD
)
CMOS Technology
Package
: SOT-23 5 pin (MTP-5)
PIN CONFIGURATION (TOP VIEW)
PRELIMINARY
PACKAGE OUTLINE
SW
V
SS
FB
1
2
3
4
5
V
DD
SHDNb
- 2 -
NJU6050
Ver.2004-02-25
PIN DESCRIPTIONS
No.
SYMBOL
TYPE
DESCRIPTION
1 SW Input Switch
Terminal
2 V
SS
Power Ground
Terminal
3 FB Input Feedback
Terminal
4 SHDNb Input
Shutdown Terminal Active "L"
"L" : Shutdown
"H" : Normal Operation
5 V
DD
Power
Power Supply terminal
BLOCK DIAGRAM
FUNCTIONAL DESCRIPTONS
(1) LED Current Control and Resistor R
LED
Selection
The NJU6050 incorporates the LED current control to regulate the LED current (I
LED
), which is programmed
by the feedback resistor (R
LED
) connected between the FB and the V
SS
terminals. The reference voltage V
REF
is
internally regulated to 1.2V typical and connected to the positive input of the built-in comparator. Formula (1) is
used to choose the value of the R
LED
, as shown below.
V
REF
=1.2V (typ.)
The I
LED
is the constant current programmed by the R
LED
. When the feedback voltage on the FB terminal
reaches above the reference voltage V
REF
(i.e., I
LED
is above the level programmed by R
LED
), the output capacitor
C2 delivers the I
LED
. Once the feedback voltage drops below the reference voltage (i.e., I
LED
drops below the level
programmed by the R
LED
), the MOS switch is turned on, then the current of the inductor L1 begins increasing.
When the switch current reaches 350mA, the MOS switch is turned off, then the L1 delivers current to the output
through the diode D1 as the inductor current drops. After that, the MOS switch is turned on again and the switch
current increases up to 350mA. This switching cycle continues until the I
LED
reaches the level programmed by the
R
LED
, then the I
LED
current is maintained constant.
LED
REF
LED
I
V
R
=
--- Formula (1)
FB
C2
SHDNb
D1
C1
Shutdown
circuit
V
DD
V
SS
DC/DC
Control
V
REF
SW
-
+
V
IN
V
OUT
L1
R
LED
1.2V
Bias
I
LED
D2
- 3 -
NJU6050
Ver.2004-02-25
(2) Inductor Selection
The following equation gives the optimum inductor value:
L = t
OFF
Formula (2)
VD
:
Schottky
diode
Voltage
I
CL
:Switch current limit
The power supply voltage V
IN
may fluctuate in battery-powered applications. For this reason, the minimum
voltage should be applied to the V
IN
in Formula (2).
The NJU6050 has about 100ns of delay time (T
DELAY
), which is defined as the period from the reach of the
current limit 350mA to the MOS-switch-off. The T
DELAY
may cause an overshoot-inductor-current, which is called
the peak current I
L,PEAK
, and calculated by Formula (3). Therefore, it is recommended that an inductor with a
rating twice of the I
L,PEAK
and a low DCR (DC resistance) be used for high efficiency.
I
L,PEAK
= I
CL
+ t
DELAY
Formula (3)
V
DS
: Drain-Source voltage of MOS switch (=I
LIMIT
*R
ON
)
V
IN(MAX)
: Maximum of V
IN
Voltage
(3) Diode Selection
A schottky diode with a low forward-voltage-drop and a fast switching-speed is ideal for the D1. And the D1
must have a rating greater than the output voltage and output current in the system.
(4) Capacitor Selection
A low ESR (Equivalent Series Resistance) capacitor should be used as the output capacitor C2 to minimize
output ripples. A multi-layer ceramic capacitor is the best selection for the NJU6050 application because of not
only the low ESR but its small package. A ceramic capacitor is also recommended for the input
decoupling-capacitor C1, and should be placed as close to the NJU6050 as possible.
(5) Open-circuit Protection
The zener diode D2 works as an open-circuit protection. When the LED is disconnected from the circuit, the
feedback voltage on the FB terminal is pulled down to 0V, which makes the switching frequency maximum.
Without the diode D2, the SW voltage exceeds the absolute maximum rating, and this voltage stress may cause a
permanent damage to the IC.
The zener voltage must be greater than the maximum forward-voltage of the LED string.
V
OUT
-V
IN(MIN)
+V
D
I
CL
V
IN(MAX)
+V
DS
L
- 4 -
NJU6050
Ver.2004-02-25
ABSOLUTE MAXIMUMN RATINGS
Ta=25
C
PARAMETERS
SYMBOL
CONDITIONS
RATINGS
UNIT
VDD Power Supply
V
DD
-0.3 to +7.0
V
SHDNb Terminal Voltage
V
SHDNb
SHDNb terminal
-0.3 to +7.0
V
SW Terminal Voltage
V
SW
SW terminal
-0.3 to +30.0
V
FB Terminal Voltage
V
FB
FB terminal
-0.3 to V
DD
V
Power Dissipation
PD MTP-5
200
mW
Operating Temperature
T
opr
-40
to
+85
C
Storage Temperature
T
stg
-65
to
+150
C
Note1)
All voltages are relative to V
SS
= 0V reference.
Note2)
Do not exceed the absolute maximum ratings, otherwise the stress may cause a permanent damage to the IC. It is
also recommended that the IC be used in the range specified in the DC electrical characteristics, or the electrical
stress may cause mulfunctions and impact on the reliability.
DC ELECTRICAL CHARACTERISTICS
(V
DD
=3.6V, V
SHDNb
=3.6V, V
SS
=0V, Ta=25
C)
PARAMETERS
SYMBOL
CONDITIONS
MIN.
TYP.
MAX.
Unit
V
DD
Power Supply
V
DD
2.5
6.5
V
I
STBY1
No
switching
20
30
Quiescent Current
I
STBY2
V
SHDNb
=0V
0.1
1
A
FB Comparator Trip point
V
FBP
1.18 1.2 1.22 V
Output Voltage Line Regulation
V
K
2.5V<V
IN
<6.5V
-0.05 %/V
FB Terminal Bias Current
I
FB
V
FB
= 1.2V
30.0
80.0
nA
V
FB
> 1.0V
500
ns
Switch Off Time
t
OFF
V
FB
< 0.6V
1.6
s
Switch VDS(ON)
V
DS
I
SW
=300mA
250
350
mV
Switch Current Limit
I
CL
300 350 400 mA
SHDNb Terminal Current
I
SHDNb
0.1
1.0
A
SHDNb Input Voltage High
V
SHDNH
0.9 V
SHDNb Input Voltage Low
V
SHDNL
0.25
V
Switch Leakage Current
I
L
Switch Off, V
SW
=28V
0.01
5.0
A
- 5 -
NJU6050
Ver.2004-02-25
TYPICAL PERFORMANCE
(V
DD
=3.6V, V
SHDNb
=3.6V, V
SS
=0V, L=10
H, Ta=25
C)
Output Voltage vs. Input Voltage
Output Voltage vs. Load Current
Efficiency vs. Load Current
Quiescent Current vs. Temperature
V
DS(ON)
vs. Temperature
Feedback Voltage vs. Temperature
PDF 
ZIP