NJU6050
PRELIMINARY
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.
PACKAGE OUTLINE
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)
SW
V
SS
FB
1
2
3
5
V
DD
4
SHDNb
Ver.2004-02-25
-1-
NJU6050
PIN DESCRIPTIONS
No.
1
2
3
4
5
SYMBOL
SW
V
SS
FB
SHDNb
V
DD
TYPE
Input
Power
Input
Input
Power
Switch Terminal
Ground Terminal
Feedback Terminal
Shutdown Terminal Active “L”
“L” : Shutdown
“H” : Normal Operation
Power Supply terminal
DESCRIPTION
BLOCK DIAGRAM
L1
V
IN
C1
Bias
DC/DC
Control
-
+
V
REF
1.2V
R
LED
V
SS
I
LED
V
DD
SW
FB
D2
C2
D1
V
OUT
SHDNb
Shutdown
circuit
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.
R
LED
=
V
REF
I
LED
--- Formula (1)
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.
-2-
Ver.2004-02-25
NJU6050
(2) Inductor Selection
The following equation gives the optimum inductor value:
L=
V
OUT
-V
IN(MIN)
+V
D
I
CL
t
OFF
Formula (2)
VD
I
CL
: Schottky diode Voltage
: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
+
V
IN(MAX)
+V
DS
L
t
DELAY
Formula (3)
V
DS
V
IN(MAX)
: Drain-Source voltage of MOS switch (=I
LIMIT
*R
ON
)
: 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.
Ver.2004-02-25
-3-
NJU6050
ABSOLUTE MAXIMUMN RATINGS
Ta=25
°C
PARAMETERS
VDD Power Supply
SHDNb Terminal Voltage
SW Terminal Voltage
FB Terminal Voltage
Power Dissipation
Operating Temperature
Storage Temperature
Note1)
Note2)
SYMBOL
V
DD
V
SHDNb
V
SW
V
FB
PD
T
opr
T
stg
CONDITIONS
SHDNb terminal
SW terminal
FB terminal
MTP-5
RATINGS
-0.3 to +7.0
-0.3 to +7.0
-0.3 to +30.0
-0.3 to V
DD
200
-40 to +85
-65 to +150
UNIT
V
V
V
V
mW
°C
°C
All voltages are relative to V
SS
= 0V reference.
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
V
DD
Power Supply
Quiescent Current
FB Comparator Trip point
Output Voltage Line Regulation
FB Terminal Bias Current
Switch Off Time
Switch VDS(ON)
Switch Current Limit
SHDNb Terminal Current
SHDNb Input Voltage High
SHDNb Input Voltage Low
Switch Leakage Current
SYMBOL
V
DD
I
STBY1
I
STBY2
V
FBP
V
K
I
FB
t
OFF
V
DS
I
CL
I
SHDNb
V
SHDNH
V
SHDNL
I
L
No switching
V
SHDNb
=0V
1.18
2.5V<V
IN
<6.5V
V
FB
= 1.2V
V
FB
> 1.0V
V
FB
< 0.6V
I
SW
=300mA
300
0.9
Switch Off, V
SW
=28V
0.01
0.25
5.0
CONDITIONS
MIN.
2.5
20
0.1
1.2
-0.05
30.0
500
1.6
250
350
0.1
TYP.
MAX.
6.5
30
1
1.22
80.0
Unit
V
µA
V
%/V
nA
ns
µs
mV
mA
µA
V
V
µA
350
400
1.0
-4-
Ver.2004-02-25
NJU6050
TYPICAL PERFORMANCE
(V
DD
=3.6V, V
SHDNb
=3.6V, V
SS
=0V, L=10µH, 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
Ver.2004-02-25
-5-
测试/测量
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