NJU6052
PRELIMINARY
White LED Driver with Automatic Dimming Control
!
GENERAL DESCRIPTION
The
NJU6052
is a white LED driver with an automatic
dimming control. It contains an output driver, a PWM
controller, a luminance sensor control (power supply for
sensor & A/D converter), a step-up DC/DC converter, a serial
interface, etc.
The output driver ensures a 60mA maximum capability
which allows the connection of 12 white LEDs (4 series x 3
parallels). Depending on the ambient light sensed with an
external luminance sensor, the PWM controller controls
PWM duty in 8 steps preselected out of 64 steps. In addition,
the frequency of the DC/DC converter is high so that it
permits the use of small, low-profile inductors and capacitors
to minimize the footprint in space-conscious applications.
All of these benefits make the
NJU6052
suitable for the
battery-powered portable applications such as a cellular
phone, a camcorder, PDA, etc.
!
PACKAGE OUTLINE
NJU6052KN1
NJU6052V
!
FEATURES
#
Drives up to 12 white LEDs (4 series x 3 parallels)
#
#
#
#
#
#
#
#
V
SW
= 18.0V(Max.), I
OUT
= 60mA
Built-in PWM Dimming Control
(Selectable 8 out of 64 steps)
Built-in Luminance Sensor Control (Power Supply for Sensor & A/D converter)
(No MPU-access required after initial setting)
Built-in Temperature Compensation Circuit to Suppress the Characteristic Degradation of LEDs
Uses Small Inductor and Capacitors
1.8V to 3.6V Operating Voltage for Logic Circuits (V
DDL
)
3.0V to 5.5V Operating Voltage for Step-up Circuits (V
DD
)
CMOS Technology
Package
: QFN28 / SSOP20
Ver.2004-02-26
-1-
NJU6052
!
PIN DESCRIPTION
No.
QFN
SSOP
4
5
25
26
27
10
9
2
6
12
11
24
23
18
19
20
16
13
17
1
3
7
8
14
15
21
22
28
6
7
1
2
3
10
9
4
8
12
11
20
19
16
17
18
14
13
15
SYMBOL
V
DD
V
DDL
TYPE
Power
Power
DESCRIPTIONS
V
DD
Power Supply
- Power supply for step-up voltage
V
DDL
Power Supply
- Power supply for logic voltage.
- Relation:1.8V
≤
V
DDL
≤
V
DD
should be maintained.
Switch
- All these terminals should be connected together.
Shift Clock
- Serial data is latched on the rising edge of SCK.
Serial Data
Test
- This terminal must be open.
Data Request
“L” : Writing command data
“H” : Reading sensor data
Luminance Sensor Connection
Reset
- Active “L”.
Input
- This terminal is connected to LED anode.
Feedback
Ground
- All these terminals should be connected together.
Oscillator Capacitor Connection / External Clock Input
V
SO
Power Supply
- Power supply for luminance sensor
- 2.4V typical
Reference Voltage
- This terminal must be open.
SW
SCK
DATA
TEST
REQ
SENS
RSTb
V
OUT
FB
V
SS
CX/TCLK
V
SO
REF
Input
Input
Input / Output
Output
Input
Input
Input
Input
Input
Power
Input
Output
Input
5
NC
-
Non Connection
- These terminals must be open.
Ver.2004-02-26
-3-
NJU6052
!
FUNCTIONAL DESCRIPTONS
(1) LED CURRENT CONTROL
The
NJU6052
incorporates the LED current control circuit to regulate the LED current (I
LED
), which is
programmed by the feedback resistor (R
LED
) connected between the FB and V
SS
terminals. The reference voltage
V
REF
is internally regulated to 0.6V typical and connected to the positive input of the built-in comparator A1.
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
=0.6V (TYP.)
Referring to the block diagram is recommended for understanding the operation of the LED current control.
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
on the REF terminal (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 R
LED
), the comparator A1 detects it and turns on the internal MOS switch,
then the current of the inductor L1 begins increasing. When this switch current reaches 720mA and the
comparator A2 detects it, or when the predetermined switch-on-period expires, the MOS switch is turned off.
The L1 then 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 720mA. This switching cycle continues until the
I
LED
reaches the level programmed by the R
LED
, then the I
LED
is maintained constant.
When the feedback voltage is less than 1/2*V
REF
, the current limit of the MOS switch is reduced to 500mA
typical. This action reduces the average inductor-current, minimizes the power dissipation and protects the IC
against high current at start-up.
The total forward-voltage of the LEDs must be greater than the power supply voltage V
DD
, otherwise the
LEDs remain lighting up, being out of control.
(2) OSCILLATOR
The built-in oscillator incorporates a reference power supply, so its frequency is independent from the V
DD
.
The frequency is varied by the external capacitor CX, as shown in Figure 7.
(3) LUMINANCE SENSOR CONTROL
The luminance sensor control circuits consist of the power supply for sensor and the A/D converter. The A/D
converter senses the voltage on the SENS terminal and selects 1 out of 8 registers (PWM REGISTER 0–7). And
the data in the selected register is reflected to the PWM duty (PWM dimming control). The contents of the
registers can be programmed through the serial interface, in other words, the dimming control is user-settable.
The voltage sense and the register selection are updated at regular intervals, and the interval period is set by
the “DIVIDE” bits. The selected register is held by setting “1” at the “HOLD” bit of the command data.
Ver.2004-02-26
-5-
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