Maximum Junction Temperature (Plastic Package . . . . . . . . . 150°C
Maximum Storage Temperature Range . . . . . . . . . .-65°C to +150°C
Maximum Lead Temperature (Soldering, 10s). . . . . . . . . . . . +300°C
(TSOT - Lead Tips Only)
CAUTION: Stresses above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the
device at these or any other conditions above those indicated in the operational sections of this specification is not implied.
IMPORTANT NOTE: All parameters having Min/Max specifications are guaranteed over the specified temperature range. All parameters are based on pulsed tests,
therefore: T
J
= T
C
= T
A
NOTE:
1.
θ
JA
is measured with the component mounted on a high effective thermal conductivity test board in free air. See Tech Brief TB379 for details.
Electrical Specifications
PARAMETER
V
IN-MIN
V
IN-MAX
V
FB
I
FB
I
IN
V
IN
= 3V, V
ENAB
= 3V, T
A
= -40°C to 85°C unless otherwise specified.
CONDITION
V
OUT
= 16V, I
LED
= 20mA
V
OUT
= 25V, I
LED
= 20mA
80
100
MIN
2.7
5.5
120
100
ENAB = 3V, output not switching
ENAB = 0V
0.6
1.0
1
0.8
T
A
= 25°C
85
82
1.35
90
90
350
350
750
0.01
0.01
0.75
2.5
0.6
1
V
IN
= 2.7V to 5V
I
LED
= 20mA, 3 LEDs
0.2
85
1
1
0.9
1.8
TYP
MAX
UNIT
V
V
mV
nA
mA
µA
MHz
%
%
mA
mA
mΩ
µA
µA
V
V
V
µA
%/V
%
DESCRIPTION
Minimum Operating Voltage
Maximum Operating Voltage
Feedback Voltage
FB Pin Bias Current
Supply Current
F
OSC
D
MAX
Switching Frequency
Maximum Duty Cycle
I
LIM
Switch Current Limit
T
A
= 25°C
280
250
r
DS(ON)
I
LEAK(LX)
I
LEAK(VOUT)
V
DIODE
V
ENAB-HI
V
ENAB-LO
I
ENAB
∆I
LED
/∆V
IN
Eff
Switch On Resistance
Switch Leakage Current
Diode Leakage Current
LX-V
OUT
Diode Forward Voltage
ENAB Voltage High
ENAB Voltage Low
ENAB Pin Bias Current
Line Regulation
Efficiency
I
LX
= 100mA
V
LX
= 27V, Vout = 27V
V
OUT
= 27V
I
DIODE
= 100mA, T
A
= 25°C
2
FN7370.1
December 21, 2005
ISL97631
Typical Application
L1
22µH
V
IN
2.7V~5.5V
C1
1µF
OFF/ON
ENAB
FB
GND
R
SET
4.75Ω
V
IN
LX
V
OUT
ISL97631
C2
0.22µF
LEDs
EFFICIENCY (%)
90
85
80
75
70
65
60
55
50
0
5
10
15
I
LED
(mA)
20
25
30
22µH, V
IN
= 4V
FIGURE 1. TYPICAL APPLICATION CIRCUIT
FIGURE 2. EFFICIENCY vs LED CURRENT (V
IN
= 4V)
Typical Performance Curves
700
600
500
400
300
200
100
0
0
2
V
IN
(V)
4
6
I
O
(mA)
Iq (µA)
18.244
18.242
18.240
18.238
18.236
18.234
18.232
18.230
18.228
0
5
10
V
OUT
(V)
15
20
FIGURE 3. QUIESCENT CURRENT vs V
IN
(ENAB = hi)
FIGURE 4. LOAD REGULATION (V
IN
= 4V)
18.33
18.32
18.31
18.30
I
O
(mA)
18.29
18.28
18.27
18.26
18.25
18.24
18.23
2.5
3.5
V
IN
(V)
4.5
5.5
FREQUENCY (MHz)
1.37
1.36
1.35
1.34
1.33
1.32
1.31
1.30
1.29
1.28
1.27
-60
-40
-20
0
20
40
60
TEMPERATURE (°C)
80
100
FIGURE 5. LINE REGULATION
FIGURE 6. SWITCHING FREQUENCY vs TEMPERATURE
3
FN7370.1
December 21, 2005
ISL97631
Block Diagram
Vin
Enable
LX
1.35MHz
Oscillator and Ramp
1.2MHz
Generator
ISL97631
Vout
PWM
Comparator
PWM Logic
Controller
FET
Driver
Current
Sense
-
GND
GM
Amplifier
FB
95mV
Bandgap
Reference
Generator
GM Amp
Compensation
FIGURE 7. ISL97631 BLOCK DIAGRAM
Pin Descriptions
PIN
PIN
NUMBER NAME
1
2
3
4
5
6
DESCRIPTION
VOUT Output Pin. Connect to the anode of the top
LED and the output filter capacitor .
GND
LX
Ground Pin. Connect to local ground.
Switching Pin. Connect to inductor.
current, the converter operates in either continuous
conduction mode or discontinuous conduction mode. Both
are normal. The forward current of the LED is set using the
R
SET
resistor. In steady state mode, this current is given by
the equation:
V
FB
I
LED
= --------------
-
R
SET
(EQ. 1)
ENAB Enable Pin. Connect to enable signal to turn-on
or off the device.
FB
VIN
Feedback Pin. Connect to the cathode of
bottom LED and the sense resistor.
Input Supply Pin. Connect to the input supply
voltage, the inductor and the input supply
decoupling capacitor.
Shut-Down
When taken low the ENAB pin places the ISL97631 into
power down mode. When in power down, the supply current
is reduced to less than 1µA.
Dimming Control
PWM DIMMING
The ENAB pin also doubles as a brightness control. There
are two different possible dimming control methods. The
first dimming method is controlled through the duty-cycle of
the ENAB input PWM waveform, which can operate at
frequencies up to 1kHz. For frequencies greater than 1kHz,
see Analog Dimming. The LEDs operate at either zero or
full current. This is the PWM dimming control method. The
relationship between the average LED current and the
duty-cycle (D) of the ENAB pin’s waveform is as follows:
V
FB
-
average I
LED
= --------------
⋅
D
R
SET
(EQ. 2)
Detailed Description
The ISL97631 uses a constant frequency, current mode
control scheme to provide excellent line and load
regulation. It can drive up to 6 LEDs in series or 15 LEDs in
parallel/series configuration, with efficiencies of up 85%.
The ISL97631 operates from an input voltage of 2.7V to
5.5V and can boost up to 27V.
Steady-State Operation
The ISL97631 operates with constant frequency PWM. The
switching frequency is around 1.35MHz. Depending on the
input voltage, inductance, number of LEDs and the LED
4
FN7370.1
December 21, 2005
ISL97631
The magnitude of the PWM signal should be higher than the
minimum ENAB voltage high. The bench PWM dimming test
results are shown in Figure 8. In the test, two PWM
frequencies 400Hz and 1kHz are chosen to compare the
linear dimming range. It is clear that there is a wider linear
dimming range for the lower PWM frequency than for the
higher one, due to the self discharge of the output capacitor
through the LEDs during the low ENAB periods. To achieve
a better linearity with high frequencies an NMOS FET can be
placed between the FB pin and the LED stack, with its gate
driven by the same signal as ENAB. This acts to prevent self
discharge of the output capacitor during the off periods. In
the PWM dimming test, the output capacitor is 0.22µF.
20
18
16
14
I
O
(mA)
12
10
8
6
4
2
0
0
20
40
60
80
100
DUTY-CYCLE (%)
1kHz
400Hz
L1
22µH
V
IN
2.7V~5.5V
C1
1µF
OFF/ON
V
IN
LX
V
OUT
ISL97631
ENAB
GND
FB
R1
3.3k
R
SET
4.75
Ω
C2
0.22µF
LEDs
R2
V
Dim
FIGURE 9. ANALOG DIMMING CONTROL APPLICATION
CIRCUIT
The analog dimming circuit can be tailored to a desired
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