ZXLD381
Single or Multi Cell LED Driver Solution
Summary
The ZXLD381 is a single cell LED driver
designed for applications where step-up
voltage conversion from a very low input
voltage is required. These applications mainly
operate from 1.5V or 1.2V cells. The IC
generates constant current pulses that are
ideal for driving single or multiple LEDs over
a wide range of operating voltages.
The ZXLD381 uses a PFM control technique to
drive an internal switching transistor which
exhibits
a low saturation resistance. This ensures high
efficiency, even for input voltages as low as 1V.
The IC can start up under full load and operates
down to an input voltage of only 0.9V.
The ZXLD381 is offered in the space saving
SOT23 package or in die form, offering an
excellent cost vs performance solution for
single cell LED driving applications.
Features
•
•
•
•
•
•
•
85% Efficiency
User adjustable output current
Single cell operation (0.9V minimum)
Low
saturation
transistor
SOT23-3 package
Available also in Die form
Simple Application circuit
voltage
switching
Applications
•
•
•
LED flashlights and torches
LED backlights
White LED driver
V
IN
L1
V
C C
LED
V
OUT
GN D
ZXLD381
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ZXLD381
Absolute maximum ratings
Supply Voltage (V
CC
)
Output Voltage (V
OUT
)
Supply Current
Output Switch Current
Power Dissipation SOT23-3
Power Dissipation Die
Operating Temperature Range
Storage Temperature Range
-0.6V to 10V
-0.6V to 20V
20mA
800mA
450mW
1W
0°C to +85°C
-55°C to +150°C
Electrical Characteristics
Measured at T
AMB
= 25°C, L = 4.7μH and VCC = 1.5V unless otherwise specified.
Parameter
Conditions
Min
Supply Voltage
Operating Range
Minimum Supply
Start-up Voltage
Switch Current
Switch Saturation
Voltage
Switch Leakage
Current
Mean LED Current
Efficiency
L = 10μH
L = 10μH
At turn-off
I
OUT
= 200mA
V
OUT
= 20V
V
LED
= 3.5V
V
LED
= 3.5V
40
40
250
0.9
0.8
320
100
70
55
85
Limits
Typ
Max
2.2
0.9
400
300
120
70
V
V
mA
mV
μA
mA
%
Units
Operating
Frequency
Discharge Pulse
Width
V
LED
= 3.5V
0.7
350
1.5
2.5
kHz
μs
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ZXLD381
Typical Characteristics
ZXLD 381 I
OUT(AVE RAGE )
v s V
CC
fo r L = 2.2uH to 47uH
100
90
80
2. 2uH
70
3. 3uH
60
50
40
30
20
10
0
0.8
1
1.2
1.4
1. 6
1. 8
2
4. 7uH
6. 8uH
10uH
15uH
22uH
47uH
70
3. 3uH
100
90
80
2. 2uH
ZXLD381 Ef ficiency vs V
C C
fo r L = 2.2uH to 47uH
Efficie ncy (%)
I
OUT
(m A)
60
50
40
30
20
10
0
0. 8
1
1.2
1.4
1.6
1. 8
2
4. 7uH
6. 8uH
10uH
15uH
22uH
47uH
V
CC
(V)
V
CC
(V)
ZXLD381 Operating Waveforms for L = 4.7uH, VCC = 1.5V
ZXLD381 I
I N
vs V
CC
for L = 2.2uH to 47uH
35
0
30
0
2.2uH
3.3uH
4.7uH
6.8uH
10uH
15
0
15uH
22uH
10
0
47uH
25
0
I
IN
(m A)
20
0
5
0
0
0.8
1
1.2
1.4
V
CC
(V)
1.6
1
.8
2
Channel-1 (Upper): I
LED
@ 100mA/cm
Channel-2 (Lower): V
OUT
@ 1V/cm
Timebase: 500ns/cm
ZXLD381 f
OSC
vs V
CC
for L = 4.7uH
45
0
40
0
35
0
30
0
f
OSC
(k Hz)
25
0
20
0
15
0
10
0
5
0
0
0.8
1
1.2
1.4
V
CC
(V)
1.6
1.8
2
Issue 1 - April 2008
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ZXLD381
Device Description
The ZXLD381 is a simple PFM, DC-DC controller combined with a high performance internal
switching transistor, enabling the production of a high efficiency boost converter for use in single
cell applications. A block diagram is shown for the ZXLD381 in Fig 1.
VCC
L1
ZXLD 381
VOU T
Pulse
Cont rol
1. 5V
LED
Con
Coff
R sense
GND
Figure 1 ZXLD381 Block Diagram
When power is applied, an oscillator within the pulse control block forces the internal switching
transistor to switch on to start an energy charge cycle. The low saturation voltage switch pulls the
V
OUT
pin close to ground which forces the supply voltage across the external inductor L1. This
causes a current to build up, storing energy in the inductor. During this phase, switch current and
supply voltage are monitored and used by the pulse control circuit to determine the optimum
drive conditions and on-time. At the end of the energy charge cycle, the internal switch is turned
off rapidly, interrupting the current flow through L1 which causes the voltage on V
OUT
to rise
dramatically. When the voltage on V
OUT
reaches the load LED's forward (on) voltage, the inductor
current is transferred from the internal switch to the LED, starting the energy discharge cycle.
With the voltage across the inductor reversed, the current flowing through it (and the LED) now
falls. When the inductor current reaches zero, the voltage on the V
OUT
pin falls back towards V
CC
.
This action is sensed by the pulse control circuit, which initiates the next energy charge cycle.
Except for low level losses, all the energy stored in the inductor during a charge cycle will be
channelled to the load LED during the following discharge cycle.
The current fed into the load LED has a sawtooth waveform, the average (DC) value of which is
kept constant by the pulse control circuit for varying supply voltage and temperature. It is
possible to change the output current given by the ZXLD381 by changing the value of inductor
L1. The larger the inductance of L1, the lower the output current. A table/graph showing the
relationship between inductance and output current is given later in this datasheet. Since the
Issue 1 - April 2008
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ZXLD381
output current of the ZXLD381 is a sawtooth waveform, its peak value is substantially larger than
the DC/average value. The table also provides this data.
The internal switching transistor has a minimum collector-emitter breakdown voltage of 20V and
this sets the maximum load voltage allowable. The minimum value is set by a feature of the pulse
control circuit that requires the load voltage to be at least 0.8V greater than V
CC
. (The device will
function with load voltages smaller than this but output current regulation will be impaired.)
Higher than nominal load voltages will lower the average (DC) output current generated for a
given inductor value.
Application Examples
Standard Operating Mode
L
(uH)
VCC
I
LED
(peak)
(mA)
I
LED
(avg)
(mA)
L1
47
1.5V
35
80
120
190
260
380
510
640
6.5
15
20
30
45
55
67
76
ZXLD381
VOUT
22
15
10
LED
GND
6.8
4.7
3.3
2.2
Note: V
LED
= 3.5V
Low Ripple LED Current Mode
L
(uH)
VCC
I
LED
(mA)
L1
47
D1
1.5V
6
13.5
18
27
41
50
61
69
ZXLD381
VOUT
22
15
C1
10
LED
GND
6.8
4.7
3.3
2.2
Note: V
LED
= 3.5V, D1 = ZHCS1000, C1 = 1μF (low ESR)
Issue 1 - April 2008
© Zetex Semiconductors plc 2008
5
www.zetex.com
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