DIODES ZXLD1366

A Product Line of
Diodes Incorporated
ZXLD1366
High accuracy 1A, 60V LED driver with internal switch
Description
The ZXLD1366 is a continuous mode
inductive step-down converter, designed for
driving single or multiple series connected
LEDs efficiently from a voltage source higher
than the LED voltage. The device operates
from an input supply between 6V and 60V and
provides an externally adjustable output
current of up to 1A. Depending upon supply
voltage and external components, this can
provide up to 48 watts of output power.
The ZXLD1366 includes the output switch and
a high-side output current sensing circuit,
which uses an external resistor to set the
nominal average output current.
Output current can be adjusted above, or
below the set value, by applying an external
control signal to the 'ADJ' pin.
The ADJ pin will accept either a DC voltage or a
PWM waveform. Depending upon the control
frequency, this will provide either a continuous
(dimmed) or a gated output current. Soft-start
can be forced using an external capacitor from
the ADJ pin to ground.
Applying a voltage of 0.2V or lower to the ADJ
pin turns the output off and switches the device
into a low current standby state.
Features
Applications
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Typically better than 0.8% output current
accuracy
Available in thermally enhanced DFN
package
Simple and with low part count
Single pin on/off and brightness control
using DC voltage or PWM
PWM resolution up to 1000:1
High efficiency (up to 97%)
Wide input voltage range: 6V to 60V
Inherent open-circuit LED protection
Pin connections
Low voltage halogen replacement LEDs
Automotive lighting
Low voltage industrial lighting
LED back-up lighting
Illuminated signs
Emergency lighting
SELV lighting
LCD TV backlighting
Refrigeration lights
Typical application circuit
D1
Rs
VIN (24V)
0.2⍀
5 VIN
LX 1
GND 2
4 ISENSE
ADJ 3
LX
VIN
GND
GND
L1
ISENSE
ADJ
C1
TSOT23-5
Top view
4.7␮F
DFN633
Top view
100nF
VIN
ISENSE
ADJ
ZXLD1366
LX
GND
GND
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ZXLD1366
Absolute maximum ratings (voltages to GND unless otherwise stated)
Input voltage (VIN)
ISENSE voltage (VSENSE)
LX output voltage (VLX)
Adjust pin input voltage (VADJ)
Switch output current (ILX)
Power dissipation (Ptot)
-0.3V to +60V (65V for 0.5 sec)
+0.3V to -5V (measured with respect to VIN)
-0.3V to +60V (65V for 0.5 sec)
-0.3V to +6V
1.25A
SOT23-5; 1W: DFN; 1.8W
(Refer to package thermal de-rating curve on page 26)
Operating temperature (TOP)
Storage temperature (TST)
Junction temperature (Tj MAX)
-40 to 125°C
-55 to 150°C
150°C
These are stress ratings only. Operation outside the absolute maximum ratings may cause device failure. Operation
at the absolute maximum ratings, for extended periods, may reduce device reliability.
Thermal resistance
Junction to ambient (R⍜JA)
Junction to board (RΨJB)
Junction to case (R⍜JC)
TSOT23-5
DFN
82°C/W
33°C/W
-
44°C/W
14°C/W
Electrical characteristics (test conditions: VIN=24V, Tamb=25°C unless otherwise stated)(a)
Symbol
VIN
Parameter
Input voltage
VSU
Internal regulator start-up threshold
VSD
Internal regulator shutdown threshold
IINQoff
Quiescent supply current
with output off
Quiescent supply current
with output switching(c)
IINQon
Conditions
See note(b)
Min.
6
Typ.
Max.
60
4.85
5.2
4.4
4.75
ADJ pin grounded
VSENSE
ADJ pin floating,
L=68␮H, 3 LEDs,
f = 260kHz
Mean current sense threshold voltage Measured on ISENSE pin
(Defines LED current setting accuracy)
with respect to VIN
VADJ = 1.25V; VIN=18V
VSENSEHYS
Sense threshold hysteresis
ISENSE
ISENSE pin input current
VSENSE = VIN -0.2
VREF
Internal reference voltage
Measured on ADJ pin
with pin floating
65
VADJoff
External control voltage range on ADJ
pin for DC brightness control(d)
DC voltage on ADJ pin to switch
device from active (on) state to
quiescent (off) state
108
4
205
10
0.15
0.2
␮A
V
50
VADJ falling
mV
%
1.25
0.3
␮A
mA
±15
⌬VREF /⌬T Temperature coefficient of VREF
VADJ
200
V
V
1.6
195
Unit
V
ppm/°C
2.5
V
0.27
V
NOTES:
(a) Production testing of the device is performed at 25°C. Functional operation of the device and parameters specified over
a -40°C to +105°C temperature range, are guaranteed by design, characterization and process control.
(b) VIN > 16V to fully enhance output transistor. Otherwise out current must be derated - see graphs. Operation at low
supply may cause excessive heating due to increased on-resistance. Tested at 7V guaranteed for 6V by design.
(c) Static current of device is approximately 700 ␮A, see Graph, Page 17
(d) 100% brightness corresponds to VADJ = VADJ(nom) = VREF. Driving the ADJ pin above VREF will increase the VSENSE.
threshold and output current proportionally.
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ZXLD1366
Electrical characteristics (test conditions: VIN=24V, Tamb=25°C unless otherwise stated) (cont.)
Symbol
VADJon
RADJ
Parameter
DC voltage on ADJ pin to switch
device from quiescent (off) state
to active (on) state
Resistance between ADJ pin
and VREF
ILXmean
Continuous LX switch current
RLX
LX Switch ‘On’ resistance
ILX(leak)
LX switch leakage current
Conditions
VADJ rising
Min.
0.2
Typ.
0.25
Max.
0.3
Unit
V
0< VADJ < VREF
VADJ > VREF +100mV
30
10.4
50
14.2
65
18
k⍀
k⍀
1
A
0.75
⍀
5
␮A
@ ILX = 1 A
DPWM(LF) Duty cycle range of PWM signal
applied to ADJ pin during low
frequency PWM dimming mode
Brightness control range
DCADJ (*) DC Brightness control range
PWM frequency
<300Hz
PWM amplitude = VREF
Measured on ADJ pin
TSS
Soft start time
fLX
Operating frequency
(See graphs for more detail)
0.001
TONmin
Minimum switch ‘ON’ time
TOFFmin
Minimum switch ‘OFF’ time
LX switch ‘OFF’
1
1000:1
5:1
See note (*)
Time taken for output
current to reach 90% of
final value after voltage
on ADJ pin has risen
above 0.3V
Requires external
capacitor 22nF. See
graphs for more details
ADJ pin floating
L = 68␮H (0.2⍀)
IOUT = 1A @ VLED = 3.6V
Driving 3 LEDs
LX switch ‘ON’
TPWmin_REC Recommended minimum switch
pulse width
Recommended maximum
fLXmax
operating frequency
DLX
Recommended duty cycle range
of output switch at fLXmax
0.5
LX switch 'ON' or ‘OFF’
2
ms
260
kHz
130 (†)
ns
70 (†)
800
ns
ns
500
0.3
kHz
0.7
NOTES:
(*) Ratio of maximum brightness to minimum brightness before shutdown VREF = 1.25/0.3. VREF externally driven to 2.5V,
ratio 10:1.
(†) Parameters are not tested at production. Parameters are guaranteed by design, characterization and process control.
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ZXLD1366
Pin description
LX 1
5
VIN
GND 2
4 ISENSE
ADJ 3
LX
VIN
GND
GND
ISENSE
ADJ
DFN633
Top view
TSOT23-5
Top view
Name
Pin no.
LX
1
Drain of NDMOS switch
GND
2
Ground (0V)
ADJ
3
Multi-function On/Off and brightness control pin:
• Leave floating for normal operation.(VADJ = VREF = 1.25V giving nominal
average output current IOUTnom = 0.2V/RS)
• Drive to voltage below 0.2V to turn off output current
• Drive with DC voltage (0.3V < VADJ < 2.5V) to adjust output current from
25% to 200% of IOUTnom
• Connect a capacitor from this pin to ground to set soft-start time.
Soft start time increases approximately 0.2ms/nF
ISENSE
4
Connect resistor RS from this pin to VIN to define nominal average output
current IOUTnom = 0.2V/RS
(Note: RSMIN = 0.2⍀ with ADJ pin open-circuit)
VIN
5
Input voltage (6V to 60V). Decouple to ground with 4.7␮F or higher X7R
ceramic capacitor close to device
Tab
Description
Not internally connected. Connect to ground plane to improve thermal
efficiency
Ordering information
Device
Reel size
(inches)
Reel width
(mm)
Quantity
per reel
ZXLD1366ET5TA
7”
8
3,000
1366
ZXLD1366DACTC
13”
12
3,000
1366
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Device mark
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ZXLD1366
D1
VIN
L1
RS
5
5V
C1
4.7␮F
4
VIN
1
ISENSE
LX
R1
Voltage
regulator
+
0.2V
+
Low voltage
detector
MN
+
Adj
3
R4
50K
D1
1.25V
Gnd
R5
20K
600KHz
R2
+
R3
1.35V
2
Figure 1
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Block diagram - Pin connection for TSOT package
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ZXLD1366
Device description
The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a selfoscillating continuous-mode buck converter.
Device operation (refer to Figure 1 - Block diagram and Figure 2 Operating waveforms)
Operation can be best understood by assuming that the ADJ pin of the device is unconnected and
the voltage on this pin (VADJ) appears directly at the (+) input of the comparator.
When input voltage VIN is first applied, the initial current in L1 and RS is zero and there is no
output from the current sense circuit. Under this condition, the (-) input to the comparator is at
ground and its output is high. This turns MN on and switches the LX pin low, causing current to
flow from VIN to ground, via RS, L1 and the LED(s). The current rises at a rate determined by VIN
and L1 to produce a voltage ramp (VSENSE) across RS. The supply referred voltage VSENSE is
forced across internal resistor R1 by the current sense circuit and produces a proportional current
in internal resistors R2 and R3. This produces a ground referred rising voltage at the (-) input of
the comparator. When this reaches the threshold voltage (VADJ), the comparator output switches
low and MN turns off. The comparator output also drives another NMOS switch, which bypasses
internal resistor R3 to provide a controlled amount of hysteresis. The hysteresis is set by R3 to be
nominally 15% of VADJ.
When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to VIN. The current
decays at a rate determined by the LED(s) and diode forward voltages to produce a falling voltage
at the input of the comparator. When this voltage returns to VADJ, the comparator output switches
high again. This cycle of events repeats, with the comparator input ramping between limits of
VADJ ± 15%.
Switching thresholds
With VADJ = VREF, the ratios of R1, R2 and R3 define an average VSENSE switching threshold of
200mV (measured on the ISENSE pin with respect to VIN). The average output current IOUTnom is
then defined by this voltage and RS according to:
IOUTnom = 200mV/RS
Nominal ripple current is ±30mV/RS
Adjusting output current
The device contains a low pass filter between the ADJ pin and the threshold comparator and an
internal current limiting resistor (50k⍀ nom) between ADJ and the internal reference voltage. This
allows the ADJ pin to be overdriven with either DC or pulse signals to change the VSENSE
switching threshold and adjust the output current.
Details of the different modes of adjusting output current are given in the applications section.
Output shutdown
The output of the low pass filter drives the shutdown circuit. When the input voltage to this circuit
falls below the threshold (0.2V nom.), the internal regulator and the output switch are turned off.
The voltage reference remains powered during shutdown to provide the bias current for the
shutdown circuit. Quiescent supply current during shutdown is nominally 60␮A and switch
leakage is below 5␮A.
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ZXLD1366
VIN
LX voltage
0V
Toff
Ton
VIN
170mV
230mV
200mV
VSENSE-
SENSE voltage
VSENSE+
IOUTnom +15%
IOUTnom
Coil current
IOUTnom -15%
0V
Comparator
input voltage
0.15VADJ
VADJ
0.15VADJ
Comparator
output
5V
0V
Figure 2
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Theoretical operating waveforms
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ZXLD1366
Actual operating waveforms [VIN=15V, RS=0.2⍀, L=68µH]
Normal operation. Output current (Ch3) and LX voltage (Ch2)
Actual operating waveforms [VIN=30V, RS=0.2⍀, L=68µH]
Normal operation. Output current (Ch3) and LX voltage (Ch2)
Actual operating waveforms [VIN=60V, RS=0.2⍀, L=68µH]
Normal operation. Output current (Ch3) and LX voltage (Ch2)
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ZXLD1366
Intentionally left blank
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ZXLD1366
Typical operating conditions
ZXLD1366 Output Current
L=68μH
1.100
01 LEDs
03 LEDs
1.080
05 LEDs
07 LEDs
09 LEDs
11 LEDs
Output Current (A)
1.060
13 LEDs
15 LEDs
1.040
1.020
1.000
0.980
0
10
20
30
40
50
60
40
50
60
40
50
60
Supply Voltage (V)
ZXLD1366 Output Current Deviation
L=68μH
10%
8%
Output Current Deviation (%)
6%
4%
2%
0%
01 LEDs
-2%
03 LEDs
05 LEDs
-4%
07 LEDs
09 LEDs
-6%
11 LEDs
13 LEDs
-8%
15 LEDs
-10%
0
10
20
30
Supply Voltage (V)
ZXLD1366 Efficiency
L=68μH
100%
01 LEDs
95%
03 LEDs
05 LEDs
90%
07 LEDs
09 LEDs
Efficiency (%)
85%
11 LEDs
13 LEDs
80%
15 LEDs
75%
70%
65%
60%
55%
50%
0
10
20
30
Supply Voltage (V)
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ZXLD1366
Typical operating conditions
ZXLD1366 Switching Frequency
L=68μH
500
450
01 LEDs
03 LEDs
400
Switching Frequency (kHz)
05 LEDs
07 LEDs
350
09 LEDs
11 LEDs
300
13 LEDs
15 LEDs
250
200
150
100
50
0
0
10
20
30
40
50
40
50
60
Supply Voltage (V)
ZXLD1366 Duty Cycle
L=68μH
100%
90%
80%
Duty Cycle (%)
70%
60%
50%
01 LEDs
03 LEDs
40%
05 LEDs
07 LEDs
30%
09 LEDs
11 LEDs
20%
13 LEDs
15 LEDs
10%
0%
0
10
20
30
60
Supply Voltage (V)
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ZXLD1366
Typical operating conditions
ZXLD1366 Output Current
L=100μH
1.100
01 LEDs
03 LEDs
1.080
05 LEDs
07 LEDs
09 LEDs
Output Current (A)
1.060
11 LEDs
13 LEDs
15 LEDs
1.040
1.020
1.000
0.980
0.960
0
10
20
30
40
50
60
Supply Voltage (V)
ZXLD1366 Output Current Deviation
L=100μH
10%
8%
Output Current Deviation (%)
6%
4%
2%
0%
01 LEDs
-2%
03 LEDs
05 LEDs
-4%
07 LEDs
09 LEDs
-6%
11 LEDs
13 LEDs
-8%
15 LEDs
-10%
0
10
20
30
40
50
60
Supply Voltage (V)
ZXLD1366 Efficiency
L=100μH
100%
01 LEDs
95%
03 LEDs
05 LEDs
90%
07 LEDs
09 LEDs
Efficiency (%)
85%
11 LEDs
13 LEDs
80%
15 LEDs
75%
70%
65%
60%
55%
50%
0
10
20
30
40
50
60
Supply Voltage (V)
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ZXLD1366
Typical operating conditions
ZXLD1366 Switching Frequency
L=100μH
500
450
01 LEDs
03 LEDs
400
Switching Frequency (kHz)
05 LEDs
07 LEDs
350
09 LEDs
11 LEDs
300
13 LEDs
15 LEDs
250
200
150
100
50
0
0
10
20
30
40
50
60
Supply Voltage (V)
ZXLD1366 Duty Cycle
L=100μH
100%
90%
80%
Duty Cycle (%)
70%
60%
50%
01 LEDs
40%
03 LEDs
05 LEDs
30%
07 LEDs
09 LEDs
20%
11 LEDs
13 LEDs
10%
15 LEDs
0%
0
10
20
30
40
50
60
Supply Voltage (V)
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ZXLD1366
Typical operating conditions
ZXLD1366 Output Current
L=150μH
1.100
01 LEDs
03 LEDs
1.080
05 LEDs
07 LEDs
Output Current (A)
09 LEDs
1.060
11 LEDs
13 LEDs
15 LEDs
1.040
1.020
1.000
0.980
0
10
20
30
40
50
60
50
60
Supply Voltage (V)
ZXLD1366 Output Current Deviation
L=150μH
10%
8%
Output Current Deviation (%)
6%
4%
2%
0%
01 LEDs
-2%
03 LEDs
05 LEDs
-4%
07 LEDs
09 LEDs
-6%
11 LEDs
13 LEDs
-8%
15 LEDs
-10%
0
10
20
30
40
Supply Voltage (V)
ZXLD1366 Efficiency
L=150μH
100%
01 LEDs
95%
03 LEDs
05 LEDs
90%
07 LEDs
09 LEDs
Efficiency (%)
85%
11 LEDs
13 LEDs
80%
15 LEDs
75%
70%
65%
60%
55%
50%
0
10
20
30
40
50
60
Supply Voltage (V)
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ZXLD1366
Typical operating conditions
ZXLD1366 Switching Frequency
L=150μH
500
450
01 LEDs
03 LEDs
Switching Frequency (kHz)
400
05 LEDs
07 LEDs
350
09 LEDs
11 LEDs
300
13 LEDs
15 LEDs
250
200
150
100
50
0
0
10
20
30
40
50
40
50
60
Supply Voltage (V)
ZXLD1366 Duty Cycle
L=150μH
100%
90%
80%
Duty Cycle (%)
70%
60%
50%
01 LEDs
40%
03 LEDs
05 LEDs
30%
07 LEDs
09 LEDs
20%
11 LEDs
13 LEDs
10%
15 LEDs
0%
0
10
20
30
60
Supply Voltage (V)
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ZXLD1366
Typical operating conditions
ZXLD1366 Output Current
L=220μH
1.100
01 LEDs
1.080
03 LEDs
05 LEDs
Output Current (A)
07 LEDs
09 LEDs
1.060
11 LEDs
13 LEDs
15 LEDs
1.040
1.020
1.000
0.980
0
10
20
30
40
50
60
Supply Voltage (V)
ZXLD1366 Output Current Deviation
L=220μH
10%
8%
Output Current Deviation (%)
6%
4%
2%
0%
-2%
01 LEDs
-4%
05 LEDs
03 LEDs
07 LEDs
09 LEDs
-6%
11 LEDs
13 LEDs
-8%
15 LEDs
-10%
0
10
20
30
40
50
60
40
50
60
Supply Voltage (V)
ZXLD1366 Efficiency
L=220μH
100%
95%
01 LEDs
03 LEDs
90%
05 LEDs
07 LEDs
85%
09 LEDs
Efficiency (%)
11 LEDs
80%
13 LEDs
15 LEDs
75%
70%
65%
60%
55%
50%
0
10
20
30
Supply Voltage (V)
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ZXLD1366
Typical operating conditions
ZXLD1366 Switching Frequency
L=220μH
500
450
01 LEDs
03 LEDs
400
Switching Frequency (kHz)
05 LEDs
07 LEDs
350
09 LEDs
11 LEDs
300
13 LEDs
15 LEDs
250
200
150
100
50
0
0
10
20
30
40
50
60
40
50
60
Supply Voltage (V)
ZXLD1366 Duty Cycle
L=220μH
100%
90%
80%
Duty Cycle (%)
70%
60%
50%
40%
01 LEDs
03 LEDs
30%
05 LEDs
07 LEDs
20%
09 LEDs
11 LEDs
10%
13 LEDs
15 LEDs
0%
0
10
20
30
Supply Voltage (V)
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ZXLD1366
Typical operating conditions
LED Current vs Vadj
1200
1000
LED Current (mA)
800
600
400
200
0
0
1
2
3
ADJ Pin Voltage (V)
R=200mΩ
R=300mΩ
R=680mΩ
Supply current
800
Supply current (␮A)
700
600
500
400
Output transistor
fully enhanced
300
Output transistor
not fully enhanced
200
100
0
0
10
20
30
40
50
60
70
Supply voltage (V)
Vref
ADJ pin voltage (V)
1.243
1.2425
1.242
1.2415
1.241
1.2405
1.24
1.2395
1.239
1.2385
1.238
0
10
20
30
40
50
60
70
Supply voltage (V)
Shutdown current
Shutdown current (␮A)
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
Supply voltage (V)
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ZXLD1366
Typical operating conditions
Lx on-resistance vs supply voltage
O n -resistan ce (O h m s)
2 .5
2
1 .5
-4 0 oC
2 5 oC
1 2 5 oC
1
0 .5
0
0
10
20
30
40
50
60
70
S u p p ly V o lta g e (V )
Vadj vs Temperature
1.262
1.26
1.258
Vadj (V)
1.256
7V
9V
12V
20V
30V
1.254
1.252
1.25
1.248
1.246
1.244
-50
0
50
100
150
200
Temperature (C)
Lx on-resistance vs die temperature
1.6
On-resistance (Ohms)
1.4
1.2
1
7V
9V
12V
20V
30V
0.8
0.6
0.4
0.2
0
-50
0
50
100
150
200
Die Temperature (C)
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ZXLD1366
Application notes
Setting nominal average output current with external resistor RS
The nominal average output current in the LED(s) is determined by the value of the external
current sense resistor (RS) connected between VIN and ISENSE and is given by:
IOUTnom = 0.2/RS [for RS ⱖ 0.2⍀]
The table below gives values of nominal average output current for several preferred values of
current setting resistor (RS) in the typical application circuit shown on page 1:
RS (⍀)
Nominal average
output current (mA)
0.20
1000
0.27
740
0.56
357
The above values assume that the ADJ pin is floating and at a nominal voltage of VREF (=1.25V).
Note that RS = 0.2⍀ is the minimum allowed value of sense resistor under these conditions to
maintain switch current below the specified maximum value.
It is possible to use different values of RS if the ADJ pin is driven from an external voltage. (See
next section).
Output current adjustment by external DC control voltage
The ADJ pin can be driven by an external dc voltage (VADJ), as shown, to adjust the output current
to a value above or below the nominal average value defined by RS.
+
ADJ
ZXLD1366
GND
DC
GND
The nominal average output current in this case is given by:
IOUTdc = (VADJ /1.25) x (0.2/RS) [for 0.3< VADJ <2.5V]
Note that 100% brightness setting corresponds to VADJ = VREF. When driving the ADJ pin above
1.25V, RS must be increased in proportion to prevent IOUTdc exceeding 1A maximum.
The input impedance of the ADJ pin is 50k⍀ ±25% for voltages below VREF and 14.2k⍀ ±25% for
voltages above VREF +100mV.
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ZXLD1366
Output current adjustment by PWM control
Directly driving ADJ input
A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the ADJ pin, as
shown below, to adjust the output current to a value above or below the nominal average value
set by resistor RS:
PWM
VADJ
ADJ
0V
ZXLD1366
GND
GND
Driving the ADJ input via open collector transistor
The recommended method of driving the ADJ pin and controlling the amplitude of the PWM
waveform is to use a small NPN switching transistor as shown below:
ADJ
PWM
ZXLD1366
GND
GND
This scheme uses the 50k resistor between the ADJ pin and the internal voltage reference as a
pull-up resistor for the external transistor.
Driving the ADJ input from a microcontroller
Another possibility is to drive the device from the open drain output of a microcontroller. The
diagram below shows one method of doing this:
MCU
3.3k
ADJ
ZXLD1366
GND
If the NMOS transistor within the microcontroller has high Gate / Drain capacitance, this
arrangement can inject a negative spike into ADJ input of the ZXLD1366 and cause erratic
operation but the addition of a Schottky clamp diode (eg Diodes Inc. SD103CWS) to ground and
inclusion of a series resistor (3.3k) will prevent this. See the section on PWM dimming for more
details of the various modes of control using high frequency and low frequency PWM signals.
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ZXLD1366
Shutdown mode
Taking the ADJ pin to a voltage below 0.2V for more than approximately 100µs will turn off the
output and supply current to a low standby level of 65µA nominal.
Note that the ADJ pin is not a logic input. Taking the ADJ pin to a voltage above VREF will increase
output current above the 100% nominal average value. (See page 18 graphs for details).
Soft-start
An external capacitor from the ADJ pin to ground will provide a soft-start delay, by increasing the
time taken for the voltage on this pin to rise to the turn-on threshold and by slowing down the
rate of rise of the control voltage at the input of the comparator. Adding capacitance increases this
delay by approximately 0.2ms/nF. The graph below shows the variation of soft-start time for
different values of capacitor.
Soft Start Time vs Capacitance from ADJ pin to Ground
16
14
Soft Start Time (ms)
12
10
8
6
4
2
0
-2
0
20
40
60
Capacitance (nf)
80
100
120
Actual operating waveforms [VIN=60V, RS=0.2⍀, L=68µH, 22nF on ADJ]
Soft-start operation. LX voltage (CH2) and Output current (CH3) using a 22nF external capacitor
on the ADJ pin.
2
3
Issue 2 - September 2008
© Diodes Incorporated 2008
Ch3 500mA
M 400μs 5.0 S/s
Ch 2 20.0V
A Ch2 \ 12.0 V
22
200 ns/pt
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ZXLD1366
VIN capacitor selection
A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in
series with the supply source impedance and lowers overall efficiency. This capacitor has to
supply the relatively high peak current to the coil and smooth the current ripple on the input
supply.
To avoid transients into the IC, the size of the input capacitor will depend on the VIN voltage:
VIN = 6 to 40V
CIN = 2.2␮F
VIN = 40 to 50V
CIN = 4.7␮F
VIN = 50 to 60V
CIN = 10␮F
When the input voltage is close to the output voltage the input current increases which puts more
demand on the input capacitor. The minimum value of 2.2␮F may need to be increased to 4.7␮F;
higher values will improve performance at lower input voltages, especially when the source
impedance is high. The input capacitor should be placed as close as possible to the IC.
For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better
dielectric is recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this
application and should NOT be used.
When higher voltages are used with the CIN = 10␮F, an electrolytic capacitor can be used provided
that a suitable 1␮F ceramic capacitor is also used and positioned as close to the VIN pin as
possible.
A suitable capacitor would be NACEW100M1006.3x8TR13F (NIC Components).
The following web sites are useful when finding alternatives:
www.murata.com
www.niccomp.com
www.kemet.com
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ZXLD1366
Inductor selection
Recommended inductor values for the ZXLD1366 are in the range 68 µH to 220 µH.
Higher values of inductance are recommended at higher supply voltages in order to minimize
errors due to switching delays, which result in increased ripple and lower efficiency. Higher
values of inductance also result in a smaller change in output current over the supply voltage
range. (see graphs pages 10-17). The inductor should be mounted as close to the device as
possible with low resistance connections to the LX and VIN pins.
The chosen coil should have a saturation current higher than the peak output current and a
continuous current rating above the required mean output current.
Suitable coils for use with the ZXLD1366 may be selected from the MSS range manufactured by
Coilcraft, or the NPIS range manufactured by NIC components. The following websites may be
useful in finding suitable components
www.coilcraft.com
www.niccomp.com
www.wuerth-elektronik.de
The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times
within the specified limits over the supply voltage and load current range.
The graph Figure 3 below can be used to select a recommended inductor based on maintaining
the ZXLD1366 case temperature below 60°C. For detailed performance characteristics for the
inductor values 68, 100, 150 and 220µH see graphs on pages 10-17.
Minimum Recommended Inductor
2% Accuracy, <60°C Case Temperature
15
Legend
14
68uH
13
100uH
12
150uH
Number of LEDs
11
220uH
10
9
8
7
6
5
4
3
2
1
0
10
20
30
40
50
60
Supply Voltage (V)
Figure 3
Issue 2 - September 2008
© Diodes Incorporated 2008
ZXLD1366 Minimum recommended inductor (TSOT)
24
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ZXLD1366
M in im u m R e c o m m e n d e d In d u c to r
2% A ccuracy, <60°C C ase T em perature
16
Legend
15
68µH
14
100µH
13
150µH
12
220µH
N u m b er o f L E D s
11
10
9
8
7
6
5
4
3
2
1
0.00
10.00
20.00
30.00
40.00
50.00
60.00
S u p p ly V o ltag e (V )
Figure 4 ZXLD1366 Minimum recommended inductor (DFN)
Diode Selection
For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance
Schottky diode* with low reverse leakage at the maximum operating voltage and temperature.
They also provide better efficiency than silicon diodes, due to a combination of lower forward
voltage and reduced recovery time.
It is important to select parts with a peak current rating above the peak coil current and a
continuous current rating higher than the maximum output load current. It is very important to
consider the reverse leakage of the diode when operating above 85°C. Excess leakage will
increase the power dissipation in the device and if close to the load may create a thermal runaway
condition.
The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will
increase the peak voltage on the LX output. If a silicon diode is used, care should be taken to
ensure that the total voltage appearing on the LX pin including supply ripple, does not exceed the
specified maximum value.
*A suitable Schottky diode would be B3100 (Diodes Inc).
Issue 2 - September 2008
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ZXLD1366
Reducing output ripple
Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor Cled
across the LED(s) as shown below:
D1
VIN
Rs
LED
Cled
L1
VIN
ISENSE
LX
ZXLD1366
A value of 1␮F will reduce the supply ripple current by a factor three (approx.). Proportionally
lower ripple can be achieved with higher capacitor values. Note that the capacitor will not affect
operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of
LED voltage.
By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp
to a more sinusoidal version without altering the mean current value .
Operation at low supply voltage
Below the under-voltage lockout threshold (VSD) the drive to the output transistor is turned off to
prevent device operation with excessive on-resistance of the output transistor. The output
transistor is not full enhanced until the supply voltage exceeds approximately 17V. At supply
voltages between VSD and 17V care must be taken to avoid excessive power dissipation due to
the on-resistance.
If the supply voltage is always less than 30V continuous (or less than 40V for less than 0.5s) an
alternative device is available, the ZXLD1360.
Note that when driving loads of two or more LEDs, the forward drop will normally be sufficient
to prevent the device from switching below approximately 6V. This will minimize the risk of
damage to the device.
Thermal considerations
When operating the device at high ambient temperatures, or when driving maximum load
current, care must be taken to avoid exceeding the package power dissipation limits. The graph
below gives details for power derating. This assumes the device to be mounted on a 25mm2 PCB
with 1oz copper standing in still air.
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ZXLD1366
Maximum Power Dissipation
2000
1800
DFN
1600
Power (mW)
1400
1200
1000
TSOT
800
600
400
200
0
-50
-30
-10
10
30
50
70
90
110
130
150
Ambient Temperature (Deg C)
Note that the device power dissipation will most often be a maximum at minimum supply
voltage. It will also increase if the efficiency of the circuit is low. This may result from the use of
unsuitable coils, or excessive parasitic output capacitance on the switch output.
Thermal compensation of output current
High luminance LEDs often need to be supplied with a temperature compensated current in order
to maintain stable and reliable operation at all drive levels. The LEDs are usually mounted
remotely from the device so, for this reason, the temperature coefficients of the internal circuits
for the ZXLD1366 have been optimized to minimize the change in output current when no
compensation is employed. If output current compensation is required, it is possible to use an
external temperature sensing network - normally using Negative Temperature Coefficient (NTC)
thermistors and/or diodes, mounted very close to the LED(s). The output of the sensing network
can be used to drive the ADJ pin in order to reduce output current with increasing temperature.
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ZXLD1366
Layout considerations
LX pin
The LX pin of the device is a fast switching node, so PCB tracks should be kept as short as
possible. To minimize ground 'bounce', the ground pin of the device should be soldered directly
to the ground plane.
Coil and decoupling capacitors and current sense resistor
It is particularly important to mount the coil and the input decoupling capacitor as close to the
device pins as possible to minimize parasitic resistance and inductance, which will degrade
efficiency. It is also important to minimize any track resistance in series with current sense resistor
RS. Its best to connect VIN directly to one end of RS and Isense directly to the opposite end of RS
with no other currents flowing in these tracks. It is important that the cathode current of the
Schottky diode does not flow in a track between RS and VIN as this may give an apparent higher
measure of current than is actual because of track resistance.
ADJ pin
The ADJ pin is a high impedance input for voltages up to 1.35V so, when left floating, PCB tracks
to this pin should be as short as possible to reduce noise pickup. A 100nF capacitor from the ADJ
pin to ground will reduce frequency modulation of the output under these conditions. An
additional series 3.3k⍀ resistor can also be used when driving the ADJ pin from an external circuit
(see below). This resistor will provide filtering for low frequency noise and provide protection
against high voltage transients.
3.3k
ADJ
100nF
ZXLD1366
GND
GND
High voltage tracks
Avoid running any high voltage tracks close to the ADJ pin, to reduce the risk of leakage currents
due to board contamination. The ADJ pin is soft-clamped for voltages above 1.35V to desensitize
it to leakage that might raise the ADJ pin voltage and cause excessive output current. However,
a ground ring placed around the ADJ pin is recommended to minimize changes in output current
under these conditions.
Evaluation PCB
ZXLD1366 evaluation boards are available on request. Terminals allow for interfacing to
customers own LED products.
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ZXLD1366
Dimming output current using PWM
Low frequency PWM mode
When the ADJ pin is driven with a low frequency PWM signal (eg 100Hz), with a high level voltage
VADJ and a low level of zero, the output of the internal low pass filter will swing between 0V and
VADJ, causing the input to the shutdown circuit to fall below its turn-off threshold (200mV nom)
when the ADJ pin is low. This will cause the output current to be switched on and off at the PWM
frequency, resulting in an average output current IOUTavg proportional to the PWM duty cycle.
(See Figure 4 - Low frequency PWM operating waveforms).
VADJ
Ton
PWM Voltage
Toff
0V
IOUTnom
0.2 / Rs
IOUTavg
Output Current
0
Figure 4
Low frequency PWM operating waveforms
The average value of output current in this mode is given by:
IOUTavg 0.2DPWM/RS [for DPWM >0.001]
This mode is preferable if optimum LED 'whiteness' is required. It will also provide the widest
possible dimming range (approx. 1000:1) and higher efficiency at the expense of greater output
ripple.
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ZXLD1366
Package outline - TSOT23-5
DIM
A
A1
A2
b
c
D
E
E1
e
e1
L
L2
a°
Millimeters
Min.
0.01
0.84
0.30
0.12
Inches
Max.
1.00
0.10
0.90
0.45
0.20
Min.
0.0003
0.0330
0.0118
0.0047
2.90 BSC
2.80 BSC
1.60 BSC
0.95 BSC
1.90 BSC
0.30
0.114 BSC
0.110 BSC
0.062 BSC
0.0374 BSC
0.0748 BSC
0.50
0.0118
12°
4°
0.25 BSC
4°
Max.
0.0393
0.0039
0.0354
0.0177
0.0078
0.0196
0.010 BSC
12°
Note: Controlling dimensions are in millimeters. Approximate dimensions are provided in inches
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ZXLD1366
Package outline - DFN633
e1
D
E
E2
L
D2
b
TOP VIEW
e
BOTTOM VIEW
A3
A1
A
PIN 1 DOT
BY MARKING
PIN #1 IDENTIFICATION
CHAMFER 0.300X45°
SIDE VIEW
DIM
Millimeters
Inches
DIM
Min.
Max.
Min.
Max.
A
0.700
0.800
0.0275
0.0315
D2
A1
0.000
0.050
0.000
0.00197
e
A3
0.203 REF
0.008
Millimeters
Inches
Min.
Max.
Min.
Max.
1.950
2.050
0.0768
0.0807
0.950 BSC
0.0374 BSC
E
2.950
3.050
0.116
0.120
1.150
1.250
0.0452
0.0492
b
0.300
0.400
0.0118
0.0157
E2
D
2.950
3.050
0.116
0.120
e1
L
1.900REF
0.450
0.550
0.0748
0.0177
0.0216
Note controlling dimensions in millimetres. Approximate dimensions are provided in inches
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ZXLD1366
Definitions
Product change
Diodes Incorporated reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product or
service. Customers are solely responsible for obtaining the latest relevant information before placing orders.
Applications disclaimer
The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for
the user’s application and meets with the user’s requirements. No representation or warranty is given and no liability whatsoever is
assumed by Diodes Inc. with respect to the accuracy or use of such information, or infringement of patents or other intellectual property
rights arising from such use or otherwise. Diodes Inc. does not assume any legal responsibility or will not be held legally liable (whether
in contract, tort (including negligence), breach of statutory duty, restriction or otherwise) for any damages, loss of profit, business,
contract, opportunity or consequential loss in the use of these circuit applications, under any circumstances.
Life support
Diodes Inc. products are specifically not authorized for use as critical components in life support devices or systems without the express
written approval of the Chief Executive Officer of Diodes Incorporated. As used herein:
A. Life support devices or systems are devices or systems which:
1. are intended to implant into the body
or
2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the
labelling can be reasonably expected to result in significant injury to the user.
B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to
cause the failure of the life support device or to affect its safety or effectiveness.
Reproduction
The product specifications contained in this publication are issued to provide outline information only which (unless agreed by the
company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a
representation relating to the products or services concerned.
Terms and Conditions
All products are sold subjects to Diodes Inc. terms and conditions of sale, and this disclaimer (save in the event of a conflict between the
two when the terms of the contract shall prevail) according to region, supplied at the time of order acknowledgement.
For the latest information on technology, delivery terms and conditions and prices, please contact your nearest Diodes sales office.
Quality of product
Diodes Zetex Semiconductors Limited is an ISO 9001 and TS16949 certified semiconductor manufacturer.
To ensure quality of service and products we strongly advise the purchase of parts directly from Diodes Zetex or one of our regionally
authorized distributors. For a complete listing of authorized distributors please visit: www.zetex.com or www.diodes.com
Diodes Inc does not warrant or accept any liability whatsoever in respect of any parts purchased through unauthorized sales channels.
ESD (Electrostatic discharge)
Semiconductor devices are susceptible to damage by ESD. Suitable precautions should be taken when handling and transporting devices.
The possible damage to devices depends on the circumstances of the handling and transporting, and the nature of the device. The extent
of damage can vary from immediate functional or parametric malfunction to degradation of function or performance in use over time.
Devices suspected of being affected should be replaced.
Green compliance
Diodes Inc. is committed to environmental excellence in all aspects of its operations which includes meeting or exceeding regulatory
requirements with respect to the use of hazardous substances. Numerous successful programs have been implemented to reduce the use
of hazardous substances and/or emissions.
All Diodes Zetex components are compliant with the RoHS directive, and through this it is supporting its customers in their compliance
with WEEE and ELV directives.
Product status key:
“Preview”
Future device intended for production at some point. Samples may be available
“Active”
Product status recommended for new designs
“Last time buy (LTB)”
Device will be discontinued and last time buy period and delivery is in effect
“Not recommended for new designs” Device is still in production to support existing designs and production
“Obsolete”
Production has been discontinued
Datasheet status key:
“Draft version”
This term denotes a very early datasheet version and contains highly provisional information, which
may change in any manner without notice.
“Provisional version”
This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance.
However, changes to the test conditions and specifications may occur, at any time and without notice.
“Issue”
This term denotes an issued datasheet containing finalized specifications. However, changes to
specifications may occur, at any time and without notice.
Diodes Zetex sales offices
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Corporate Headquarters
Zetex GmbH
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Balanstraße 59
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Telefon: (49) 89 45 49 49 0
Fax: (49) 89 45 49 49 49
europe.sales@zetex.com
Zetex Inc
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Hauppauge, NY 11788
USA
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Hing Fong Road, Kwai Fong
Hong Kong
Diodes Incorporated
15660 N. Dallas Parkway
Suite 850, Dallas,
X57248, USA
Telephone: (1) 631 360 2222
Fax: (1) 631 360 8222
usa.sales@zetex.com
Telephone: (852) 26100 611
Fax: (852) 24250 494
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Telephone: (1) 972 385 2810
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