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ZXLD1362 1A LED driver with internal switch
Description
The ZXLD1362 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 ZXLD1362 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
* * * * * * * * * * * * * Simple low parts count Internal 60V NDMOS switch Up to 1A output current Single pin on/off and brightness control using DC voltage or PWM PWM resolution up to 3000:1 Soft start capability High efficiency (up to 95%) Wide input voltage range: 6V to 60V 65V transient capability Low power shutdown Up to 1MHz switching frequency Inherent open-circuit LED protection Typical 4% output current accuracy
Applications
* * * * * * * * Low voltage halogen replacement LEDs Automotive lighting Low voltage industrial lighting LED back-up lighting Illuminated signs Emergency lighting SELV lighting LCD TV backlighting
Pin connections
LX 1 GND 2 ADJ 3
TSOT23-5 Top view 4 ISENSE 5
Typical application circuit
VIN
VIN (24V) Rs 0.1
L1 68 H C1 4.7 F D1
VIN
ISENSE
LX
100nF
ADJ
ZXLD1362
GND
GND
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ZXLD1362
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)
(Refer to package thermal de-rating curve on page 16)
-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 1W -40 to 125C -55 to 150C 150C
Operating temperature (TOP) Storage temperature (TST) Junction temperature (Tj MAX)
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 case (R JC) 82C/W TBD
Electrical characteristics (test conditions: VIN=24V, Tamb=25C unless otherwise stated)(a)
Symbol VIN VSD IINQoff IINQon Parameter Input voltage Internal regulator shutdown threshold Quiescent supply current with output off Quiescent supply current with output switching Conditions See note
(c)
Min. 6
Typ. 4.7
Max. 60
Unit V V
ADJ pin grounded
65 1.8
TBD TBD
A mA
VSENSE
ADJ pin floating, L=68 H, 3 LEDs, f = 260kHz Mean current sense threshold Measured on ISENSE voltage pin with respect to VIN (Defines LED current setting accuracy) V ADJ = 1.25V ISENSE pin input current Internal reference voltage VSENSE = VIN -0.1 Measured on ADJ pin with pin floating
95
100
105
mV
VSENSEHYS Sense threshold hysteresis ISENSE VREF
15 4 1.25 50 0.3 0.15 0.2 2.5 0.25 TBD
% A V ppm/C V V
VREF / T Temperature coefficient of VREF VADJ VADJoff External control voltage range on ADJ pin for DC brightness control(b) DC voltage on ADJ pin to switch VADJ falling device from active (on) state to quiescent (off) state
NOTES: (a) Production testing of the device is performed at 25C. Functional operation of the device and parameters specified over a -40C to +105C temperature range, are guaranteed by design, characterization and process control. (b) 100% brightness corresponds to VADJ = VADJ(nom) = VREF. Driving the ADJ pin above VREF will increase the VSENSE. threshold and output current proportionally. (c) 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.
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Electrical characteristics (test conditions: VIN=24V, Tamb=25C unless otherwise stated) (cont.)
Symbol VADJon Parameter DC voltage on ADJ pin to switch device from quiescent (off) state to active (on) state Resistance between ADJ pin and VREF Continuous LX switch current LX Switch `On' resistance LX switch leakage current PWM frequency <300Hz PWM amplitude = VREF Measured on ADJ pin 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 (0.1 ) IOUT = 1A @ VLED = 3.6V Driving 3 LEDs LX switch `ON' LX switch `OFF' LX switch 'ON' 0.001 @ ILX = 1 A 0.5 Conditions VADJ rising Min. 0.2 Typ. 0.25 Max. 0.3 Unit V
RADJ ILXmean RLX ILX(leak)
0< VADJ < VREF VADJ > VREF +100mV
30 13.5
50 20
65 25 1 1.0 5 1
k A 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 TSS Soft start time
1000:1 5:1 2 ms
fLX
Operating frequency (See graphs for more detail)
260 240 () TBD 200 () TBD 800 1 0.3 50 0.7
KHz ns ns ns MHz
TONmin TOFFmin
Minimum switch `ON' time Minimum switch `OFF' time
TONmin_REC Recommended minimum switch 'ON' time Recommended maximum fLXmax operating frequency DLX Recommended duty cycle range of output switch at fLXmax TPD Internal comparator propagation delay
ns
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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Pin description
LX 1 GND 2 ADJ 3
TSOT23-5 Top view
Name LX GND ADJ Pin no. 1 2 3
5
VIN
4 ISENSE
Description Drain of NDMOS switch Ground (0V) Multi-function On/Off and brightness control pin: * Leave floating for normal operation.(VADJ = VREF = 1.25V giving nominal average output current IOUTnom = 0.1/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 Connect resistor RS from this pin to VIN to define nominal average output current IOUTnom = 0.1/RS (Note: RSMIN = 0.1 with ADJ pin open-circuit) Input voltage (6V to 60V). Decouple to ground with 4.7 F or higher X7R ceramic capacitor close to device
ISENSE
4
VIN
5
Ordering information
Device ZXLD1362ET5TA Reel size (mm) 180 Reel width (inches) 8 Quantity per reel 3,000 Device mark 1362
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Block diagram
D1 VIN RS L1
5
VIN
4
ISENSE
1
LX
5V C1 4.7 F
Voltage regulator
R1 +
0.2V +
Low voltage detector
MN Adj
3
R4 50K R5 20K 600KHz R2
+
D1 1.25V
+ 1.35V
R3
Gnd
2
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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 block diagram and Figure 1 - 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 100mV (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 = 100mV/RS Nominal ripple current is 15mV/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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VIN
LX voltage
0V Toff VIN 115mV SENSE voltage 85mV 100mV VSENSEVSENSE+ Ton
IOUTnom +15% Coil current IOUTnom IOUTnom -15% 0V
Comparator input voltage
0.15VADJ VADJ 0.15VADJ
Comparator output
5V
0V
Figure 1 - Operating waveforms
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Actual operating waveforms [VIN=15V, RS=0.1 , L=100H]
Normal operation. Output current (Ch1) and LX voltage (Ch2)
Actual operating waveforms [VIN=30V, RS=0.1 , L=100H]
Normal operation. Output current (Ch1) and LX voltage (Ch2)
Actual operating waveforms [VIN=60V, RS=0.1 , L=100H]
Normal operation. Output current (Ch1) and LX voltage (Ch2)
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Typical operating conditions
ZXLD1362 Output Current
L=68H
1100 1090 1080 1070
Output Current (mA)
1060 1050 1040 1030 1020 1010 1000
0
10 1 LED 9 LED
20 2 LED 10 LED 3 LED 11 LED
30 40 Supply Voltage (V) 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED
50 7 LED 15 LED
60 8 LED 16 LED
70
ZXLD1362 Output Current
L=68H
10% 8% 6%
Output Current Deviation
4% 2% 0% -2% -4% -6% -8% -10%
0
10 1 LED 9 LED
20 2 LED 10 LED 3 LED 11 LED
30 4 LED 12 LED
40 5 LED 13 LED 6 LED 14 LED
50 7 LED 15 LED
60 8 LED 16 LED
70
Supply Voltage (V)
ZXLD1362 Efficiency
L=68H
100.0%
90.0%
Efficiency (%)
80.0%
70.0%
60.0%
50.0%
0
10 1 LED 9 LED
20 2 LED 10 LED 3 LED 11 LED
30 40 Supply Voltage (V) 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED
50 7 LED 15 LED
60 8 LED 16 LED
70
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Typical operating conditions
ZXLD1362 Switching Frequency
500
L=68H
400
Switching Frequency (kHz)
300
200
100
0 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
ZXLD1362 Duty Cycle
L=68H
100 90 80 70
Duty Cycle (%)
60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
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ZXLD1362
Typical operating conditions
ZXLD1362 Output Current
L=100H
1100 1090 1080 1070
Output Current (mA)
1060 1050 1040 1030 1020 1010 1000 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
ZXLD1362 Output Current
L=100H
10% 8% 6%
Output Current Deviation
4% 2% 0% 0 -2% -4% -6% -8% -10% Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED 10 20 30 40 50 60 70
ZXLD1362 Efficiency
L=100H
100.0%
90.0%
Efficiency (%)
80.0%
70.0%
60.0%
50.0% 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
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Typical operating conditions
ZXLD1362 Switching Frequency
L=100H
600
500
Switching Frequency (kHz)
400
300
200
100
0 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
ZXLD1362 Duty Cycle
L=100H
100 90 80 70
Duty Cycle (%)
60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
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ZXLD1362
Typical operating conditions
ZXLD1362 Output Current
L=150H
1100 1090 1080 1070
Output Current (mA)
1060 1050 1040 1030 1020 1010 1000 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
ZXLD1362 Output Current
L=150H
10% 8% 6%
Output Current Deviation
4% 2% 0% 0 -2% -4% -6% -8% -10% Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED 10 20 30 40 50 60 70
ZXLD1362 Efficiency
L=150H
100.0%
90.0%
Efficiency (%)
80.0%
70.0%
60.0%
50.0% 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 11 LED 2 LED 12 LED 3 LED 13 LED 4 LED 14 LED 5 LED 15 LED 6 LED 16 LED 7 LED 8 LED 9 LED 10 LED
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ZXLD1362
Typical operating conditions
ZXLD1362 Switching Frequency
L=150H
500 450 400
Switching Frequency (kHz)
350 300 250 200 150 100 50 0 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
ZXLD1362 Duty Cycle
L=150H
100 90 80 70
Duty Cycle (%)
60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
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ZXLD1362
Typical operating conditions
ZXLD1362 Output Current
L=220H
1100 1090 1080 1070
Output Current (mA)
1060 1050 1040 1030 1020 1010 1000 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
ZXLD1362 Output Current
L=220H
10% 8% 6%
Output Current Deviation
4% 2% 0% 0 -2% -4% -6% -8% -10% Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED 10 20 30 40 50 60 70
ZXLD1362 Efficiency
L=220H
100%
90%
Efficiency (%)
80%
70%
60%
50% 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
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ZXLD1362
Typical operating conditions
ZXLD1362 Switching Frequency
L=220H
500
400
Switching Frequency (kHz)
300
200
100
0 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
ZXLD1362 Duty Cycle
L=220H
100 90 80 70
Duty Cycle (%)
60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
ZXLD1362 Duty Cycle
L=220H
100 90 80 70
Duty Cycle (%)
60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 9 LED 2 LED 10 LED 3 LED 11 LED 4 LED 12 LED 5 LED 13 LED 6 LED 14 LED 7 LED 15 LED 8 LED 16 LED
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Typical operating conditions
LED Current vs Vadj
1200
1000
LED Current (mA)
800
600
400
200
0
0
1 ADJ Pin Voltage (V)
R=100m R=150m
2
R=330m
3
800 700
Supply current
Supply current ( A)
600 500 400 300 200 100 0 0 10 20 30 40 50 60 70
Supply voltage (V) Vref
1.243
ADJ pin voltage (V)
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) Shutdow n current
90
Shutdown current ( A)
80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70
Supply voltage (V)
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Typical operating conditions
Lx on-resistance vs supply voltage
1.6 1.4
On-resistance (Ohms)
1.2 1 0.8 0.6 0.4 0.2 0 0 5 10 15 20 25 30 35 -40C 20C 150C
Supply Voltage (V)
Vadj vs Temperature
1.262 1.26 1.258 1.256 7V 9V 12V 20V 30V
Vadj (V)
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 1.4
On-resistance (Ohms)
1.2 1 0.8 0.6 0.4 0.2 0 -50 0 50 100 150 200 7V 9V 12V 20V 30V
Die Temperature (C)
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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 = 0.1/RS [for RS 0.1 ]
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 ( ) 0.1 0.13 0.15 Nominal average output current (mA) 1000 760 667
The above values assume that the ADJ pin is floating and at a nominal voltage of VREF (=1.25V). Note that RS = 0.1 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
ZXLD1362
GND
DC
GND
The nominal average output current in this case is given by: IOUTdc = (VADJ /1.25) x 100mV x 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 voltages above VREF +100mV. 25% for voltages below VREF and 20k 25% for
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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
ZXLD1362
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
ZXLD1362
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
ZXLD1362
GND
If the NMOS transistor within the microcontroller has high Drain / Source capacitance , this arrangement can inject a negative spike into ADJ input of the 1362 and cause erratic operation but the addition of a Schottky clamp diode (cathode to ADJ) 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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ZXLD1362
Shutdown mode Taking the ADJ pin to a voltage below 0.2V for more than approximately 100s, will turn off the output and supply current will fall to a low standby level of 60A 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 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 12
Soft Start Time (ms)
10 8 6 4 2 0 -2 0 20 40 60 Capacitance (nf) 80 100 120
Actual operating waveforms [VIN=24V, RS=0.1 , L=68H, 22nF on ADJ] Soft-start operation. Output current (Ch2) and LX voltage (Ch1)
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ZXLD1362
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 40 VIN = 40 to 50 VIN = 50 to 60 CIN = 2.2 F CIN = 4.7 F 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. If higher voltages are used and the CIN is 10 F. This can be an electrolytic capacitor provide a suitable 1 F ceramic capacitor is also used and positioned as close the VIN of the IC as possible. A suitable TDK capacitor would be CK657NX7R2AXXM. The following web sites are useful when finding alternatives: www.tdk.com www.murata.com www.t-yuden.com www.kemet.com www.avxcorp.com
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ZXLD1362
Inductor selection
Recommended inductor values for the ZXLD1362 are in the range 68 H to 220 H. 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). 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 ZXLD1362 may be selected from the MSS range manufactured by Coilcraft, or the NPIS range manufactured by NIC components. 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 following equations can be used as a guide, with reference to Figure 1 - Operating waveforms.
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ZXLD1362
LX Switch 'On' time
LI T ON = ------------------------------------------------------------------------------------V IN - V LED - I avg ( R S + rL + R LX )
Note: TONmin>240ns LX Switch 'Off' time
LI T OFF = --------------------------------------------------------------------V LED + VD + I avg ( R S + rL )
Note: TOFFmin>200ns Where: L is the coil inductance (H) rL is the coil resistance ( ) RS is the current sense resistance Iavg is the required LED current (A) I is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg} VIN is the supply voltage (V) VLED is the total LED forward voltage (V) RLX is the switch resistance ( ) {=0.5 nominal} VD is the diode forward voltage at the required load current (V)
Example:
For VIN =24V, L=68H, rL=0.034 , RS=0.1 , RLX=0.5 , VLED=3.6V*3, Iavg =1A and VD =0.49V TON = (68e-6 x 0.3)/(24 - 10.8 - 0.904) = 1.66s TOFF = (68e-6 x 0.3)/(10.8 + 0.49 + 0.404)= 1.75s This gives an operating frequency of 294 kHz and a duty cycle of 0.49. These and other equations are available as a spreadsheet calculator from the Zetex website at www.zetex.com/ZXLD1362 Note that, in practice, the duty cycle and operating frequency will deviate from the calculated values due to dynamic switching delays, switch rise/fall times and losses in the external components. Optimum performance will be achieved by setting the duty cycle close to 0.5 at the nominal supply voltage. This helps to equalize the undershoot and overshoot and improves temperature stability of the output current.
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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 85C. 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.
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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:
VIN Rs
LED
Cled
L1 D1
VIN
ISENSE
LX
ZXLD1362
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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Maximum Power Dissipation
1100 1000 900 800 700
Power (mW)
600 500 400 300 200 100 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 ZXLD1362 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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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 100nF GND
High voltage tracks
ADJ
ZXLD1362
GND
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 ZXLD1362 evaluation boards are available on request. These boards contain LEDs to allow quick testing of the 1362 device. Additional terminals allow for interfacing to customers own LED products.
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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 2 - Low frequency PWM operating waveforms).
VADJ
PWM Voltage
Ton
Toff
0V
IOUTnom Output Current
0.1/Rs IOUTavg
0
Figure 2 Low frequency PWM operating waveforms The average value of output current in this mode is given by: IOUTavg 0.1DPWM/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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Intentionally left blank
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Package outline - TSOT23-5
DIM A A1 A2 b c D E E1 e e1 L L2 a Min. 0.01 0.84 0.30 0.12
Millimeters Max. 1.00 0.10 0.90 0.45 0.20 2.90 BSC 2.80 BSC 1.60 BSC 0.95 BSC 1.90 BSC 0.30 0.25 BSC 4 12 4 0.50 0.0118 Min. 0.0003 0.0330 0.0118 0.0047
Inches Max. 0.0393 0.0039 0.0354 0.0177 0.0078 0.114 BSC 0.110 BSC 0.062 BSC 0.0374 BSC 0.0748 BSC 0.0196 0.010 BSC 12
Note: Controlling dimensions are in millimeters. Approximate dimensions are provided in inches
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Definitions Product change Zetex Semiconductors 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 Zetex 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. Zetex 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 Zetex 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 Zetex Semiconductors plc. 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 Zetex' 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 Zetex sales office. Quality of product Zetex 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 Zetex Semiconductors or one of our regionally authorized distributors. For a complete listing of authorized distributors please visit: www.zetex.com/salesnetwork Zetex Semiconductors 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 Zetex Semiconductors 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 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. Zetex sales offices Europe Zetex GmbH Kustermann-park Balanstrae 59 D-81541 Munchen Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 europe.sales@zetex.com Americas Zetex Inc 700 Veterans Memorial Highway Hauppauge, NY 11788 USA Telephone: (1) 631 360 2222 Fax: (1) 631 360 8222 usa.sales@zetex.com Asia Pacific Zetex (Asia Ltd) 3701-04 Metroplaza Tower 1 Hing Fong Road, Kwai Fong Hong Kong Telephone: (852) 26100 611 Fax: (852) 24250 494 asia.sales@zetex.com Corporate Headquarters Zetex Semiconductors plc Zetex Technology Park, Chadderton Oldham, OL9 9LL United Kingdom Telephone: (44) 161 622 4444 Fax: (44) 161 622 4446 hq@zetex.com
(c) 2007 Published by Zetex Semiconductors plc
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