ZXLD1352 Datasheet by Diodes Incorporated

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ALBSGO ZXLD1352 DC D1 “WES:
ZXLD1352
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ZXLD1352
OBSOL ETE PA RT D I SCONT INU E D
30V 350mA LED DRIVER WITH 1000:1 PWM DIMMING AND AEC-Q100
Description
The ZXLD1352 is a continuous mode inductive step-down
converter with integrated switch and high side current sense.
It operates from an input supply from 7V to 30V driving
single and multiple series connected LEDs effciently
externally adjustable output current up to 350mA.
The ZXLD1352 has been qualified to AEC-Q100 Grade
2 allowing it to operate at ambient temperatures from -40 to
+105°C.
DC voltage or a PWM waveform. 1000:1 adjustment of
output current is possible using PWM control. 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
Single Pin on/off and Brightness Control Using DC
Voltage or PWM
1000:1 PWM Dimming Range
High Efficiency (Up to 95%)*
Wide Input Voltage Range: 7V to 30V
40V Transient Capability
Up to 1MHz Switching Frequency
Typical 4% Output Current Accuracy
Qualified to AEC-Q100 Grade 2
Available in Green Molding (no Br, Sb) with Lead Free
Finish/RoHS Compliant
Pin Assignments
Applications
Low Voltage Halogen Replacement LEDs
Automotive Lighting
Low Voltage Industrial Lighting
LED Back-up Lighting
Illuminated Signs
* Using standard external components as specified under electrical characteristics. Efficiency is dependent upon the number of LEDs driven and on external
component types and values.
Typical Applications Circuit
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VIN ISENSE LX
GND
ZXLD1352
ADJ
VIN
(12V - 30V)
Rs
0.33
1µF
C1
GND
D1
ZLLS1000
47mHL1
PW M
m5! m ZXLD1352 H U 9 . QQ % ——| [1 [WW]?
ZXLD1352
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Pin Descriptions
Name
Pin No.
Description
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=0.1/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
Drive with PWM signal from open-collector or open-drain transistor, to adjust output current.
o Adjustment range 0.1% to 100% of IOUTnom for 100Hz < f < 1kHz
Connect a capacitor from this pin to ground to define soft-start time. Soft-start time is
approx.0.5ms/nF)
ISENSE
4
Connect resistor RS from this to VIN to define nominal average output current IOUTnom=0.1/RS
(Note: RSMIN=0.27V with ADJ pin open circuit)
VIN
5
Input voltage (7V to 30V). Decouple to ground with 1µF of higher X7R ceramic capacitor close to
device
Functional Block Diagram
MN
-
+
VIN
Comparator
R1
R2
R3
GND
-
+
LX
VIN ISENSE
Current sense circuit
VIN
ADJ
RS
L1
D1
5V
Voltage
regulator
Shutdown
circuit
Vref
50k
1.25V
600KHz
C1
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Absolute Maximum Ratings (@TA = +25°C, unless otherwise specified.)
Symbol
Parameter
Rating
Unit
VIN
Input Voltage
-0.3 to +30
(40V for 0.5 sec)
V
VSENSE
ISENSE Voltage
+0.3 to -5
(measured with respect to VIN)
V
VLX
LX Output Voltage
-0.3 to +30
(40V for 0.5 sec)
V
VADJ
Adjust Pin Input Voltage
-0.3 to +6
V
ILX
Switch Output Current
500
mA
PTOT
Power Dissipation
(Refer to Package thermal de-rating curve on page 17)
450
mW
TST
Storage Temperature
-55 to 150
°C
TJ MAX
Junction Temperature
150
°C
These are stress ratings only. Operation above the absolute maximum rating may cause device failure. Operation at the absolute maximum ratings, for extended
periods, may reduce device reliability.
Thermal Resistance
Symbol
Parameter
Rating
Unit
θJA
Junction to Ambient
200
°C/W
Recommended Operating Conditions
Symbol
Parameter
Min
Max
Unit
VIN
Input voltage
7
30
V
tOFFMIN
Minimum switch off-time
-
200
ns
tONMIN
Minimum switch on-time
-
200
ns
fLXmax
Recommended maximum operating frequency
-
1
MHz
DLX
Duty cycle range
0.01
0.99
-
TA
Ambient Temperature range
-40
+105
°C
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ALSSGO ZXLD1352 Vsu w VADJ VSENSEHVS SENSE SENSE SENSE Vw VREF AVREF A Ram REF LXmm RLX LXUeak] PD
ZXLD1352
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Electrical Characteristics (Test conditions: VIN = 12V, Tamb = +25°C, unless otherwise specified.)
Symbol
Parameter
Conditions
Min.
Typ.
Max.
Unit
VSU
Internal regulator start-up threshold
VIN rising
-
4.8
-
V
IINQoff
Quiescent supply current
with output off
ADJ pin grounded
-
20
30
µA
IINQon
Quiescent supply current
with output switching
ADJ pin floating
f = 250kHz
-
250
500
µA
VSENSE
Mean current sense threshold voltage
(defines LED current setting accuracy)
Measured on ISENSE pin with respect
to VIN VADJ =1.25V
95
100
105
mV
VSENSEHYS
Sense threshold hysteresis
-
-
±15
-
%
ISENSE
ISENSE pin input current
VSENSE = VIN -0.1
-
1.25
10
µA
VREF
Internal reference voltage
Measured on ADJ pin with pin floating
1.21
1.25
1.29
V
VREF /T
Temperature coefficient of VREF
-
-
50
-
ppm/°C
VADJ
External control voltage range on ADJ
pin for dc brightness control1
-
0.3
-
2.5
V
VADJoff
DC voltage on ADJ pin to switch device
from active (on) state to quiescent (off)
state
VADJ falling
0.15
0.2
0.25
V
VADJon
DC voltage on ADJ pin to switch device
from quiescent (off) state to active (on)
state
VADJ rising
0.2
0.25
0.3
V
RADJ
Resistance between ADJ pin and VREF
-
35
-
65
kΩ
ILXmean
Continuous LX switch current
-
-
-
0.37
A
RLX
LX Switch ‘On’ resistance
-
-
1.5
2
ILX(leak)
LX switch leakage current
-
-
-
1
µA
DPWM(LF)
Duty cycle range of PWM signal
applied to ADJ pin during PWM
dimming mode
PWM frequency 100Hz 1kHz
PWM amplitude = VREF
Measured on ADJ pin
0.001
-
1
-
Brightness control range
-
-
1000:1
-
-
fLX
Operating frequency
(See graphs for more detail)
ADJ pin floating
L=100µH (0.82V)
IOUT=350mA @ VLED=3.4V
Driving 1 LED
-
250
-
kHz
tPD
Internal comparator propagation delay
-
-
50
-
ns
Notes: 1. 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.
2. 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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Device Description
The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a self-oscillating 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 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 for noise suppression 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 PWM signals to adjust the output current. The filter is first order, comprising one section with a cut-off frequency of
nominally 600kHz.
Details of the different modes of adjusting output current are given in the applications section.
Output shutdown
The ADJ pin 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 20mA and switch leakage is below 1mA.
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+15%
ZXLD1352
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Device Description
Figure 2. Operating Waveforms
0V
VIN
100mV115mV
0V
SENSE voltage
VSENSE+
VSENSE-
Toff Ton
85mV
0V
5V
VIN
0.15VADJ
0.15VADJ
IOUTnom
IOUTnom +15%
IOUTnom -15%
VADJ
LX voltage
Coil current
Comparator
input voltage
Comparator
output
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ALBSGO mnes. ZXLD1352 Tek Run: 50.0MS/s Hi Res [4,”, M 1.00115 Ch3 .f 8mv C3 Max 396mV C3 Mll’l 282mV C3 Period 4‘40pus Low signal amplltude C3 Mean 333.6mV 22 Dec 2005 10:58:55 ‘ ‘ ‘ ‘ ’w”’r”r”1”fl***r** ‘ ‘ ‘ ‘ ‘ ‘ ‘ w ‘ ‘ ‘ ‘ 7 77777 \fi’kiiriifiiifi ‘7" ‘ ‘ ‘ ‘ ‘ ‘ ‘ w ‘ ‘ ‘ ‘ Chum MADOpSSDSIs zoom/pt m2 20.1w Achznzov
ZXLD1352
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Typical Operating Waveforms [VIN = 12V, RS = 0.3Ω, L = 100µH]
Normal Operation. Output Current (Ch3) and LX Voltage (Ch1)
Start-up Waveforms. Output Current (Ch3), LX Voltage (Ch2)
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Typical Characteristics
For typical application circuit driving 1W Luxeon® white LED(s) at VIN=12V and Tamb=25°C unless otherwise stated.
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07
57
08
58
09
59
001
03520251015
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Efficiency (%)
DEL1
DEL2
DEL3
DEL4
DEL5
DEL6
DEL7
DEL8
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0
2.0
4.0
6.0
8.0
1
2.1
0352
0
251015
)V(NIV
Duty Cycle
DEL1
DEL2
DEL3
DEL4
DEL5
DEL6
DEL7
DEL8
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0
001
002
003
004
005
006
03520251015
)V(NIV
Frequency (kHz)
DEL1
DEL2
DEL3
DEL4
DEL5
DEL6
DEL7
DEL8
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8-
6-
4-
2-
0
2
4
6
8
03520251015
)V(NIV
Deviation from nominal set current (%)
DEL1
DEL2
DEL3
DEL4
DEL5
DEL6
DEL7
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07
57
08
58
09
59
001
03520251015
)V(NIV
Efficiency (%)
DEL1
DEL2
DEL3
DEL4
DEL5
DEL6
DEL7
egatloVtupnIsvelcyCytuD smhO33.0=sR,Hu74=L
0
2.0
4.0
6.0
8.0
1
03520251015
)V(NIV
Duty Cycle
DEL1
DEL2
DEL3
DEL4
DEL5
DEL6
DEL7
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0
001
002
003
004
005
006
007
00
8
035
2
0
25
101
5
)V(NIV
Frequency (kHz)
DEL1
DE
L
2
DEL3
DEL4
DEL5
DEL6
DEL7
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7
4
=
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51-
01-
5-
0
5
01
51
02
03520251015
)V(NIV
Deviation from nominal
set current (%)
DEL1
DEL2
DEL3
DEL4
DEL5
DEL6
DEL7
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Typical Characteristics (Cont.)
sDELfo.oNsvycneiciffE smhO33.0=sR,Hu022=L
57
08
58
09
59
001
03520251015
)V(NIV
Efficiency (%)
DEL1
DEL2
DEL3
DEL4
DEL5
DEL6
DEL7
DEL8
egatloVtupnIsvelcyCytuD smhO33.0=sR,Hu022=L
0
2.0
4.0
6.0
8.0
1
03520251015
)V(NIV
Duty Cycle
DEL1
DE
L
2
DEL3
DEL4
DEL5
DEL6
DEL7
DEL8
egatloVtupnIsvycneuqerFgnitarepO smhO33.0=sR,Hu022=L
0
05
001
051
002
052
003
053
03520251015
)V(NIV
Frequency (kHz)
DEL1
DE
L
2
DEL3
DEL4
DEL5
DEL6
DEL7
DEL8
egatloVtupnIsvnoitairaVtnerruCtuptuO smhO33.0=sR,Hu022=L
6-
5-
4-
3-
2-
1-
0
1
2
03520251015
)V(NIV
Deviation from nominal set current (%)
DEL1
DEL2
DEL3
DEL4
DEL5
DEL6
DEL7
DEL8
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ZXLD1352
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Typical Characteristics (Cont.)
Vref vs Vin at low supply voltage
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4 5 6 7 8 9 10
Vin (V)
Vref (V)
egnaregatlovylppuslanimonrevoniVsvferV
5142.1
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5242.1
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0
05
001
051
002
052
003
053
35.225.115.00
)V(JDAV
Iout mean (mA)
mhO65.0=sR
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0
001
002
003
004
005
035202510150
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Iin (uA)
)tnecseiuQ(niVsvtnerruCylppuS
0
5
01
51
02
035202510150
)V(niV
Iin (uA)
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ALBSGO ZXLD1352 VADJ u Tmnlmluri mun-m, Rn”: ohm ‘ 255 . :5 - _m" z —m { M 3 WWW —WW 2. m a so mu m "nu-mum um or u mm 'mr mm," vs Immm "mum. c: 0mm eumm Changn vs 1mp-umn - nmm‘ Rs-nm Ohms E E s 2 -§ 1 5 mm" m... nnmma‘ gamma: 3 o nmmomwmmmn‘znw .2 mum... (mg c) Dlllpul cumm chlnge u, Tun per-tun vm-uv, L-1 onuH, Runs: ohm: 495— m 41 5 “my.“ [Deg c) 0mm Currlnl chum v. rump-mm- vm. w. L=1DUuH. ago 33 Ohm! ‘.— E E; 2 a 2 i E E 49— am An 20 a z: 40 an an we ‘20 «a mwmumc)
ZXLD1352
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Typical Characteristics (Cont.)
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Application Information
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>0.27
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)
0.27
370
0.3
333
0.33
300
0.39
256
The above values assume that the ADJ pin is floating and at a nominal voltage of VREF (=1.25V). Note that RS=0.27 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.
The nominal average output current in this case is given by:
IOUTdc = 0.08*VADJ/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 370mA maximum.
The input impedance of the ADJ pin is 50k ±25%.
GND
ZXLD1352
ADJ
GND
+
DC
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Application Information (Cont.)
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:
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:
This scheme uses the 50k resistor between the ADJ pin and the internal voltage reference as a pull-up resistor for the external
transistor eg MMBT3904.
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:
If the NMOS transistor within the microcontroller has high Gate / Drain capacitance, this arrangement can inject a negative spike
into ADJ input of the ZXLD1352 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.
PWM
GND
0V
VADJ
GND
ZXLD1352
ADJ
GND
ZXLD1352
ADJ
MCU 10k
PWM
GND
ZXLD1352
ADJ
GND
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sonsunnmncan-cmc-mm ADJ vlnmflmum «% m m swan-e ("II 0Q)
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Application Information (Cont.)
Shutdown Mode
Taking the ADJ pin to a voltage below 0.2V will turn off the output and supply current will fall to a low standby level of 20µA
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 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. The
graph below shows the variation of soft-start time for different values of capacitor.
Inherent open-circuit LED protection
If the connection to the LED(s) is open-circuited, the coil is isolated from the LX pin of the chip, so the device will not be damaged,
unlike in many boost converters, where the back EMF may damage the internal switch by forcing the drain above its breakdown
voltage.
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. A minimum value of F is acceptable if the input source is close to the device, but 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 are recommended. Capacitors
with Y5V dielectric are not suitable for decoupling in this application and should NOT be used.
A table of recommended manufacturers is provided below:
Manufacturer
Website
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Kemet
www.kemet.com
AVX
www.avxcorp.com
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Application Information (Cont.)
Inductor Selection
Recommended inductor values for the ZXLD1352 are in the range 47µH to 220µH.
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 ZXLD1352 are listed in the table below:
Part No.
L (μH)
DCR ()
ISAT (A)
Manufacturer
DO1608C
47
0.64
0.5
CoilCraft
MSS6132ML
47
0.38
0.56
68
0.58
0.47
100
0.82
0.39
CD104-MC
220
0.55
0.53
Sumida
NP04SB470M
47
0.27
0.38
Taiyo Yuden
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.
LX Switch 'On' time
Note: TONmin>200ns
LX Switch 'Off' time
Note: TOFFmin>200ns
Where:
L is the coil inductance (H)
rL is the coil resistance ()
Iavg is the required LED current (A)
DI 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 ()
VD is the diode forward voltage at the required load current (V)
TON LI
VIN VLED
Iavg RSrL RLX
+ + 
----------------------------------------------------------------------------------------
=
TOFF LI
VLED VD Iavg RSrL+ + +
-----------------------------------------------------------------------
=
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Application Information (Cont.)
Example
For VIN =12V, L=47μH, rL=0.64, VLED=3.4V, Iavg =350mA and VD =0.36V
TON = (47e-6 x 0.105)/(12 - 3.4 - 0.672) = 0.622μs
TOFF = (47e-6 x 0.105)/(3.4 + 0.36 + 0.322)= 1.21μs
This gives an operating frequency of 546kHz and a duty cycle of 0.34.
These and other equations are available as a spreadsheet calculator from the Zetex website. Go to
www.diodes.com/ZXLD1352
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.
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. The recommended diode for use with this part is the ZLLS1000.
This has approximately ten times lower leakage than standard Schottky diodes, which are unsuitable for use above 85°C. It also
provides better efficiency than silicon diodes, due to a combination of lower forward voltage and reduced recovery time.
The table below gives the typical characteristics for the ZLLS1000:
Diode
Forward Voltage at
100mA
(mV)
Continuous Current
(mA)
Reverse Leakage
At 30V 85ºC
(μA)
Package
ZLLS1000
310
1000
300
TSOT23-5
If alternative diodes are used, 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.
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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Application Information (Cont.)
Reducing Output Ripple
Peak to peak ripple current in the LED can be reduced, if required, by shunting a capacitor Cled across the LED(s) as shown
below:
A value of F will reduce nominal 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.
Operation at low supply voltage
The internal regulator disables the drive to the switch until the supply has risen above the start-up threshold (VSU). Above this
threshold, the device will start to operate. However, with the supply voltage below the specified minimum value, the switch duty
cycle will be high and the device power dissipation will be at a maximum. Care should be taken to avoid operating the device
under such conditions in the application, in order to minimize the risk of exceeding the maximum allowed die temperature. (See
next section on thermal considerations).
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.
VIN
VIN
ISENSE LX
ZXLD1352
Rs
L1
CledLED
D1
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Application Information (Cont.)
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 ZXLD1352 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.
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
It is particularly important to mount the coil and the input decoupling capacitor close to the device to minimize parasitic resistance
and inductance, which will degrade efficiency. It is also important to take account of any track resistance in series with current
sense resistor RS.
ADJ pin
The ADJ pin is a high impedance input, 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 10kΩ 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.
High voltage tracks
Avoid running any high voltage tracks close to the ADJ pin, to reduce the risk of leakage due to board contamination. Any such
leakage may raise the ADJ pin voltage and cause excessive output current. A ground ring placed around the ADJ pin will
minimize changes in output current under these conditions.
GND
ZXLD1352
ADJ
10k
100nF
GND
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Dimming Output Current Using PWM
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 current will 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)
Figure 3. Low Frequency PWM Operating Waveforms
The average value of output current in this mode is given by:
The average value of output current is given by:
IOUTavg = 0.1DPWM/RS for DPWM >0.01
PWM dimming is preferable to DC dimming 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.
VADJ
PWM Voltage Ton
IOUTavg
0V
0
Toff
0.1/RsIOUTnom
Output Current
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ZXLD1352 *l I+ . Fl F4 F3|_' A? 1—1— A A2 j1_ a FA1 —1_ DIM Millimeters Inches Min. Max. Min. Max. A 0.90 1.45 0.0354 0.0570 A1 0.00 0.15 0.00 0.0059 A2 0.90 1.30 0.0354 0.0511 b 0.20 0.50 0.0078 0.0196 C 0.09 0.26 0.0035 0.0102 D 2.70 3.10 0.1062 0.1220 E 2.20 3.20 0.0866 0.1181 E1 1.30 1.80 0.0511 0.0708 9 0.95 REF 0.0374 REF 91 1.90 REF 0.0748 REF L 0.10 0.60 0.0039 0.0236 :3o 0" 30° 0° 30° Note: comronmg dimensions are m mulimsters. Approximate dimensions are provmea m mches
ZXLD1352
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Ordering Information
Device
Package
Reel Size
(mm)
Tape Width
(mm)
Quantity
(per reel)
Part
Marking
ZXLD1352ET5TA
TSOT23-5
180
8
3000
1352
Package Outline Dimensions
TSOT23-5
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IMPORTANT NOTICE
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INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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website, harmless against all damages.
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This document is written in English but may be translated into multiple languages for reference. Only the English version of this document is the
final and determinative format released by Diodes Incorporated.
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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
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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.
Customers represent that they have all necessary expertise in the safety and regulatory ramifications of their life support devices or systems, and
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