ZXLD1350
HIGH POWER LED DRIVER WITH INTERNAL SWITCH
DESCRIPTION
The ZXLD1350 is a continuous mode PWM inductive
converter, designed for driving 1 watt white LEDs
efficiently from a voltage source higher than the LED
voltage. The device operates from an input supply of
between 9V and 30V and provides an externally
adjustable output current of up to 350mA.
The ADJ pin will accept either a dc voltage or a PWM
waveform. Depending upon the control frequency, this
will provide either a continuous or a gated output
current. The PWM filter components are contained
within the chip.
A capacitor from the ADJ pin to ground provides an
adjustable soft-start function.
The ZXLD1350 includes the output switch and an
output current sensing circuit, which uses an external
resistor to set the nominal output current.
A continuous logic low signal on the ADJ pin will turn
the device off. (shutdown current <1μA)
Once set, the nominal output current can be adjusted
to a lower value, by applying an external control signal
to the ‘ADJ’ pin.
The device is assembled in a SOT23 5 pin package.
FEATURES

Internal 30V NDMOS switch

Up to 350mA output current

Can drive up to 3 series connected 1Watt LEDs

Brightness control using dc voltage or PWM

Internal PWM filter

Optional soft-start

High efficiency (80% typ)

Wide input voltage range: 9V to 30V

Low quiescent current: (100A typ)

1A max shutdown current

Up to 1MHz switching frequency
PIN CONNECTIONS
APPLICATIONS

Low voltage halogen replacement LEDs

Automotive lighting

Low voltage industrial lighting

LED flashlights

Mining LED backup light
TYPICAL APPLICATION CIRCUIT
SOT23-5 pack
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ZXLD1350
ABSOLUTE MAXIMUM RATINGS
(Voltages to GND unless otherwise stated)
Input Voltage (VIN)
30V
(40V for 20 sec)
-0.3V to 30V
500mA
320mW
-40 to 105C
-55 to 150C
150C
LX Output Voltage (VLX)
Switch output current (ILX)
Power Dissipation (Ptot)
Operating Temperature (TOP)
Storage Temperature (TST)
Junction Temperature (Tj MAX)
ELECTRICAL CHARACTERISTICS:
Test conditions: VIN=VEN=12V, TAMB=25C unless otherwise stated1
Symbol Parameter
VIN
IIN
Input voltage
Supply current
ISD
VSENSE
VHYS
ISENSE
VADJ(nom)
VADJ
Shutdown current
VSENSE pin threshold voltage
VSENSE pin hysteresis
Isense pin input current
Internal voltage reference
External dc control voltage
applied to ADJ pin
Minimum dc voltage on ADJ pin
to hold device out of shutdown
mode
Minimum duty cycle of PWM
signal on ADJ pin to hold device
out of shutdown mode
Operating frequency
VADJoff
D
fLX
fLXmax
DLX
ILXpk
RLX
ILX(leak)
Conditions
Min Typ
9
ILX = 0,
Output not switching
ADJ pin held at 0V
Measured with respect to VIN
Measured as proportion of VSENSE
VIN-100mV <VSENSE < VIN
ADJ pin floating
12
100
Max
Unit
s
30
V
µA
1
µA
mV
%
µA
V
V
100
30
10
1.23
0
VADJ(nom)
0.5
PWM frequency >10kHz
0.4
ADJ pin floating
L=47µH, IOUT=350mA @ 3.4V
Maximum operating frequency
Duty cycle of output switch at
fLXmax
LX Switch max current
LX Switch ‘On’ resistance
LX Switch leakage current
V
400
0.2
KHz
1
0.8
MHz
0.5
A

µA
1
1
Note 1: Production testing of the device is performed at 25 C. Functional operation of the device over a –40C to +105C
temperature range is guaranteed by design, characterisation and process control.
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PIN DESCRIPTION
Name
LX
GND
ADJ
ISENSE
VIN
Pin #
Description
Drain of NDMOS switch
Ground (0V)
Internal voltage reference pin (1.23V)
Connect to GND to shut-down the device
Drive with dc voltage ((0.5<V<1.23) or PWM signal
to adjust output current.
Connect a capacitor from this pin to ground to
provide a soft-start function.
Connect resistor Rs from this pin to VIN to sense
nominal output current IOUT=0.1/ Rs
Input voltage (9V to 30V). Decouple to ground with
capacitor close to device.
BLOCK DIAGRAM
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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 inductive converter.
With reference to the typical application and block diagram, operation is as follows:
Control loop
When an input voltage is first applied to the device, the current in the coil will be zero and there will be
no output from the current sense circuit. Under this condition, the output of the comparator will be
high. This will turn the NDMOS transistor on and the LX pin will be switched low, causing current to
flow from VIN to ground, via the external sense resistor Rs and LED. This current will rise at a rate
determined by the coil and VIN to produce a voltage drop across Rs. This voltage drop is sensed at
the ISENSE pin and forced across internal resistor RA. The current in RA is mirrored into internal
resistors RB and RC and produces a rising voltage at the input to the comparator. As the current
rises, the voltage at the inverting input of the comparator increases
<<TBC>>
the internal current sense circuit will produce a voltage dropcompared against an upper threshold
voltage VTH+. When this threshold is reached, the comparator will switch and reset the latch, causing
the LX output switch to turn off. At this point, the current in the coil will continue to flow via the LED
and schottky diode and will ramp down linearly at a rate determined by the coil and the forward drop
of the LED. The falling current produces a decreasing voltage between VIN and ISENSE, which is
compared against a second threshold voltage VTH-. When this threshold is reached, the lower
comparator will switch and set the latch This turns the switch back on again and the cycle repeats.
The loop is therefore self-oscillating and the average current in the LED will be determined by the
threshold voltages.
The thresholds VTH+ and VTH- are derived from an internal reference voltage Vref. This reference is
brought out to the ADJ pin and may be overdriven to adjust the output current. For stable oscillation,
within the specified frequency range, the thresholds VTH+ and VTH- are set nominally 15% above
and below the reference voltage. This defines the output ripple current to be nominally 30% of the
nominal output current.
The device contains a low pass filter between the ADJ pin and the threshold comparators. This allows
the ADJ pin to be overdriven with either dc or ac signals. The filter has a cut-off frequency of
nominally 4kHz
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ZXLD1350
TYPICAL OPERATING CHARACTERISTICS
(For typical application circuit at Vin=12V and TAMB=25 C unless otherwise stated)
[TBD]
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APPLICATION NOTES
Adjusting output current
When connected as shown in the typical application circuit, with Rs=0.28, the ZXLD1350 will
produce a nominal output current of 350mA. This can be adjusted by one of the two methods
described below.
1) Output current adjustment by external resistor Rs
The nominal output current is determined by the internal reference voltage and the value of the
external current sense resistor (RS) from VIN to the current sense pin ISENSE and is given by:
IOUTdc = 0.1/Rs
2) Output current adjustment by external dc control voltage
The ADJ pin of the device is connected to the internal voltage reference (1.23V nom). This pin can be
overdriven by an external dc voltage (VADJ) in order to override this reference and adjust the output
current to a value below the nominal value in 1) above.
The nominal output current is then given by:
IOUTdc = 0.0813*VADJ/Rs
3) Output current adjustment by PWM control
A Pulse Width Modulated (PWM) signal with duty cycle D can be applied to the ADJ pin in order to
adjust the output current to a value below the nominal value in 1) above.
If the PWM frequency is less than approximately 10kHz, the device will be gated ‘on’ and ‘off’ and the
output will be discontinuous with an average value given by:
IOUTavg  0.1D/Rs
This mode may be preferred when optimum output ‘whiteness’ is required when adjusting LED
brightness. It will also provide higher efficiency at the expense of greater output ripple.
If the PWM frequency is higher than approximately 10kHz and the duty cycle above the specified
minimum value, the device will remain active and the output will be continuous, with a nominal value
given by:
IOUTnom  0.1D/Rs
This mode will give minimum output ripple and reduced radiated emission.
Shutdown mode
Taking the ADJ to a voltage below 0.5V for more than approximately 100µs, will turn off the device
and supply current will fall to a low standby level.
Soft-start
A capacitor from the ADJ pin to ground will provide a soft-start function, by delaying the time taken for
the voltage to rise to the turn-on threshold and by slowing down the rate of rise of the internal
reference voltage.
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Capacitor selection
A good quality, low ESR capacitor should also be used for input decoupling, as the ESR of this
capacitor is effectively in series with the 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 1F is acceptable if the input source is close to the device, but
higher values will improve performance at lower input voltages, when the source impedance is high.
The input capacitor should be mounted as close as possible to the IC
For maximum stability over temperature, capacitors with X7R dielectric are recommended, as these
have a much smaller temperature coefficient than other types.
A table of recommended manufacturers is provided below
Manufacturer
Website
Murata
Taiyo Yuden
Kemet
AVX
www.murata.com
www.t-yuden.com
www.kement.com
www.avxcorp.com
Inductor selection
The choice of inductor will depend on available board space as well as required performance. Small
value inductors have the advantage of smaller physical size and may offer lower series resistance and
higher saturation current compared to larger values. A disadvantage of smaller inductors is that they
can result in reduced efficiency due to switch losses. Higher inductor values can provide better
performance at lower supply voltages.
<insert equations for duty cycle and ripple here>
Recommended inductor values for the ZXLD1350 are in the range 10H to 100H. The inductor
should be mounted as close to the device as possible with low resistance connections to the LX and
VIN pins.
Suitable coils for use with the ZXLD1350 are given in the table below:
Part No.
CMD4D11-100MC
DO1608C
L
(H)
10
DCR
()
0.457
ISAT
(A)
0.5
10
22
33
47
0.16
0.37
0.51
0.64
1.1
0.7
0.58
0.5
Manufacturer
Sumida
www.sumida.com
Coilcraft
www.coilcraft.com
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ZXLD1350
Diode selection
The rectifier diode (D1) should be a fast low capacitance schottky diode with low reverse leakage at
the working voltage. It should also have a peak current rating above the peak coil current and a
continuous current rating higher than the maximum output load current.
The table below gives some typical characteristics for diodes that can be used with the ZXLD1350
Diode
ZHCS1000
ZHCS2000
Forward voltage at
100mA
(mV)
300
300
Peak
current
(mA)
1000
1000
Continuous
current
(mA)
1000
2000
Reverse leakage
at 30V
(A)
15
15
Package
TSOT23
TSOT23
Layout considerations
PCB tracks should be kept as short as possible to minimise ground bounce and the ground pin of the
device should be soldered directly to the ground plane. It is particularly important to mount the coil
and the input/output capacitors close to the device to minimise parasitic resistance and inductance,
which will degrade efficiency. The FB pins are high impedance inputs, so PCB track lengths to these
should also be kept as short as possible to reduce noise pickup. Excess capacitance from the FB pins
to ground should be avoided and the FB2 pin should be grounded if not required.
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ZXLD1350
PACKAGE OUTLINE
SOT235
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ZXLD1350
ORDERING INFORMATION
DEVICE
REEL SIZE
ZXLD1350E5TA
180mm
REEL WIDTH
8mm
QUANTITY PER REEL
PART MARK
3000
1350
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© Zetex plc 2000
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This publication is 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 contact or be regarded as a representation relating to the
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TARGET SPEC ISSUE F September 2004
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