ZXLD1370EV2 User Guide Issue 3

ZXLD1370EV2 BUCK/BOOST LED DRIVER
USER GUIDE
BUCK/BOOST
0-500mA
DC
GND
8-48V 2A
DC PSU
1 - 15 LEDs
350mA
4.5V
0 – 4.5V Status steps
0V
Open Drain output
2.5V = 200%
125mV = 10%
-t°
Thermal connection
Fig.1 ZXLD1370EV2 Evaluation board connection diagram
Issue 3 – May 2011
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FLAG
ZXLD1370EV2
Fig2. Schematic diagram
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ZXLD1370EV2
PARTS LIST
Ref
U1
Q1
Q2 Q3
D1
D2
D3
Z1
L1
C1
C2 C5 C8
C3 C4 C9
C10
C6 C7
C11
R1 R2
R4
R7
R9
R10
R3 R5 R6
R8 R11
R12 R14
R15
R13 R18
R16 R17
R19
Value
LED Driver
Controller
60V N-ch
MOSFET
60V General
Purpose
Mosfet
Freewheeling
diode 3A
100V
51V 200mW
Zener Diodes
Freewheeling
diode 3A
100V
47V 300mW
Zener Diode
33uH 2.3A
Package
TSSOP16L
Part Number
ZXLD1370
Manufacturer
Diodes Zetex
Contact Details
www.diodes.com
DPAK
DMN6068LK3
Diodes Zetex
www.diodes.com
SOT23
2N7002
Diodes
www.diodes.com
PowerDI5
PDS3100
Diodes
www.diodes.com
SOD323
BZT52C51
Diodes
www.diodes.com
PowerDI5
PDS3100
Diodes
www.diodes.com
SOT23
BZX84C47
Diodes
www.diodes.com
MS1246-333MLB
NPIS24H330MTRF
744-7715330
Coilcraft
NIC Comps.
Wurth
www.coilcraft.com
www.niccomp.com
www.weonline.com
100pF 10V
1uF 100V
X7R
2.2uF 100V
X7R
Not fitted
10nF 100V
X7R
0R3
1K3
47K
120K 1%
36K 1%
0R
0805
1206
GRM31CR72A105KA01L
Generic
Murata
www.murata.com
1812
GRM43ER72A225KA01L
Murata
www.murata.com
2K
Not fitted
1K
0805
Generic
1206
0805
0805
0805
0805
0805
Generic
Generic
Generic
Generic
Generic
Generic
0805
Generic
0805
Generic
Issue 3 – May 2011
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© Diodes Incorporated 2010
3
ZXLD1370EV2
NOTES
The PCB is supplied with R3 and R8 0R0 resistors fitted.
The ‘ADJ’ pin and the ‘TADG’ pin are disabled.
Boost-only mode is selected by changing PL1
In boost mode, the total LED output voltage is <=47V.
The supply voltage for the ZXLD1370EV2 is >=6V, <= (total LED voltage).
In buck-boost operation, the input voltage range is limited by the overvoltage threshold voltage and the
LED voltage. >=6V, <=(47V – VLED).
For other reference designs or further applications information, please refer to the ZXLD1370
datasheet.
Q2 and Z1 protect the circuit from open-circuit LEDs.
The overvoltage threshold of the evaluation board is 47V. (Set by the 47V Zener diode Z1)
The overvoltage threshold can be increased by using a Zener diode with a higher voltage (56V)
Do not use a Zener diode of higher voltage than the MosFet or ZXLD1370.
Boost and Buck-boost modes, average ILED= average IINDUCTOR x R10/(R9+R10)
The nominal current, ILED for the evaluation board is set to 350mA.
OPERATION
In Boost and Buck-boost mode the LED current is sensed by the series resistor (R1 // R2). An output
from the control loop drives the input of a comparator. The comparator drives the gate of the external
NMOS switch transistor via ‘GATE’ pin. When the NMOS switch is on, current flows from VIN, via (R1 //
R2), inductor and switch to ground and increases until a high value is reached. Then, GATE goes low,
the switch turns off and the current flows through (R1 // R2), the inductor, D1 and the LED, to ‘VIN’
(Buck-boost mode), or ‘GND’ (Boost mode). When the inductor current has gone low, ‘GATE’ goes high,
and the cycle of events repeats. The circuit oscillates. The average current in the LEDs is equal to the
average of the maximum and minimum threshold currents. The ripple current (hysteresis) is equal to the
difference between the thresholds. The average current in the LED is always less than the average
current in the inductor and the ratio between these currents is set by the values of resistors R9 and
R10. The peak current in the LED is equal to the peak current in the inductor. The control loop keeps
the average LED current at the level set by the voltage on the ‘ADJ’ pin. Loop compensation is
achieved by C1.
Bootstrap Circuit
The ZXLD1370 works normally between 8-60V. For input voltages between 6-8V and high switch
currents that require a fully enhanced MOSFET, it is recommended to use the bootstrap network D2R13 and remove R11.
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ZXLD1370EV2
Fig. 3 Waveforms for Boost and Buck-boost modes
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ZXLD1370EV2
ADJ Terminal (DC output current adjustment)
On the ZXLD1370EV2, the ‘ADJ’ pin R3 connects the internal 1.25V reference (VREF) give 100% LED
current.
The ADJ pin can also be driven with an external DC voltage >=125mV and <= 2.5V to adjust the LED
current to >= 10% and <=200% of the nominal value.
To do this, remove R3, fit R5 and apply an external DC voltage between ‘ADJIN’ and ‘GND’
The voltage VADJ can be derived from a resistor-divider connected between ‘REF’ and ‘GND’.
‘ADJ’ has high impedance within its normal operating voltage range. An internal 2.6V clamp protects the
device against excessive input voltage and limits the maximum output current to approximately 4% above
the maximum current set by ‘VADJ‘ if the maximum input voltage is exceeded.
PWM Terminal (PWM output current control/dimming)
The LED current can be adjusted digitally, by applying a low frequency PWM logic signal to the ‘PWM’ pin
to turn the controller on and off. This will produce an average output current proportional to the duty cycle of
the control signal. During PWM operation, the device remains powered up and only the output switch is
switched by the control signal.
The device can be shut down by taking the PWM pin to < 0.4V with a short to 0V or suitable open collector
NPN, or open drain NMOS transistor, for >15ms. In the shutdown state, most of the circuitry inside the
device is off and the quiescent current will be typically 90µA.
TADJ Terminal (Thermal control of LED current)
The ‘Thermal control’ circuit monitors the voltage on the ‘TADJ’ pin and reduces the output current linearly
if the voltage on ‘TADJ’ < 625mV. An NTC thermistor and resistor can be connected to set the voltage on
the ‘TADJ’ pin = 625mV at the required threshold temperature. This will give 100% LED current below the
threshold temperature and <100% above it as shown in the graph. The temperature threshold can be
changed by adjusting the value of Rth and/or the thermistor to suit the LED used.
On the ZXLD1370EV2, Rth is 1K3 (R4). In order to use thermal control, remove R8, fit R6, and fit a 10K
NTC Negative Temperature Coefficient) type thermistor between ‘TADJ’ and ‘GND’. This will set the
threshold temperature to ~ 90ºC.
Thermal control by LED current reduction
Fig. 4 Thermal control
The Thermal Control feature can be disabled by connecting TADJ to REF through the jumper resistor R8.
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ZXLD1370EV2
BOARD LAYOUT
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ZXLD1370EV2
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