ZXLD1322

A Product Line of
Diodes Incorporated
ZXLD1322
BUCK/BOOST MODE DC-DC CONVERTER FOR LED DRIVING WITH 700mA OUTPUT AND CURRENT
CONTROL
Description
The ZXLD1322 is an inductive DC-DC converter, with an internal switch, designed for driving single or multiple LEDs in
series up to a total of 700mA output current. Applications cover input voltages ranging from 2.5V to 15V. Depending upon
supply voltage and external components, this can provide up to 12W of output power. The device employs a variable 'on'
and 'off' time control scheme with adjustable peak switch current limiting and operates in Buck/Boost mode, offering higher
power efficiency and lower system cost than conventional PFM buck/boost circuitry. The device includes the DC-DC
converter, a high-side current monitor and an NPN switching transistor to provide an integrated solution offering small PCB
size, competitive cost/performance, high power efficiency of DC-DC conversion and maximum LED brightness/reliability.
More importantly, it retains design flexibility to add customer specific features. The feedback control circuitry inside the
ZXLD1322 provides excellent load and current regulation, resulting in very stable LED current over the useful life of the
battery and over the full operating temperature range. The LED current can be adjusted from 100% down to 10% of the set
value by applying a dc voltage to the ADJ pin and down to 1% by applying a PWM signal. An on-chip LED protection circuit
also allows output current to be reduced linearly above a predetermined threshold temperature using an external thermistor
at the TADJ pin. External resistors set nominal average LED current and coil peak current independently. The device can
be shut down by applying a continuous low level dc voltage to the ADJ pin.
Features
•
•
•
•
•
•
•
•
•
•
•
•
Pin Assignments
2.5V to 15V Input Voltage Range
Up to 700mA output current
#
Typical efficiency >80%
User-defined thermal control of LED
output current using external thermistor
High output current stability over input
voltage and temperature
12μA typical standby current
LED current adjustable from 100%
down to 2%
Adjustable Soft-Start
Capable of driving 3 LEDs in series
(Top View)
DFN4030-14 with Exposed Pad
4mm x 3mm
0.50mm pitch
Applications
•
•
•
High power LED flashlights
LED back-up lighting
General LED lighting
1.5W @ TA = 70°C
Typical Application Circuit
DFN4030-14 Package(Bottom View)
45° chamfer denote Pin 1
Notes:
#. Using standard external components as specified under electrical characteristics. Efficiency is dependent upon external component types and
values. Higher efficiency is possible with alternative coils.
ZXLD1322
Document number: DS32166 Rev. 3 - 2
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ZXLD1322
Block Diagram
ZXLD1322
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Pin Description
Name
Pin #
ADJ
1
BIAS
2
CFB
3
N/C
4
ISENSE
5
EMITTER1
EMITTER2
COLLECTOR2
COLLECTOR1
N/C
6
7
8
9
10
M_VIN
11
VIN
12
TADJ
13
VREF
Exposed Pad
14
15
Description
Adjust input
•
Leave floating, or connect to VREF to set 100% output current.
•
Drive with dc voltage. (50mV<VADJ< VREF) to adjust output current from 10% to 100%
of set value. (DC brightness control mode)
•
Drive with low frequency (200Hz) PWM control signal to gate output ‘on’ and ‘off’ at
the PWM frequency. (PWM brightness control mode)
•
Drive with low level dc voltage (VADJ<28mV) to turn off device (Standby mode)
Bias pin for setting base current of internal switch transistor
•
Short pin to ground to define maximum base drive current for output switch
(Maximum output current condition)
•
Connect resistor (RBIAS) from this pin to ground to reduce base drive current
(Reduced output current condition)
Control input/output for feedback control loop
•
Connect 10nF capacitor from this pin to ground to provide loop compensation
Not connected internally (Open circuit)
Switch peak current sense pin
•
Connect resistor (RSENSE) from this pin to ground to define peak switch current
(ISWPEAK)=0.05/RS
Switch emitters (Connect both pins to top of RSENSE to sense emitter current)
Switch Collectors (Connect both pins to lower side of coil)
Internally connected - Do not connect to external circuitry
Input supply to high side current monitor
•
Connect output voltage (whichever is higher)
•
Connect resistor (RM) from this pin to to VIN to define nominal average output (LED)
current of 0.1/RM
Input supply voltage and load side input of high side current monitor.
Connect to sensing resistor RM.
Temperature Adjust input for LED thermal compensation
•
Connect thermistor/resistor network to this pin to reduce output current above a
preset temperature threshold.
•
Connect to VREF to disable thermal compensation function
(see section on temperature control for details)
Internal 0.5V reference voltage output
Connect to ground (0V)
ZXLD1322
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Absolute Maximum Ratings (Voltages to GND Unless Otherwise Stated)
Symbol
Parameter
Rating
Units
TOP
Operating Temperature
-40 to 125
°C
TST
Storage Temperature
-55 to 150
°C
TJ
Junction Temperature
-40 to 150
°C
1.5 at TAMB =70°C
W
Package Power Dissipation
PTOT
DFN14 with Exposed Pad: 4mm x 3mm, 0.5mm Pitch
DC-DC Converter
Supply Voltage (VIN)
-0.3V to +15V
ADJ
-0.3V to The lower of (+5.0V) or (VIN + 0.3V)
CFB
-0.3V to The lower of (+5.0V) or (VIN + 0.3V)
ISENSE
-0.3V to The lower of (+5.0V) or (VIN + 0.3V)
TADJ
-0.3V to The lower of (+5.0V) or (VIN + 0.3V)
BIAS
-0.3V to The lower of (+5.0V) or (VIN + 0.3V)
High-Side Current Monitor
Monitor Supply Voltage (M_VIN)
Continuous Sense Voltage
(M_VIN – M_LOAD)
-0.3V to +15V
-0.3V to +5V
Switching NPN Transistor
Symbol
Parameter
Rating
Units
VCBO
Collector-Base Voltage
18
V
VCEO
Collector-Emitter Voltage
18
V
3
(Pulsed Width = 300µs. Duty Cycle<=2%)
A
2
A
ICM
IC
Peak Pulse Current
Continuous Collector Current
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 (RθJC)
DFN4030-14
ZXLD1322
Document number: DS32166 Rev. 3 - 2
Nominal Value
26.3°C/W
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Electrical Characteristics (Test conditions: VIN = 4V, TAMB = 25°C unless otherwise stated(a))
DC-DC Converter Supply Parameters
Symbol
Parameter
Supply Voltage
Condition
Normal Operation
Start-up mode
VUV+
Supply voltage for start-up(b)
Under-voltage detection threshold
normal operation to start-up mode
Under-voltage detection threshold
start-up mode to normal operation
Iq
Quiescent Current
ISTBY
Standby Current
VREF
Internal Reference Voltage
TCO(REF)
Internal Reference Temperature
Coefficient
VIN
VIN(Start)
VUV-
Notes:
Min.
2.5
Typ.
1.2
Max.
15
Unit
V
2.4
V
VIN falling
1.8
V
VIN rising
2.2
V
Measured into VIN ADJ pin floating.
Excluding switch base current).
1.5
mA
Measured into VIN. ADJ pin grounded
ADJ pin floating
2.5V<VIN<15V
480
12
20
µA
500
520
mV
50
ppm/K
(a) Production testing of the device is performed at 25°C. Functional operation of the device and parameters specified from -40°C to +125°C are
guaranteed by design, characterization and process control.
(b) Between 1.2V and 2.2V the device will run in the Low Voltage Startup Mode (for details refer to section "Low Voltage Operation")
DC-DC Converter Input Parameters
Symbol
Parameter
Condition
VSENSE
Peak switch current sense voltage
Measured on ISENSE pin CFB pin
at 0V
VSENSE
(SU)
Peak switch current sense voltage in
start-up mode
Measured on ISENSE pin. Start-up
mode VIN = 1.2V
ISENSE
Sense input current
Measured into ISENSE with pin at
0V. CFB pin at 0V
CFB
Control loop compensation capacitor
External DC control voltage applied to
ADJ pin to adjust output current
Switching threshold of ADJ pin
VADJ
VADJ(th)
TCO
(VADJ)
Min.
Typ.
Max.
Unit
45
55
65
mV
10.5
-15
-7
mV
-1
µA
500
mV
30
mV
10
DC brightness control mode
50
Standby state to normal operation
26
28
Temperature coefficient of VADJ(th)
+0.3
%/K
RADJ
Internal resistor between VREF and
ADJ
VADJ<550mV
100
kΩ
VADJ
(CLMP)
Clamp voltage on ADJ pin
100μA injected into ADJ pin
575
mV
DC-DC Converter Output Parameters
Symbol
Toff(100)
Toff(10)
Condition
100% Output current
10% Output current
fLXMAX
Parameter
Discharge pulse width
Discharge pulse width
Maximum operating frequency
fSU
Switching frequency in start-up mode
VIN = 1.2V
ZXLD1322
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Min.
0.7
4
Typ.
1.2
8
50
Max.
1.7
12
600
Unit
µs
µs
KHz
KHz
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ZXLD1322
Switching NPN Transistor
Symbol
V(BR)CE
Parameter
(c)
Average continuous switch current
Maximum base current into switch
transistor from internal drive circuit(d)
Base current into switch transistor
using external resistor (RBASE) from
BIAS pin to ground
Collector-Emitter breakdown voltage
VCE(sat)
Collector-Emitter saturation voltage
hFE
Static forward current transfer ratio
COBO
t(on)
t(off)
Turn-off time
ISW
IBON(max)
IBON
Condition
2V<VIN<18V BIAS pin at 0V
Min.
Typ.
Max.
2
Unit
A
30
50
70
mA
10
RBIAS = 1680Ω
mA
15
IC = 10mA
V
IC = 0.1A, IB = 10mA
50
120
IC = 2A, IB = 50mA(e)
IC = 200mA, VCE = 2V
mV
IC = 2A, VCE = 2V
209
116
Output capacitance
VCB = 10V,f = 1MHz
64
pF
Turn-on time
Ic = 0 to IC = 2A VIN = 10V
30
ns
IC = 2A to Ic < 100μA
28
ns
High-Side Current Monitor
VM_VIN
Symbol
Parameter
Supply voltage
VMON
Sense voltage
I_M_VIN
Input current
TCO(MON)
Temperature coefficient
BW
Bandwidth
Gm
Tran conductance
ΔIOUT/ΔVMON
ACC
Accuracy
Condition
=V(VIN) – VM_VIN
Measured into M_VIN pin
Min.
2.5
0
VMON=10mV
VMON=100mV
VMON=10mV
VMON=100mV
RM = 0.1Ω
VMON = 100mV
Typ.
Max.
18
Unit
V
100
200
mV
0.08
1
µA
370
150
ppm/K
350
2.5
KHz
Mhz
1
mA/V
-3
3
%
Max.
500
Unit
mV
Reference Current Monitor
Symbol
VADJ
TCO
Parameter
Adjust Voltage
Temperature coefficient
(MON)
BW
Bandwidth
Tran conductance
ΔIOUT/ΔVADJ
Accuracy
Gm
ACC
Notes:
Condition
Min.
0
VADJ = 50mV
VADJ = 500mV
VADJ = 50mV
VADJ = 500mV
VADJ = 500mV
-3
Typ.
160
200
ppm/K
275
3
KHz
Mhz
200
µA/V
3
%
(c) Measured under pulse conditions.
(d) This current is measured via the collectors and emitters of the switch with these connected to ground (0V)
(e) Measured under pulse conditions. Peak Current = IC
ZXLD1322
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LED Thermal Control Circuit (TADJ) Parameters
Symbol
Parameter
VTADJH
Upper threshold voltage
VTADJL
Lower threshold voltage
Gm(TADJ)
Trans conductance
ΔIOUT/ΔVTADJ
Condition
Onset of output current reduction
(VTADJ falling)
Output current reduced to
<10% of set value (VTADJ falling)
Min.
Typ.
Max.
Unit
75
mV
50
mV
4
mA/V
Output Current Regulation Parameters
Symbol
Parameter
Condition
IOUT
Minimum output/ LED current
VIN > 3V
ΔIOUT
Output current accuracy(g)
3.0V < VIN < 15V, IOUT = 700mA,
VADJ = 100mV,
TCO
I(OUT)
Output current temperature drift
ΔIOUT/
Load Current Regulation
IOUT
ΔIOUT/
ΔVIN
Line Voltage Regulation of output
current
Eff
Efficiency
Notes:
(f)
3.0V < VIN < 15V, Iout = 700mA,
VADJ = 100mV
350mA < I(LED) < 700mA
350mA < I(LED) < 700mA
Min.
Typ.
Max.
0.75
-5
100
Unit
A
+5
%
200
ppm/K
2
%/A
0.5
%/V
85
%
(f) System parameter only. This value is dependent upon external components and circuit configuration.
(g) This refers to the accuracy of output current regulation under normal operation when the feedback loop incorporating the current monitor is active.
The tolerances of external components are not included in this figure.
ZXLD1322
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Device Description
The ZXLD1322 is a buck/boost mode inductive DC-DC converter, with an internal switch, designed for driving single or
multiple LEDs in series up to a total of 700mA output current. Depending upon supply voltage (VIN), LED forward voltage
drop (VLED) and circuit configuration, this can provide up to 12W of output power.
Applications cover VIN ranging from 2.5V to 15V.
The device employs a modified Pulse Frequency Modulation (PFM) control scheme, with variable "ON" and "OFF" time
control and adjustable peak switch current limiting.
General device operation (refer to block diagram)
Normal Operation
Control is achieved by sensing the LED current in a series resistor (RM), connected between the two inputs of the LED
Current Monitor. This generates a proportional current (IMON) that charges the external integrator capacitor CFB. IMON is
balanced against a reference discharge current (IADJ) generated at the output of a second voltage to current converter
driven from the demand voltage (VADJ) on the ADJ pin. The difference between IMON and IADJ is integrated by CFB to
produce an error voltage. A comparator takes a summed version of the voltage at the ISENSE pin and a fraction of this CFB
voltage and resets the latch driving the switch when the sum is greater than 50mV. The switch transistor is turned on by the
output of the SR latch, which remains set until the emitter current in the switch transistor produces a voltage drop VSENSE
(=50mV nominal) in external resistor RSENSE, defining a preset maximum switch current of 50mV/RSENCE. Operation is such
that a rising error voltage on CFB will effectively lower the voltage required on the ISENSE pin and therefore reset the latch
earlier in the switching cycle. This will reduce the 'ON' time of the switch and reduce the peak current in the switch from its
preset maximum value. Similarly, a falling error voltage will reset the latch later and the peak switch current will be
increased. The control loop therefore reduces or increases the energy stored in the coil during each switching cycle, as
necessary, to force the LED current to the set value. This results in high accuracy, as no error is needed in the LED current
to drive the servo to the required region.
The time taken for the coil current to reach the peak value depends on several factors: the supply voltage, the peak coil
current required at that particular LED power and whether the system operates in "continuous" or "discontinuous" mode.
The time allowed for the coil current to discharge into the LED is fixed by the 'Variable Off Delay' monostable, whose period
is modified by the power demand signal on the ADJ pin. This monostable determines the time for which the latch remains
reset (switch off) and provides a longer "OFF" period at lower power settings, helping to keep the parameters within an
acceptable range.
Note that the "ON" period and the "OFF" period are set by the supply voltage, LED power and external components chosen.
The frequency is therefore determined by these parameters and is NOT fixed. In this modified PFM scheme, the external
components can be chosen to keep the frequency well above the audio range for all extremes of parameters, so no audible
whistling should ever occur.
The 500mV reference voltage defines the nominal VADJ voltage and this defines the 100% output current. For lower LED
currents, the ADJ pin can be-driven from an external DC voltage (50mV<VADJ<500mV) or a low frequency Pulse Width
Modulated (PWM) waveform.
Low voltage operation (start-up mode)
For supply voltages below 2V, the normal control loop will have insufficient headroom to operate reliably. This condition is
detected by the 'under-voltage comparator', which compares a fraction of the internal supply voltage (VCC) against VREF.
When the comparator output is active (VCC<1.8V), the output of the normal switch drive circuit is disabled and an alternative
'Start-up oscillator and driver' enabled. The start-up oscillator provides a nominal 50kHz fixed frequency drive signal to the
base of the switch transistor, which is independent of VADJ and the voltage on CFB. Under low voltage conditions, the peak
current in the coil ramps to approximately 25% of the normal value and the "OFF" time is fixed.
The low voltage start-up mode allows the device to operate down to 1.2V nominal. This allows the chip to work from a single
cell.
ZXLD1322
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ADJ pin
The ADJ pin is connected to the internal 500mV reference (VREF) via a 100k resistor. This biases the ADJ pin to the
reference voltage and defines nominal 100% LED current.
The ADJ pin can be overdriven with an external dc voltage between 50mV and 500mV to reduce the LED current
proportionally between 10% and 100% of the nominal value.
LED current can also be adjusted by applying a low frequency PWM signal to the ADJ pin to turn the device On and Off.
This will produce an average output current proportional to the duty cycle of the control signal.
The device can be shut down by shorting the ADJ pin to ground, or pulling it to a voltage below 28mV with a suitable open
collector NPN or open drain NMOS transistor. In the shutdown state, most of the circuitry inside the device is switched off
and residual quiescent current will be typically 12μA.
Thermal control of LED current
The 'Thermal compensation current' circuit produces a sourcing current (Itc) which is zero for voltages above 75mV on TADJ
and increases to 100μA when TADJ falls to 50mV. This current is summed into the control node and subtracted from the
demand current, causing LED current to reduce from 100% down to zero over this input range. The potential divider,
consisting of a fixed resistor Rt and an NTC Thermistor Rth between VREF and ground, defines the voltage on TADJ and sets
the threshold temperature. Further details are given in the application notes.
The Thermal Control feature can be disabled by leaving the TADJ pin floating, or by connecting it to VREF.
Over-temperature shutdown
The ZXLD1322 incorporates an over-temperature shutdown circuit to protect the device against damage caused by excess
die temperature, resulting from excessive power dissipation in the switch. The output of the 'Over-temp Shutdown' circuit will
go high when the die temperature exceeds 150°C (nominal). This will turn off the drive to the switch during normal operation.
Operation will resume when the device has cooled to a safe level.
ZXLD1322
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Application Notes
Setting Peak Coil Current
The peak current in the coil is set by the resistor (RSENSE) between the switch emitter and ground according to:
ISWPEAK
=
50mV
RSENSE
The minimum peak current will depend on operating mode, coil inductance and supply voltage range. The maximum peak
current must not exceed the specified value for the switch. (See Application circuits for details)
Setting LED Current
The nominal average LED current is given by:
ILED (NOM) =
100mV
RM
Where RM is the external resistor connected between pins M_VIN and VIN.
This current can be adjusted to a lower value by applying a dc control voltage or PWM control signal to the ADJ pin.
DC Control
The LED current can be adjusted over a 10% to 100% range by connecting a variable resistor RADJ from the ADJ pin to
ground to vary the DC voltage at the ADJ pin. RADJ forms the lower part of a resistive divider and the internal 100kΩ resistor
between the ADJ and VREF pins forms the upper part. A value of 1MΩ for RADJ will therefore give a maximum current of
91% of ILED (nom) and the device will be turned off when the voltage on the ADJ pin falls below 28mV, corresponding to an
RADJ value of approximately 5kΩ. If required, an end-stop resistor in series with RADJ can be used to maintain the voltage on
the ADJ pin above the turn-on threshold.
Using a logarithmic potentiometer for RADJ will give an approximately linear variation of output current with shaft rotation.
(Fig 1)
If required, the maximum output current can be restored to 100% by adjusting the value of the LED current monitor resistor
(RM). The tolerance of the internal 100k resistor and RADJ should be taken into account when calculating output current.
The ADJ pin is clamped internally to a voltage of 575mV (nom), to limit maximum average output current to approximately
115% of ILED(nom).
Fig. 1
ZXLD1322
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PWM Control
A wider dimming range can be achieved by applying a PWM control signal to the ADJ pin to turn the device on and off,
giving an average output current proportional to the duty cycle of the control signal. The ADJ pin can be driven directly from
the open drain NMOS output of a microcontroller, or indirectly with a low saturation voltage NPN transistor such as the Zetex
ZXTN25015DFL. (Fig 2).
ZXTN25015DFL
Fig. 2
In the circuit of Fig 4, the average LED output current will be:
ILED (AVG) = ILED(NOM) *D
Where duty cycle
A PWM frequency of 200Hz, or lower is recommended, to minimize errors due to the rise and fall
times of the converter output.
Thermal Compensation of LED Current
High-luminance LEDs often need to be supplied with a temperature compensated current in order to maintain stable and
reliable operation at high temperatures. This is usually achieved by reducing the LED current proportionally from its nominal
set value when the LED temperature rises above a predefined threshold. (Fig.3)
Fig. 3
The 'Thermal compensation current' generator inside the ZXLD1322 provides the necessary thermal compensation current
to meet this requirement, using an NTC thermistor and resistor. (Fig 4)
Fig. 4
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The TADJ pin of the device has a voltage threshold of 75mV nominal, which is derived from the reference voltage VREF. If
the voltage (VTADJ) on the TADJ pin is held above the threshold, the thermal compensation current will be zero and no
thermal compensation is applied. However, if VTADJ falls below the threshold, a thermal compensation current (ITC) is
produced that is proportional to VTADJ. ITC is injected into the control loop in such a way as to reduce the demand current
IADJ, causing the control loop to decrease the LED current. The LED current will be reduced to less than 10% of the set
value when VTADJ falls below 50mV.
The threshold voltage has been chosen to set a nominal threshold of 105°C and the device has been optimized to operate
with a standard 103KT1608 thermistor and 5k resistor in the potential divider. Circuit details are given in the application
notes. Alternative thermistor/resistor networks can be used providing the input resistance presented to the device at the
TADJ pin is similar at the threshold temperature. If no LED thermal compensation is required, the TADJ pin should be
connected to VREF to disable this function.
ZXLD1322
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Typical Operating Conditions
Inductive converters can operate in either CONTINUOUS mode, where current always flows ithe inductor, but rises during the
ON period and falls during the OFF period, or DISCONTINUOUS mode, where the current falls to zero during the OFF period.
The mode depends on several factors, including supply voltage, output (LED) voltage and the choice of peak current and
inductor value. Calculations need to be done to determine which mode the converter will be in. The circuit should be designed
to give slightly more LED current than required under the lowest supply voltage, so the control loop can regulate the current
accurately. If the theoretical LED current is less than that required, the control loop will not be able to reach the required
value. The calculations will give an idea of the ON and OFF times and hence the operating frequency, but bear in mind that
the control loop will reduce the peak current to achieve the exact programmed LED current and this will raise the operating
frequency. In general, values in the discontinuous mode are simpler to calculate because the current can go from zero to the
theoretical maximum during the ON period and fall to zero during the OFF period. In continuous mode the current will start
from some value, so the ON time will be lower to reach the theoretical maximum and lower still when the control loop reduces
the peak current below the maximum.
Circuit Operation
Operation of buck / boost LED driver
Used when the input voltage can go higher or lower than the LED voltage, this circuit has an ON phase, where the coil is
connected from the supply to ground and an OFF phase, where the coil current flows through the LED via a Schottky diode.
The current therefore only flows into the LED circuit during the OFF phase, although the reservoir capacitor C3 should keep
current flowing in the LED(s) continuously. The important difference is that this circuit has the LED cathode taken to VIN
instead of ground.
ADJ is set between 50mV and 500mV to give between 10% and 100% power respectively. Making R2 = ZERO gives a base
current to the output transistor of 50mA nominal and making R2 = 1.68kΩ gives 10mA nominal. The reduced base current will
lower supply current and hence improve efficiency in lower power applications. Making R1 = 25mΩ gives a peak coil current
of 2 Amps. The internal power transistor turns on until the coil current builds up to the peak value. At this
point the transistor switches off and the coil current continues to flow in the LED(s) via the
Schottky diode D1.
With a buck converter, the LED is in series with the coil, so no coil current can flow until the supply voltage exceeds the LED
forward drop. The circuit will not work if the supply is less than this. With a boost converter, there is always a path from
supply to ground through the coil, Schottky diode and LED in series, so if the supply voltage is greater than the LED and
Schottky forward drops, unlimited current will flow in the LED. The circuit will not work if the supply is greater than this. Thus
neither circuit will work for both conditions, where the supply could be either higher or lower than the LED forward drop, for
example when using 3 cells to supply it.
Although it looks like a boost circuit, taking the LED cathode to the supply means that no current can flow in the LED even if
the supply is greater than the forward drop. However, because the coil is still connected straight across the supply during
ZXLD1322
Document number: DS32166 Rev. 3 - 2
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the ON phase, the current can still be established when the supply is less than the LED forward drop. Hence this circuit will
work at supply voltages above and below the forward LED drop.
This mode is useful for example when using 3 cells and a white LED, where the voltage of 3 fully charged alkaline cells is
more than the LED forward drop, but the voltage of 3 partly discharged rechargeable NiCd cells is less than the LED forward
drop.
The LED current is sensed by R3 and the controller varies this until the drop in R3 equals 20% of VADJ. Hence making
R3 = 100mΩ and VADJ = 500mV gives a LED current of 1 Amp because the 500mV VADJ results in 100mV across R3 which
equals 1 Amp. Making VADJ = 10mV gives a LED current of 100mA because the 50mV VADJ results in 10mV drop across R3
which equals 100mA.
The power is controlled by the chip backing off the peak coil current, so it is necessary to calculate the coil inductance and
current to guarantee slightly more than 100% LED power, so the circuit can control it effectively. The internal control loop is
compensated by C1, which is normally 10nF.
If the thermistor (R5) is used, the power will be backed off progressively as the TADJ pin goes low. With the TADJ pin above
75mV, power is 100% and this is reduced to zero when the TADJ pin reaches 50mV. Making R4 = 5kΩ and using a
103KT1608 thermistor, the thermistor will reach 869Ω at 105°C giving VTADJ = 74mV which will start to reduce the LED
power above 105°C. By 125°C the thermistor will reach 547Ω giving VTADJ = 50mV which gives zero power. This will protect
the LED from damage. These temperature values can be set by the customer by using a different thermistor or a different
value of R4. If protection is not required, leaving the TADJ pin open circuit will make it float to a high voltage and always give
100% power.
Bill of materials
Reference
Part No
Value
Manufacturer
Contact Details
U1
ZXLD1322
LED Driver
Zetex
www.zetex.com
D1
ZHCS2000
Schottky diode
Zetex
L1
MSS7341-103ML
10µH 2A
Coilcraft
www.coilcraft.com
L1
NPIS64D100MTRF
10µH 2A
NIC
www.niccomp
L1
744 777910
10µH 2A
Wurth
www.wurth.co.uk
C1
Generic
10nF 10V
Generic 0603
C2
GRM31CR71H475K
4.7µF 50V
Murata 1206
www.murata.com
C3
GRM31MR71E225K
2.2µF 25V
Murata 1206
www.murata.com
R1
Generic
25mΩ
Generic 0805
R2
Generic
1.5kΩ
Generic 0603
R3
Generic
100mΩ
Generic 0805
R4
Generic
5.1kΩ
Generic 0603
R5
Thermistor NTC
10k
103kt1608
ZXLD1322
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Additional Notes
Note that the ON time is set by the time it takes the coil to reach the peak current. This peak value is reduced by the control
loop to give the desired LED power, so the ON time can vary over a wide range. The minimum coil current can be zero
(discontinuous operation) or finite (continuous operation) depending on the supply voltage, LED current and the LED
voltage. The OFF time is set by an internal timer and is nominally 1.2μs at 100% LED power (VADJ = 500mV), increasing to
about 8μs at 10% LED power (VADJ = 50mV). The longer OFF time and variable peak current enables the circuit to dim the
LED whilst maintaining continuous switching, rather than "skipping" or stalling and continuous running is better for reducing
electrical noise and also for eliminating audible noise from the coil core.
Layout Considerations
As with all switching DC to DC converters, the currents can be large. Using small inductors with a reasonable high supply
voltage will cause currents to change quickly. High dI/dt can cause inductively-coupled spikes into adjacent tracks. At the
transition from of the ON phase to the OFF phase and back, where the power transistor switches, the voltage at the collector
rises and falls quickly. High dV/dt can cause capacitively coupled spikes into adjacent tracks, especially if they have a high
impedance. For this reason, all tracks on the PCB should be thick, to minimise drops, and short to keep all the components
coupled tightly together.
A double-sided board should be used with a ground plane to screen the tracks and provide a good ground return for the
various functions and the rear exposed pad on the package should have an appropriately-sized land with good ground
connections, both to reduce electrical noise due to ground drops and to improve thermal conductivity.
The input decoupling capacitor C1 should be very close to the chip pins and the LED sense resistor R3 should have Kelvin
tracks to M_VIN and VIN to achieve LED current measurement accuracy, as the PCB tracks will have comparable
resistance to the 100mΩ resistor, so taking sense tracks to the current monitor which are not connected close to the ends of
R3 will cause a measurement error.
The peak current sense resistor R1 should have short tracks to the ground at the bottom end and Kelvin tracks to ISENSE at
the top end. This resistor might need to be only 25mΩ and PCB track resistance becomes comparable if the tracks are not
very short. ISENSE is a high impedance input, so a thin track from this pin directly to the top of RSENSE resistor R1 will still
give an accurate measurement.
The ADJ pin should have short tracks, as this is a fairly low-level signal controlling the power of the system. As it needs to
be less than 28mV for shutdown, a close ground connection is needed for the pull-down device, as any ground drops could
raise the potential. In particular, if a bipolar transistor is used as a pull-down device, this will have an appreciable VSAT,
which could perhaps be half the shutdown potential.
The bottom of the thermistor must be coupled very closely to ground, as the TADJ pin varies the LED current from 100% to
0% for a voltage change of only 25mV, so any noise on the bottom of the thermistor will seriously affect the accuracy of the
Thermal Protection circuit.
Ordering Information
Device
ZXLD1322DCCTC
ZXLD1322
Document number: DS32166 Rev. 3 - 2
Reel Size
(mm)
33.02
Reel Width
(mm)
12
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Quantity
per reel
3000
Device Mark
1322
April 2010
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Diodes Incorporated
ZXLD1322
Package Outline – DFN4030-14
E
E2
A
PIN #1 IDENTIFICATION
CHAMFER 0.300 X 45°
b
A3
e
D
D2
PIN 1 DOT
BY MARKING
L
A1
DIM
Inches
Millimeters
DIM
Min
Max
Min
Max
A
0.0315
0.0354
0.80
0.90
D2
A1
0.00
0.002
0.00
0.05
e
A3
0.008 REF.
0.203 REF.
Inches
Millimeters
Min
Max
Min
Max
0.1240
0.1279
3.15
3.25
0.0197 BSC
0.50 BSC
E
0.1161
0.1201
2.95
3.05
b
0.0079
0.0118
0.20
0.30
E2
0.0650
0.0689
1.65
1.75
D
0.1555
0.1594
3.95
4.05
L
0.0138
0.0177
0.35
0.45
Note: Controlling dimensions are in millimeters. Approximate dimensions are provided in inches
ZXLD1322
Document number: DS32166 Rev. 3 - 2
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IMPORTANT NOTICE
DIODES INCORPORATED MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARDS TO THIS DOCUMENT,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION).
Diodes Incorporated and its subsidiaries reserve the right to make modifications, enhancements, improvements, corrections or other changes
without further notice to this document and any product described herein. Diodes Incorporated does not assume any liability arising out of the
application or use of this document or any product described herein; neither does Diodes Incorporated convey any license under its patent or
trademark rights, nor the rights of others. Any Customer or user of this document or products described herein in such applications shall
assume all risks of such use and will agree to hold Diodes Incorporated and all the companies whose products are represented on Diodes
Incorporated website, harmless against all damages.
Diodes Incorporated does not warrant or accept any liability whatsoever in respect of any products purchased through unauthorized sales
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Should Customers purchase or use Diodes Incorporated products for any unintended or unauthorized application, Customers shall indemnify
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directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized application.
Products described herein may be covered by one or more United States, international or foreign patents pending. Product names and
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LIFE SUPPORT
Diodes Incorporated 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
labeling 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.
Customers represent that they have all necessary expertise in the safety and regulatory ramifications of their life support devices or systems,
and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products
and any use of Diodes Incorporated products in such safety-critical, life support devices or systems, notwithstanding any devices- or systemsrelated information or support that may be provided by Diodes Incorporated. Further, Customers must fully indemnify Diodes Incorporated and
its representatives against any damages arising out of the use of Diodes Incorporated products in such safety-critical, life support devices or
systems.
Copyright © 2010, Diodes Incorporated
www.diodes.com
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