This wide range evaluation board showcases the LM3424 NFET controller used with a buck-boost current
regulator. It is designed to drive 4 to 10 LEDs at a maximum average LED current of 1A from a DC input
voltage of 10 to 70V.
The evaluation board showcases many of the LM3424 features including thermal foldback, analog
dimming, external switching frequency synchronization, and high frequency PWM dimming, among others.
There are many external connection points to facilitate the full evaluation of the LM3424 device including
inputs, outputs and test points. Refer to Table 1 for a summary of the connectors and test points.
The buck-boost circuit can be easily redesigned for different specifications by changing only a few
components (see the Alternate Designs section). Note that design modifications can change the system
efficiency for better or worse.
This application note is designed to be used in conjunction with the LM3424 datasheet as a reference for
the LM3424 buck-boost evaluation board. Refer to the LM3424 Constant Current N-Channel Controllerwith Thermal Foldback for Driving LEDs (SNVS603) data sheet for a comprehensive explanation of the
device, design procedures, and application information.
User's Guide
SNVA397A–August 2009–Revised May 2013
AN-1967 LM3424 Buck-Boost Evaluation Board
2Key Features
•Input: 10V to 70V
•Output: 4 to 10 LEDs at 1A
•Thermal Foldback / Analog Dimming
•PWM Dimming up to 10 kHz
•External Synchronization > 500 kHz
•Input Under-voltage and Output Over-voltage Protection
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SNVA397A–August 2009–Revised May 2013AN-1967 LM3424 Buck-Boost Evaluation Board
J2GNDInput GroundConnect to negative terminal of supply voltage (GND).
J3ENEnable On/OffJumper connected enables device.
J4LED+LED PositiveConnect to anode (top) of LED string.
J5LED-LED NegativeConnect to cathode (bottom) of LED string.
J6BNCDimming InputConnect a 3V to 10V PWM input signal up to 10 kHz for PWM dimming the LED load.
J7OUTOutput with NTCAlternative connector for LED+ and LED-. Pins 4 and 11 are used for connecting an
TP1SWSwitch NodeTest point for switch node (where Q1, D1, and L1 connect).
TP3SGNDSignal GroundConnection for GND when applying signals to TP5, TP8, and TP9.
TP4LED+LED PositiveTest point for anode (top) of LED string.
TP5nDIMInverted Dim SignalTest point for dimming input (inverted from input signal).
TP6V
IN
TP8SYNCSynchronizationConnect a 3V to 6V PWM clock signal > 500 kHz (pulse width of 100ns) to synchronize
TP9NTCTemp Sense InputConnect a 0V to 1.24V DC voltage to analog dim the LED current.
TP10 PGNDPower GroundTest point for GND when monitoring TP1, TP4, or TP6.
Input VoltageConnect to positive terminal of supply voltage.
external NTC thermistor. Refer to schematic for detailed connectivity.
Voltage
Voltage
Input VoltageTest point for input voltage.
Inputthe LM3424 switching frequency to the external clock.
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2
AN-1967 LM3424 Buck-Boost Evaluation BoardSNVA397A–August 2009–Revised May 2013
12OVPOver-Voltage ProtectionTurn-off threshold is 1.24V and
13DDRVDimming Gate Drive Output
14GNDGround
15GATEGate Drive Output
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Bypass with 100 nF capacitor
IN
Input Voltage
to GND as close to the device
as possible in the circuit board
layout.
Connect to > 2.4V to enable
power shutdown.
Connect a capacitor to GND to
compensate control loop.
Connect a resistor to GND to
set the signal current. Can also
be used to analog dim as
explained in the Thermal
Foldback / Analog Dimming
section of the datasheet.
Connect a resistor to GND to
set the switching frequency.
Can also be used to
explained in the SwitchingFrequency section of the
datasheet.
Connect a PWM signal for
dimming as detailed in the
PWM Dimming section of the
datasheet and/or a resistor
divider from VINto program
input under-voltage lockout
(UVLO). Turn-on threshold is
1.24V and hysteresis for turnoff is provided by 20 µA current
source.
Connect a capacitor to GND to
extend start-up time.
Connect a resistor to GND to
set the foldback slope.
Connect a resistor/ thermistor
divider from VSto sense the
temperature as explained in
the Thermal Foldback / AnalogDimming section of the
datasheet.
Temperature Foldback
Reference
Connect a resistor divider from
foldback reference voltage.
2.45V reference for
S
Voltage Referencetemperature foldback circuit
and other external circuitry.
Connect to a resistor divider
from VOto program output
over-voltage lockout (OVLO).
hysteresis for turn-on is
provided by 20 µA current
source.
Connect to gate of dimming
MosFET.
Connect to DAP to provide
proper system GND
Connect to gate of main
switching MosFET.
4
AN-1967 LM3424 Buck-Boost Evaluation BoardSNVA397A–August 2009–Revised May 2013
Figure 4. Standard OperationFigure 5. 200Hz 50% PWM Dimming
TP1 Switch Node Voltage (VSW)TP5 Dim Voltage (V
LED Current (I
)LED Current (I
LED
LED
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)
DIM
)
10Alternate Designs
Alternate designs with the LM3429 evaluation board are possible with very few changes to the existing
hardware. The evaluation board FETs and diodes are already rated higher than necessary for design
flexibility. The input UVLO, output OVP, input and output capacitance can remain the same for the designs
shown below. These alternate designs can be evaluated by changing only R9, R10, and L1.
Table 2 gives the main specifications for four different designs and the corresponding values for R9, R10,
and L1. PWM dimming can be evaluated with any of these designs.
Specification /Design 1Design 2Design 3Design 4
Component
V
IN
V
O
f
SW
I
LED
R90.05Ω0.2Ω0.04Ω0.08Ω
R1012.1 kΩ10.2 kΩ14.3 kΩ10.2 kΩ
L122µH68µH15µH33µH
Table 2. Alternate Design Specifications
10V - 45V15V - 50V20V - 55V25V - 60V
14V21V28V35V
600kHz700kHz500kHz700kHz
2A500mA2.5A1.25A
14
AN-1967 LM3424 Buck-Boost Evaluation BoardSNVA397A–August 2009–Revised May 2013
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