Texas Instruments LM25119EVAL User Manual
AN-2066 LM25119 Evaluation Board
1 Introduction
The LM25119EVAL evaluation board provides the design engineer with a fully functional dual output buck
converter, employing the LM25119 Dual Emulated Current Mode Synchronous Buck Controller. The
evaluation board is designed to provide both 3.3V and 1.8V outputs over an input range of 6.0V to 36V.
Also the evaluation board can be easily configured for a single 3.3V, 16A regulator.
2 Performance of the Evaluation Board
• Input Voltage Range: 6.0V to 36V
• Output Voltage: 3.3V (CH1), 1.8V (CH2)
• Output Current: 8A (CH1), 8A (CH2)
• Nominal Switching Frequency: 230 KHz
• Synchronous Buck Operation: Yes
• Diode Emulation Mode: Yes
• Hiccup Mode Overload Protection: Yes
• External VCC Sourcing: No
User's Guide
SNVA445B–August 2010–Revised April 2013
3 Powering and Loading Consideration
When applying power to the LM25119 evaluation board, certain precautions need to be followed. A
misconnection can damage the assembly.
3.1 Proper Board Connection
The input connections are made to the J1 (VIN) and J2 (RTN/GND) connectors. The CH1 load is
connected to the J3 (OUT1+) and J4 (OUT1-/GND) and the CH2 load is connected to the J6 (OUT2+) and
J5 (OUT2-/GND). Be sure to choose the correct connector and wire size when attaching the source power
supply and the load.
3.2 Source Power
The power supply and cabling must present low impedance to the evaluation board. Insufficient cabling or
a high impedance power supply will droop during power supply application with the evaluation board
inrush current. If large enough, this droop will cause a chattering condition during power up. During power
down, insufficient cabling or a high impedance power supply will overshoot. This overshoot will cause a
non-monotonic decay on the output.
An additional external bulk input capacitor may be required unless the output voltage droop/overshoot of
the source power is less than 0.5V. In this board design, UVLO setting is conservative while UVLO
hysteresis setting is aggressive. Minimum input voltage can goes down with an aggressive design.
Minimum operating input voltage depends on the output voltage droop/overshoot of the source power
supply and the forced off-time of the LM25119. For complete design information, see the
LM25119/LM25119Q Wide Input Range Dual Synchronous Buck Controller Data Sheet (SNVS680).
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Volt-meter
Current-meter
VIN
RTN(GND)
OUT1+(3.3V)
OUT1-(GND)
OUT2-(GND)
OUT2+(1.8V)
+ Electric Load
With Current Meter
- 0A-8A
- Electric Load
With Current Meter
+ 0A-8A
0-36V, 20A
DC Power
Supply
Volt-meter
Volt-meter
Scope
J1
J2
J6
J5
J4
J3
LM25119 DUAL BUCK
Powering and Loading Consideration
3.3 Loading
When using an electronic load, it is strongly recommended to power up the evaluation board at light load
and then slowly increase the load. If it is desired to power up the evaluation board at maximum load,
resistor banks must be used. In general, electronic loads are best suited for monitoring steady state
waveforms.
3.4 Air Flow
Prolonged operation with high input voltage at full power will cause the MOSFETs to overheat. A fan with
a minimum of 200LFM should be always provided.
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3.5 Quick Start-Up Procedure
Figure 1. Typical Evaluation Setup
1. Set the power supply current limit to at least 20A. Connect the power supply to J1 and J2.
2. Connect one load with an 8A capacity between J3 and J4. Connect another load with an 8A capacity
between J6 and J5.
3. Set input voltage to 12V and turn it on.
4. Measure the output voltages. CH1 should regulate at 3.3V and CH2 should regulate at 1.8V.
5. Slowly increase the load current while monitoring the output voltages. The outputs should remain in
regulation up to full load current.
6. Slowly sweep the input voltage from 6.0V to 36V while monitoring the output voltages. The outputs
should remain in regulation.
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4 Waveforms
4.1 Soft Start
When applying power to the LM25119 evaluation board a certain sequence of events occurs. Soft-start
capacitors and other components allow for a linear increase in output voltages. The soft-start time of each
output can be controlled independently. Figure 2 shows the output voltage during a typical start-up with a
load of 0.5Ω on the 3.3V output, and 0.33Ω on the 1.8V output, respectively.
Conditions:
Input Voltage = 12VDC
0.5Ω Load on 3.3V output
0.33Ω Load on 1.8V output
Traces:
Top Trace: 3.3V Output Voltage, Volt/div = 1V
Bottom Trace: 1.8V Output Voltage, Volt/div = 1V
Horizontal Resolution = 1 ms/div
Waveforms
Figure 2. Start-up with Resistive Load
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Waveforms
4.2 Load Transient
Figure 3 shows the transient response for a load of change from 2A to 6A on 3.3V output. The upper
waveform shows output voltage droop and overshoot during the sudden change in output current shown
by the lower waveform.
Conditions:
Input Voltage = 12VDC
Output Current 2A to 6A
Traces:
Top Trace: 3.3V Output Voltage, Volt/div = 100mV, AC coupled
Bottom Trace: Output Current Amp/Div = 2A
Horizontal Resolution = 0.5 ms/div
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Figure 3. Load Transient Response
4.3 Over Load Protection
The evaluation board is configured with hiccup mode overload protection. The restart time can be
programmed by C11. Figure 4 shows hiccup mode operation in the event of an output short on CH1
output. One channel may operate in the normal mode while the other is in hiccup mode overload
protection.
Conditions:
Input Voltage = 12VDC
Output Short on 3.3V
Traces:
Top Trace: SW voltage on CH1, Volt/div = 10V
Bottom Trace: Inductor Current Amp/div = 10A
Horizontal Resolution = 20 ms/div
Figure 4. Short Circuit
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4.4 External Clock Synchronization
A TP1 (SYNC) test point has been provided on the evaluation board in order to synchronize the internal
oscillator to an external clock. Figure 5 shows the synchronized switching operation. Each channel
operates 180° out of phase from the other.
Conditions:
Input Voltage = 12VDC
8A on 3.3V output
8A on 1.8V output
Traces:
Top Trace: SYNC pulse, Volt/div = 5V
Middle Trace: SW voltage on CH1, Volt/div = 10V
Bottom Trace: SW voltage on CH2, Volt/div = 10V
Horizontal Resolution = 1 µs/div
Figure 5. Clock Synchronization
Waveforms
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