STMicroelectronics STEVAL-L3751V12 Manuel utilisateur
Introduction
The 6 to 75 V wide input voltage range STEVAL-L3751V12 synchronous buck evaluation board features an extreme voltage
conversion ratio over switching frequency.
The board provides a regulated 5 V output at 230 kHz switching frequency. The output voltage can be easily set to 12 V using a
jumper connector. Different voltage output can be selected by changing a resistor to a lower VIN range.
The STEVAL-L3751V12 is a 100 W demo board. The default maximum current is set to 15 A. It can be easily selected by
changing a resistor.
The evaluation board is based on the L3751 synchronous buck controller, which implements the voltage mode in a VQFN
package with internal compensation to minimize the design complexity and size.
The diode emulation (DEM) implements the pulse-skipping mode, which maximizes the efficiency at light-load with a controlled
output voltage ripple.
The forced PWM (FPWM) over-the-load range makes the switching frequency constant and minimizes the output voltage ripple.
The power good open collector output validates the regulated output voltage for monitoring. It implements the output voltage
sequencing for digital ICs during the power-up phase.
The embedded gate driver is designed for standard Vth MOSFET and minimizes the number of external components.
The embedded protections, such as the output overcurrent, the input voltage UVLO, the internal voltage monitoring, and the
thermal shutdown at 150°C degrees, feature a controlled and safe operation for critical environments in telecom, networking,
and industrial applications.
You can use the eDesignSuite software tool to configure the L3751 buck converter and satisfy the application requirements.
Figure 1. STEVAL-L3751V12 evaluation board (top view)
Figure 2. STEVAL-L3751V12 evaluation board (bottom view)
Getting started with the STEVAL-L3751V12 evaluation board based on DC-DC
converter buck regulator with L3751 controller
UM3024
User manual
UM3024 - Rev 1 - October 2022
For further information contact your local STMicroelectronics sales office. www.st.com
1Getting started
1.1 Safety instructions
Caution: This board has to be used only by skilled technical personnel who are suitably qualified and familiar with the
installation, use, and maintenance of power electronic systems. The same personnel must be aware of and must
apply national accident prevention rules. The electrical installation has to be completed in accordance with the
appropriate requirements (that is, cross-sectional areas of conductors, fusing, and GND connections).
1.2 Features
• 6 to 75 V wide input voltage range
•Selectable two-output voltage level: 5 or 12 V
• 230 kHz switching frequency setting
• Low drop-out operation during line transient
• Pulse skipping or forced PWM operation at light load
• Synchronization from 185 to 345 kHz
• Power good open collector output validates the VOUT
• 7.5 V gate drivers for standard Vth MOSFETs
• Constant current protection with hiccup mode
• Current sensing on rds-on low-side
• Improved line-transient response
• Input voltage undervoltage lockout
• Internal voltage monitoring
• Thermal protection
• Operating junction temperature range: -40 to 150°C
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Getting started
UM3024 - Rev 1 page 2/24
2How to use the board
The STEVAL-L3751V12 is configured through the L3751 with adjustable VOUT. The board is set to deliver a 5 V or
12 V output voltage (see Section 3.4 Signal manager - J5) with FSW = 230 kHz.
To use the board, follow the procedure below.
Step 1. Connect the voltage supply between VIN and GND connectors.
Step 2. Connect the load (power resistor or active load) between VOUT and GND connectors.
Note: We suggest using short wires for the connections of steps 1 and 2.
Step 3. Set the supply voltage VIN from 6 to 75 V with VOUT = 5 V or VIN from 13 to 75 V with VOUT = 12 V.
Step 4. To reach IoutMax, increase the active load or decrease the power resistor.
An external air flow is suggested to decrease the board thermal heating.
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How to use the board
UM3024 - Rev 1 page 3/24
3Connectors and test points
3.1 VIN - FS1
This is the VIN connector. To use it, connect the supply voltage from 6 to 75 V. This voltage becomes the device
VIN after an EMI filter (see Section 4 for further details).
The supply voltage should be set with a current limitation compatible with the power board request. Typically:
Ilim >VOUT/VIN ×IOUT
VIN should be connected to the power source with a short wire to avoid oscillations between the cable parasitic
inductance and the input ceramic capacitor.
3.2 GND - FS2
This is the return of the terminal of the input and output capacitors. We suggest using a short wire to avoid
oscillations between the cable parasitic inductance and the input capacitor.
3.3 VOUT - FS4
This is the VOUT connector and test point. Connect the active load or power resistor between VOUT and GND test
point. We suggest using short wires for the connection.
3.4 Signal manager - J5
• Pin 1–VCC: output pin. You can monitor the VCC pin voltage (driver supply voltage).
•Pin 2–SYNCIN: input pin. You can change the switching frequency through an external waveform generator.
You can set the frequency between 50% FSW and 120% FSW.
• Pin 3–AGND: signal GND.
• Pin 4–SYNCOUT: output pin. A rectangular waveform is generated from this pin. It implements a 180° phase
shifted referred to the HO driving signal. It can be useful for synchronization with others power system
(check the L3751 datasheet for further details).
• Pin 5 - PGOOD: output signal. It is an open collector. It is short to GND once VOUT is out of the regulator
windows (±8%). Otherwise, it is short to the external pull-up voltage. In the STEVAL-L3751V12, it is pulled
up to the VCC pin.
• Pin 6–TRIM: connect to GND ( short to pin 7 through a jumper) to select VOUT1. Leave the pin floating to
select VOUT2. In the STEVAL-L3751V12, VOUT1 is set to 12.18 V and VOUT2 is set to 5.07 V.
• Pin 7–AGND: signal GND. Used to short pin6 to GND and changeVOUT. See pin 6 description.
• Pin 8 - SS/TRK: output pin. You can monitor the soft start ramp. The pin voltage is clamped at 115 mV over
the FB voltage (see the L3751 datasheet for further details).
• Pin 9–EN/Uvlo: input signal. You can turn on and off the system with an external signal (see the L3751
datasheet for further details). By default, it is connected to a partition of VIN. The system turns on once
VIN>14.5 V.
• Pin 10 - AGND: signal GND.
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Connectors and test points
UM3024 - Rev 1 page 4/24
4Input EMI filter
The STEVAL-L3751V12 is compliant with CISPR16-4-2.
An input filter helps to reduce EMI. This filter consists of:
• a double pi filter with an inductor (L2);
• a ferrite bead (L5);
• MLCC capacitors (C18, C19, C20, and C30);
• an electrolytic capacitor used for a bulk energy storage;
• an input damping.
By default, the ferrite (R25 = 0 ohm and L5 NC) is not used. Only the pi filter is set.
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Input EMI filter
UM3024 - Rev 1 page 5/24
5Board setting capability
The STEVAL-L3751V12 overcurrent limit is set to 18 A (average) through R5. Increasing or decreasing this
resistor, it is possible to increase or decrease the current limit (check the datasheet or eDesignSuite for further
details).
The switching frequency is set through R10. It can be changed from 100 KHz to 1 MHz. Considering that the
increasing of Fsw can charge a large amount of self-heating (check the datasheet or eDesignSuite for further
details).
The STEVAL-L3751V12 is a 100 W power evaluation board. So, it is considered IoutMax = 15 A for VOUT = 5 V
and IoutMax = 10 A for VOUT = 12 V. For high output power, we suggest to use a 200 LMF air flow to decrease
the self-heating and avoid that OTP occurs.
The high input current can increase the EMI inductor temperature. In this case, it is possible to shortcut the
inductor by using R18 = 0 Ohm or selecting an inductor with a higher Irms.
The VIN UVLO is set to 14.5 V. However, it is possible to change the VIN UVLO by changing the R9 and R19
partition (check the datasheet or eDesignSuite for further details).
VOUT can be selected from 5.07 V to 12.18 V by using J1.
It is possible to change VOUT1 and VOUT2 by selecting different values for R12, R13 and R21.
• VOUT1→pin 6 to GND→VOUT = (R12 +R13//R21)/(R13//R2) 1 x VFB where VFB is fixed to 0.8 V.
• VOUT2→ pin 6 to floating → VOUT = (R12 +R13)/R13 x VFB where VFB is fixed to 0.8 V (see the L3751
datasheet for further details).
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Board setting capability
UM3024 - Rev 1 page 6/24
6eDesignSuite
The eDesignSuite suite software tool helps you to configure ST products for power conversion applications.
You can use it to customize a PFC controller for a specific application. Start by entering the main specifications
for your design. Then, generate an automatic design or follow a sequential process to build a highly customized
design.
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eDesignSuite
UM3024 - Rev 1 page 7/24
Figure 5. STEVAL-L3751V12 PCB layout (3 of 4)
Figure 6. STEVAL-L3751V12 PCB layout (4 of 4)
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PCB layout
UM3024 - Rev 1 page 9/24
8Performance waveforms
VIN soft start
•Green = VOUT
• Blue = VIN
• Yellow = switching node
• Violet = PGOOD
Figure 7. VIN = 48 V, VOUT = 12 V at no load Figure 8. VIN = 48 V, VOUT = 5 V at no load
Figure 9. VIN = 48 V, VOUT = 12 V, ROUT = 10 ohm Figure 10. VIN = 48 V, VOUT = 5 V, ROUT = 10 ohm
Enable soft start
•Green = VOUT
• Blue = enable
• Yellow = switching node
• Violet = PGOOD
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Performance waveforms
UM3024 - Rev 1 page 10/24
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