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Hello, Please ask a question about SM72485MM Datasheet
# Example questions:
➢ The text explains the need for resistor r what two factors, combined, determine the minimum value required for r3?
➢ What is the primary purpose of capacitor c3 in the sm72485 application circuit?
➢ What input voltage range and output voltage are targeted in the provided example?
| Part No. | SM72485MM |
| Manufacturer | NSC |
| Size | 448 Kbytes |
| Pages | 16 pages |
| Description | SolarMagic 100V, 150 mA Constant On-Time Buck Switching Regulator |
I. Overview of the SM72485 & its Features
️· Purpose: A synchronous buck regulator (step-down converter) designed for efficiency and flexibility.
️· Key Features:
- Thermal Shutdown
- Current Limit Protection
- Adjustable Switching Frequency
- Synchronous Rectification (improves efficiency)
- Internal UVLO (Under Voltage Lockout)
️· Target Applications: Power supplies for various applications (e.g., servers, networking equipment, consumer electronics)
II. Design Example: Specifications
️· Vin: 12V to 90V
️· Vout: 10V
️· Io: 100mA to 150mA (Continuous Conduction Mode)
III. Component Selection & Design Choices (with Calculations)
Let's break down each component's selection rationale and approximate calculations:
1. Feedback Resistors (R<sub>FB1</sub> & R<sub>FB2</sub>):
- Goal: Set the output voltage (V<sub>OUT</sub>) to the desired value.
- Calculation: V<sub>FB</sub> * (R<sub>FB1</sub> + R<sub>FB2</sub>) / R<sub>FB1</sub> = V<sub>OUT</sub>. Since V<sub>FB</sub> is fixed at 2.5V, maintain a 3:1 ratio between R<sub>FB2</sub> and R<sub>FB1</sub>.
- Chosen Values: R<sub>FB1</sub> = 1.0 kΩ, R<sub>FB2</sub> = 3.01 kΩ. (Standard Values, maintaining the ratio).
2. Switching Frequency (f<sub>SW</sub>) & Inductor (L<sub>1</sub>):
- Goal: Set the switching frequency and select an inductor that minimizes ripple and avoids saturation.
- Minimum On-Time: Minimum on-time = V<sub>OUT</sub> / (V<sub>IN (max)</sub> * f<sub>SW</sub>) = 476ns.
- Maximum f<sub>SW</sub>: f<sub>SW</sub> <= V<sub>OUT</sub> / (V<sub>IN (max)</sub> * Minimum on-time) = 277 kHz
- Chosen f<sub>SW</sub>: 234 kHz (using a 309 kΩ resistor for R<sub>T</sub>).
- Inductor Selection: Minimum inductance is determined by ripple current. Calculate L based on desired ripple amplitude (I<sub>OR</sub>).
■ I<sub>OR</sub> < 2 * I<sub>OUT</sub>.
■ L > (V<sub>IN</sub> * I<sub>OUT</sub> * T<sub>on</sub>)/(V<sub>IN</sub> - V<sub>OUT</sub>)* (T<sub>on</sub> = Ton = on time)
- Chosen Value: L<sub>1</sub> = 220 µH (compromise between size and ripple).
3. Capacitor on VCC (C<sub>3</sub>):
- Goal: Prevent false triggering of UVLO.
- Minimum Value: 0.47 µF
- Chosen Value: 0.47 µF
4. Output Filter Capacitor (C<sub>2</sub>) & Resistor (R<sub>3</sub>):
- Goal: Reduce output ripple while ensuring sufficient feedback voltage.
- Minimum Ripple Required at Feedback Pin: 25 mV.
I. Overview of the SM72485 & its Features
️· Purpose: A synchronous buck regulator (step-down converter) designed for efficiency and flexibility.
️· Key Features:
- Thermal Shutdown
- Current Limit Protection
- Adjustable Switching Frequency
- Synchronous Rectification (improves efficiency)
- Internal UVLO (Under Voltage Lockout)
️· Target Applications: Power supplies for various applications (e.g., servers, networking equipment, consumer electronics)
II. Design Example: Specifications
️· Vin: 12V to 90V
️· Vout: 10V
️· Io: 100mA to 150mA (Continuous Conduction Mode)
III. Component Selection & Design Choices (with Calculations)
Let's break down each component's selection rationale and approximate calculations:
1. Feedback Resistors (R<sub>FB1</sub> & R<sub>FB2</sub>):
- Goal: Set the output voltage (V<sub>OUT</sub>) to the desired value.
- Calculation: V<sub>FB</sub> * (R<sub>FB1</sub> + R<sub>FB2</sub>) / R<sub>FB1</sub> = V<sub>OUT</sub>. Since V<sub>FB</sub> is fixed at 2.5V, maintain a 3:1 ratio between R<sub>FB2</sub> and R<sub>FB1</sub>.
- Chosen Values: R<sub>FB1</sub> = 1.0 kΩ, R<sub>FB2</sub> = 3.01 kΩ. (Standard Values, maintaining the ratio).
2. Switching Frequency (f<sub>SW</sub>) & Inductor (L<sub>1</sub>):
- Goal: Set the switching frequency and select an inductor that minimizes ripple and avoids saturation.
- Minimum On-Time: Minimum on-time = V<sub>OUT</sub> / (V<sub>IN (max)</sub> * f<sub>SW</sub>) = 476ns.
- Maximum f<sub>SW</sub>: f<sub>SW</sub> <= V<sub>OUT</sub> / (V<sub>IN (max)</sub> * Minimum on-time) = 277 kHz
- Chosen f<sub>SW</sub>: 234 kHz (using a 309 kΩ resistor for R<sub>T</sub>).
- Inductor Selection: Minimum inductance is determined by ripple current. Calculate L based on desired ripple amplitude (I<sub>OR</sub>).
■ I<sub>OR</sub> < 2 * I<sub>OUT</sub>.
■ L > (V<sub>IN</sub> * I<sub>OUT</sub> * T<sub>on</sub>)/(V<sub>IN</sub> - V<sub>OUT</sub>)* (T<sub>on</sub> = Ton = on time)
- Chosen Value: L<sub>1</sub> = 220 µH (compromise between size and ripple).
3. Capacitor on VCC (C<sub>3</sub>):
- Goal: Prevent false triggering of UVLO.
- Minimum Value: 0.47 µF
- Chosen Value: 0.47 µF
4. Output Filter Capacitor (C<sub>2</sub>) & Resistor (R<sub>3</sub>):
- Goal: Reduce output ripple while ensuring sufficient feedback voltage.
- Minimum Ripple Required at Feedback Pin: 25 mV.
| Part No. | SM72485MM |
| Manufacturer | NSC |
| Size | 448 Kbytes |
| Pages | 16 pages |
| Description | SolarMagic 100V, 150 mA Constant On-Time Buck Switching Regulator |
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