Buzzer Control Circuit¶
Purpose¶
This circuit provides controlled high-side switching for external 5 V loads connected to the “Buzzer” pin. It is designed to safely support a wide range of unknown user-supplied devices (buzzers, relays, LEDs, small solenoids) while limiting current and protecting the PCB from overloads and inductive transients.
Key features of this revision: - Active current limiting using TPS2553 (~250 mA fold-back threshold) - Flyback clamp to GND at the Buzzer pin (anode = GND, cathode = Buzzer) — sized for inductive loads where the load's own ground return carries the recirculation current - Optional ESD/TVS clamp at the external connector
Control Signal¶
- Origin: ESP32 GPIO (or any 3.3 V microcontroller)
- Voltage: 3.3 V logic level
- Logic: Active HIGH
- GPIO HIGH → Load ON
- GPIO LOW → Load OFF
Main Components¶
| Component | Part Number | Function |
|---|---|---|
| Q6 | FDN340P | P-channel MOSFET high-side switch |
| Q5 | BC847 | NPN transistor gate driver |
| J5 | TPS2553 | High-side current-limited power switch |
| R22 | 113 kΩ 0.1% | Sets TPS2553 current limit (~250 mA) |
| R38 | 1 kΩ 1% | Base current limiting resistor |
| R65 | 6.8 kΩ | MOSFET gate pull-up |
| D1 | SS14 | Flyback clamp to GND (anode = GND, cathode = Buzzer) |
| C29 | 0.1 µF | TPS2553 input bypass capacitor |
| C41 | 1 µF | TPS2553 output stability capacitor |
| D50 | TVS/ESD diode | Clamp at external buzzer pin to GND |
Circuit Architecture¶
The load path is:
5 V rail → Q6 (P-MOSFET switch) → TPS2553 (current limiter) → Buzzer pin → external device → GND
Control path:
ESP32 GPIO → Q5 (NPN) → pulls Q6 gate low → enables 5 V power to the TPS2553 and load.
Protection elements: - TPS2553 limits load current to ~250 mA. - SS14 provides inductive kickback return to the 5 V rail. - TVS/ESD diode clamps external spikes to ground.
Circuit Connections¶
Power Path¶
- Q6 Source (Pin 2): 5 V rail
- Q6 Drain (Pin 3): TPS2553 IN
- TPS2553 OUT (Pin 6): Buzzer output pin
- Load return: External device returns to GND
TPS2553 Configuration¶
- IN (Pin 1): Fed from Q6 drain
- EN (Pin 3): Tied to IN (always enabled when Q6 is on)
- GND (Pin 2): Ground
- ILIM (Pin 5): 113 kΩ to GND → sets ~250 mA current limit
- FAULT (Pin 4): Not used
- C29: 0.1 µF from IN to GND
- C41: 1 µF from OUT to GND
Gate Drive¶
- BuzzerControl → R38 (1 kΩ) → Q5 base
- Q5 emitter → GND
- Q5 collector → Q6 gate
- R65 (6.8 kΩ) pulls Q6 gate up to 5 V
Flyback Protection¶
- D1 (SS14, SMA, 40 V Schottky):
- Cathode → Buzzer output node (TPS2553 OUT, RJ3 cable 4 pin 15)
- Anode → GND
When the user's external load is inductive (a relay coil, magnetic buzzer) and the switch turns OFF, the inductor drives the Buzzer pin below ground to try to maintain current flow. D1 forward-biases and clamps that negative excursion to one Schottky drop (~0.3 V below GND), giving the stored inductive energy a safe path to recirculate through the load's own ground return.
Note (audit 2026-05-23): Earlier versions of this document described the diode as returned to the 5 V rail (cathode = 5 V, anode = Buzzer). That is wrong for this circuit: the TPS2553 is a high-side switch and the user's load returns to its own GND, so the inductive kick is negative at the Buzzer pin, and the correct clamp is to GND, not to 5 V. The V9 build has D1 anode = GND, cathode = Buzzer, which is what is documented above. The doc has been corrected to match.
ESD Protection (Connector Side)¶
- D50 TVS diode:
- Cathode → Buzzer pin
- Anode → GND
This absorbs static discharge or cable-induced transients at the external interface.
Circuit Operation¶
Load OFF (GPIO = LOW)¶
- Q5 is OFF.
- R65 pulls Q6 gate up to 5 V.
- VGS ≈ 0 V → Q6 OFF.
- TPS2553 receives no input power.
- Buzzer pin is unpowered → Load OFF.
Load ON (GPIO = HIGH)¶
- Base current into Q5:
(3.3 V − 0.7 V) / 1 kΩ ≈ 2.6 mA - Q5 turns ON.
- Q5 pulls Q6 gate to GND.
- VGS ≈ −5 V → Q6 fully ON.
- 5 V applied to TPS2553 input.
- TPS2553 supplies load, limiting current to ~250 mA.
- Load energizes.
Turn-Off Event (Inductive Load)¶
- Q6 turns OFF; TPS2553 input loses power and OUT high-Zs.
- The external inductive load (e.g., relay coil) keeps current flowing toward GND through itself.
- With no source at the Buzzer pin, the inductor drags the Buzzer node below GND to maintain that current.
- D1 (SS14) forward-biases when the Buzzer pin falls ~0.3 V below GND.
- Current circulates: GND → D1 (forward) → Buzzer pin → external load → external GND → back through ground plane to D1 anode.
- Stored inductive energy dissipates in the load's own series resistance over the L/R time constant. The diode forward-drop accounts for a small fraction of the energy.
Current Capability¶
Hard Limit¶
- TPS2553 current limit: ~250 mA (set by 113 kΩ)
This is now the primary protection mechanism and defines safe operation.
MOSFET Capability (Secondary)¶
FDN340P ratings (not the limiting factor anymore): - Continuous current: ~2 A - RDS(on): ~65 mΩ @ VGS = −2.5 V - Power dissipation at 250 mA: - P ≈ I²R = 0.25² × 0.065 ≈ 4 mW (negligible)
System-Level Limit¶
- External load is electronically capped at ~250 mA.
- Prevents trace, via, and connector overheating.
- Protects against unknown customer devices.
Load Compatibility¶
Supported Loads (Safe)¶
| Load Type | Typical Current | Result |
|---|---|---|
| Piezo buzzer | 5–15 mA | Excellent |
| Magnetic buzzer | 20–80 mA | Excellent |
| Small relay (5 V) | 50–150 mA | Good |
| LEDs / indicators | 5–200 mA | Good |
| Small solenoids | ≤250 mA | At limit |
Loads That Will Trip Protection¶
| Load Type | Behavior |
|---|---|
| Large relay (>250 mA) | TPS2553 limits/shuts down |
| Short circuit | Current limited automatically |
Idle Power Consumption¶
Load OFF¶
- Only R65 conducts:
- 5 V / 6.8 kΩ ≈ 0.74 mA
- Power ≈ 3.7 mW
Load ON Overhead¶
- Base drive ≈ 2.6 mA
- Gate pullup current ≈ 0.74 mA
- TPS2553 quiescent current: small
Total overhead ≈ 3–4 mA plus load current.
Bill of Materials (BOM)¶
| Reference | Part Number | Description | Package | Qty |
|---|---|---|---|---|
| Q6 | FDN340P | P-MOSFET, −20 V, −2 A | SOT-23 | 1 |
| Q5 | BC847 | NPN transistor | SOT-23 | 1 |
| J5 | TPS2553 | Current-limited high-side switch | SOT-23-6 | 1 |
| R22 | 113 kΩ 0.1% | ILIM set resistor | 0603 | 1 |
| R38 | 1 kΩ 1% | Base resistor | 0603 | 1 |
| R65 | 6.8 kΩ | Gate pull-up | 0603 | 1 |
| D1 | SS14 | Flyback diode | SMA/SMB | 1 |
| C29 | 0.1 µF | Input bypass | 0402/0603 | 1 |
| C41 | 1 µF | Output stability cap | 0402/0603 | 1 |
| D50 | PTVS5V5D1BL (or similar) | TVS/ESD clamp | DFN/SOD | 1 |
Design Considerations¶
Current Limiting Strategy¶
The TPS2553 is now the dominant protection element.
It prevents:
- Trace overheating
- Via overheating
- Connector damage
- User wiring faults
Flyback Strategy¶
Returning the diode to GND (anode = GND, cathode = Buzzer pin): - Clamps negative inductive kick at the Buzzer pin to ~0.3 V below GND - Recirculates current through the load's own ground return — keeps the high-side switch and the 5 V rail isolated from the recirculation loop - Protects Q6 and the TPS2553 OUT pin from negative voltage that would otherwise exceed absolute-max ratings
ESD Strategy¶
The TVS at the connector: - Handles cable-induced static - Protects internal silicon - Not critical for function, but improves robustness
PCB Layout Notes¶
- Keep TPS2553 IN/OUT caps physically close to pins.
- Keep Q6–TPS2553 path low resistance.
- Place flyback diode near the buzzer node if possible.
- Place TVS near connector entry point.
Performance Goals¶
- Controlled load current (≈250 mA max)
- Robust against unknown external devices
- Safe with inductive loads
- Minimal thermal stress on PCB
- Electrically tolerant of field wiring and ESD
Notes¶
- Logic is active HIGH.
- Suitable for DC switching and low-frequency PWM.
- Designed to tolerate arbitrary customer-connected 5 V loads safely.