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 additions in this revision: - Active current limiting using TPS2553 - Flyback diode returned to the 5 V rail (not ground) - 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 diode to 5 V rail
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):
Cathode → 5 V rail
Anode → Buzzer output node (TPS2553 OUT)

This clamps inductive spikes to slightly above 5 V and recirculates energy back into the supply.

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.
Inductor forces current to continue flowing.
Buzzer node rises above 5 V.
SS14 forward-biases.
Current circulates: load → diode → 5 V rail.
Spike is clamped and dissipated safely.

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 the 5 V rail: - Prevents large negative spikes - Keeps energy localized - Protects Q6 and TPS2553

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.