Hercules — Dual H-Bridge Motor Controller

Project Overview

The Hercules board is a standalone motor driver module designed to act as an interface between a microcontroller (such as a Raspberry Pi or STM32) and two DC brushed motors. The name "Hercules" reflects the board's purpose: brute mechanical force delivery, controlled precisely by digital signals.

The design goal was to build the smallest possible board that could drive two motors bidirectionally at meaningful current levels, while being easy to interface with via standard 0.1-inch pitch headers. Every component choice — from the IC to the capacitor dielectrics — was deliberate and grounded in understanding why, not just following a reference schematic.

Bill of Materials

The TB6612FNG Motor Driver IC

The TB6612FNG is Toshiba's dual H-bridge motor driver IC, a popular choice for hobby robotics and embedded motor control. Each of the two H-bridges is built from four N-channel and P-channel MOSFETs internally, with separate logic and power supply domains, integrated protection circuitry, and low on-resistance output stages.

Why TB6612FNG Over Discrete MOSFETs?

Building a dual H-bridge from discrete MOSFETs requires 8 switching devices, 8 gate drivers, dead-time insertion circuitry (to prevent shoot-through), and 8 protection diodes. The TB6612FNG integrates all of this — two complete H-bridges with gate drive, dead-time control, thermal shutdown, and back-EMF protection diodes — in a 24-pin SSOP package. For a two-motor drive application at these current levels, integration is the correct engineering choice.

Complete Pin Map (SSOP-24)

H-Bridge Topology Physics

An H-bridge is the fundamental circuit for bidirectional motor control. Understanding its operation at the transistor level reveals why specific control logic sequences are needed and why some switch combinations are actively forbidden.

The Four-Switch Bridge

Each H-bridge channel consists of four transistors: an upper-left (UL), upper-right (UR), lower-left (LL), and lower-right (LR) switch. The motor winding connects between the midpoints of the two half-bridges.

Forward Rotation (CW Mode)

To drive the motor clockwise: upper-left and lower-right switches close simultaneously. Current flows from VBATT+ through UL, through the motor left-to-right, through LR, to GND. The motor sees the full rail voltage (minus transistor drops), which creates a magnetic field in the winding that spins the rotor forward.

The current magnitude depends on the motor's resistance: I = (VBATT − V_drop) / R_motor. At startup, R_motor is just the DC winding resistance (typically 1–10Ω for small motors). This is why startup current is much higher than running current — during acceleration, no back-EMF exists yet to oppose supply voltage.

Why Shoot-Through Must Be Prevented

If both UL and LL (or UR and LR) turn on simultaneously, a direct short circuit forms from VBATT+ to GND through only the transistor on-resistances. This creates a near-unlimited current path that will destroy both transistors in microseconds.

Current in the Motor During Switching

When a PWM cycle turns off the drive transistors, current cannot instantly drop to zero — the motor winding's inductance resists any abrupt change in current. This is Faraday's law: V = L × dI/dt. The winding acts as an energy-storage element; when the transistors switch off, the stored magnetic energy must flow somewhere.

Without a current path, the inductor would generate a voltage spike large enough to punch through any transistor in the circuit. The solution is recirculation diodes — also called flyback or freewheeling diodes — which provide a safe current path when the switches open.

Back-EMF and Flyback Protection

Back-EMF: The Motor as a Generator

When a DC motor spins, the rotating armature cuts through the magnetic field of the stator. By Faraday's law, this generates a voltage — the back-EMF (BEMF) — that opposes the applied supply voltage. The motor's actual operating current is:

This has critical implications for motor driver design: the driver must handle full stall current at startup, and the instantaneous current at reversal (when BEMF and supply both push current in the same direction) can briefly reach 2× stall current.

Inductive Kickback During Switching

During PWM switching (100kHz on this board), the drive switches open and close 100,000 times per second. Each time switches open, the winding inductance generates a voltage spike. The magnitude depends on the switch speed:

Integrated Flyback Diodes

The TB6612FNG integrates four catch diodes per H-bridge (eight total per chip) across each output transistor. These diodes are forward-biased exactly when needed: when the drive switches open and the winding tries to sustain current via a voltage reversal, the diodes clamp the spike to VBATT+ + V_diode (≈ 0.7V) on the high side, or to GND − 0.7V on the low side.

Short-Brake Mode Uses This Physics Intentionally

When both low-side switches (LL and LR) are turned on with both high-side switches off, the motor terminals are shorted to GND through the low-side MOSFETs. The motor's rotating inertia continues spinning the armature, which generates back-EMF. This back-EMF now drives a braking current through the low-impedance path (LL and LR), creating a strong electromagnetic braking torque that rapidly decelerates the motor. This is regenerative short braking — far more effective than simply cutting power (coast mode).

Power Architecture: Two Independent Rails

The board operates from two distinct supply rails, kept separate by design to prevent motor noise from corrupting the logic signals and to allow independent voltage selection.

Why Separate Supplies Are Essential

A brushed DC motor is an electrically noisy device. Each time the brushes commutate (switch between armature segments), a spark occurs at the brush-commutator interface. This generates broadband conducted and radiated electromagnetic noise. At 100kHz PWM, the switching transients on VBATT+ can exceed hundreds of millivolts peak-to-peak.

If VCC and VBATT+ were the same net, this switching noise would appear directly on the logic supply and could cause spurious state changes in the control logic, false readings on input pins, or false STBY assertions. By using separate supplies (even from the same source, separated by a ferrite bead in larger systems), this interference is isolated.

Three Motor Supply Pins (VM1, VM2, VM3)

The TB6612FNG has three VM pins (pins 13, 14, and 24), all connected to VBATT+. This is intentional for the same reason output pins are doubled: distributing high current across multiple package leads reduces the resistance-per-pin to ground, lowers I²R heating in the package leads, and improves current handling. On the PCB, all three must connect to the VBATT+ plane with adequate copper.

Decoupling Capacitor Strategy

Four capacitors are placed on this board — two on VCC and two on VBATT+ — each pair consisting of a 100nF (0603) and a 10µF (0805). This two-value strategy is intentional and covers two different frequency domains of power supply noise.

How Capacitors Filter Different Frequencies

A capacitor's impedance is Z = 1/(2πfC). A large capacitor has low impedance at low frequencies but behaves poorly at high frequencies because its equivalent series inductance (ESL) — the parasitic inductance of its leads and body — starts dominating. A small capacitor has high impedance at DC but excels at filtering high-frequency noise because its ESL is very small.

Why X5R Dielectric?

The X5R rating in EIA dielectric classification means the capacitance stays within ±15% of its nominal value across a temperature range from −55°C to +85°C. This contrasts with Y5V or Z5U dielectrics, which can lose 50–80% of their capacitance at temperature extremes or under DC bias — making their stated value essentially fictitious under real operating conditions.

Per-Cap Placement Requirement

The Hercules schematic includes a specific application note (visible in the schematic title block): "Condensers for noise absorption (C1, C2, C3, and C4) should be connected as close as possible to the IC." This is because every millimeter of PCB trace between capacitor and IC pin adds ESL (~1nH/mm), which degrades the high-frequency effectiveness of the bypass capacitor.

Control Logic: Direction and Enable

Each H-bridge channel is controlled by three input signals: IN1, IN2 (direction), and PWM (enable/speed). The STBY pin controls both channels simultaneously.

Complete Truth Table

Understanding Each State

CW / CCW (Driven Rotation)

When IN1=H, IN2=L, PWM=H: the internal logic activates the upper-left and lower-right MOSFETs, driving current through the motor in the "forward" direction. Setting IN1=L, IN2=H reverses the active switch pair, reversing current direction and thus motor rotation. The PWM input acts as an enable: when PWM=H, the selected drive switches are active. When PWM=L, both output pins are pulled to GND (short brake) regardless of IN1/IN2.

Short Brake (IN1=H, IN2=H)

Both low-side switches activate simultaneously, connecting both motor terminals to GND. The motor generates its own braking current via back-EMF as previously described. This state also occurs automatically when PWM goes LOW during a driven state — meaning the PWM signal can be used for speed control via duty cycle without ever entering the high-impedance (coast) state.

Stop / Coast (IN1=L, IN2=L)

All four MOSFETs turn off, leaving both motor terminals floating (high-impedance). The motor freewheels — it continues spinning due to mechanical inertia with no electrical braking torque. This is the slowest braking mode. Current regenerated by back-EMF must flow through the internal flyback diodes to the VBATT+ rail.

Speed Control via PWM

For speed control in CW or CCW direction, set the appropriate IN1/IN2 state and then vary the PWM duty cycle. The TB6612FNG's internal logic sees IN1=H, IN2=L on each cycle. When PWM=H (ON phase): current flows through the motor. When PWM=L (OFF phase): the circuit enters short-brake (both low-side switches on). The motor sees an average voltage equal to VBATT+ × duty cycle — controlling rotational speed.

PWM Speed Control Physics

Pulse-Width Modulation (PWM) controls average power delivery to a load by rapidly switching the supply on and off. The ratio of on-time to total period is the duty cycle, D.

Why 100kHz Switching Frequency?

The 100kHz PWM frequency on the Hercules board (noted in the schematic: Fsw = 100kHz) was selected for two reasons:

1. Above audible range. PWM frequencies below 20kHz cause the motor windings to vibrate at the switching frequency, producing an audible whine. At 100kHz, this vibration is completely inaudible to humans (typical upper limit ~20kHz). The motor runs silently.

2. Current ripple reduction. The motor's inductance filters the PWM waveform into an approximately DC current. Higher switching frequency means smaller current ripple (ΔI = V×t_on/L). At 100kHz vs 10kHz, current ripple is 10× lower for the same inductance, giving smoother torque, less motor heating, and better low-speed control.

The Motor as a Low-Pass Filter

A DC motor winding has both resistance (R) and inductance (L). Together they form an RL low-pass filter with a time constant τ = L/R. For a typical small DC motor (L ≈ 1mH, R ≈ 5Ω), τ = 200µs. At 100kHz, the PWM period is only 10µs — much shorter than τ. This means the inductor strongly attenuates the switching ripple, and the motor's rotor (which has far more mechanical inertia) sees only the DC average, resulting in smooth rotation.

Braking Modes: Short Brake vs. Coast

The TB6612FNG supports two distinct stop behaviors, and choosing between them depends on the application's requirements for deceleration rate and positional precision.

Short Brake (Active Braking)

Both low-side switches (LL and LR) on simultaneously. Both motor terminals connected to GND. The motor acts as a generator: back-EMF drives a braking current through the short circuit (LL → motor → LR). The braking torque is proportional to this current and to motor speed — very effective at high speeds, tapering to zero as the motor decelerates. Stopping distance is much shorter than coast mode.

Coast (High-Impedance Stop)

All four switches open. Motor terminals float. No electrical braking torque — only mechanical friction decelerates the motor. The motor "freewheels" to a slow stop. Useful when you need the motor to spin down gradually, or when you want zero power draw in the stopped state.

PWM-Synchronized Braking

A subtlety in the TB6612FNG control scheme: when the PWM input is used for speed control with a fixed IN1/IN2 direction, the "off" phase of each PWM cycle engages short brake rather than coast. This means during each switching cycle:

Because the current never fully collapses during the OFF phase (the brake keeps it recirculating), the average current at a given duty cycle is higher than if coast mode were used for the OFF phase. This gives the TB6612FNG better low-duty-cycle torque than coast-mode switching schemes.

Standby Mode (STBY Pin)

The STBY (Standby) pin is active-HIGH: pulling it LOW forces both H-bridges into a complete shutdown state, where all output transistors turn off (high-impedance) and most internal logic is powered down to reduce quiescent current consumption.

This is useful in battery-powered applications: when the motors are not needed, asserting STBY LOW can reduce IC quiescent current from the active-mode value (~1mA) to the standby value. On this board, STBY (pin 19) is connected via J1 and must be driven HIGH by the controlling microcontroller to enable motor operation.

Connector Interface (J1 and J2)

The board uses two standard 2.54mm pitch 8-pin headers (same Digi-Key part 732-5321-ND) for complete signal and power interfacing. The connectors are arranged with J1 carrying control and supply inputs, and J2 carrying power and motor outputs.

Usage Pattern

A host microcontroller connects to J1 with 5 GPIO pins (AIN1, AIN2, BIN1, BIN2 for direction, plus STBY for enable/standby) and 2 PWM outputs (PWMA, PWMB for speed). Two DC motors connect to J2 — polarity determines initial direction convention. VBATT+ and VCC enter through J1, with VBATT+ and motor outputs accessible on J2.

LED Power Indicator (R1 + D1)

A simple power-on indicator consists of a green LED (D1, LTST-C171KGKT) and a 5.1kΩ current-limiting resistor (R1), connected between VCC and GND. When VCC is present, the LED illuminates, confirming the logic supply is active.

Current Limiting Resistor Calculation

The forward voltage of the green LTST-C171KGKT LED is approximately 2.2V at 5mA. For 3.3V VCC, the current through R1 is:

At 0.2–0.5mA, the LED will be visibly lit but dimly — which is appropriate for a power indicator that should not distract or consume significant current. The LTST-C171KGKT (Lite-On 0805 clear lens green) is rated for 20mA maximum, so the 0.5mA operating current provides a 40× safety margin against LED destruction.

Thermal Design and Power Dissipation

Motor driver ICs dissipate power as heat through three mechanisms: transistor on-resistance during conduction, switching losses during transitions, and quiescent supply current. At 100kHz and 1.2A continuous, conduction losses dominate.

Conduction Loss Calculation

Each H-bridge in CW or CCW mode has two transistors conducting simultaneously — one high-side and one low-side. The TB6612FNG's internal on-resistance per switch is approximately 0.5Ω typical (1.0Ω maximum). The total series resistance seen by the motor current is the sum of upper and lower switches:

Package Thermal Limits

The TB6612FNG in SSOP-24 has a thermal resistance θ_JA (junction to ambient) of approximately 75°C/W in free air. The schematic's power dissipation table (visible on the schematic sheet) shows:

PCB Copper as Heat Spreader

The motor output traces on the Hercules PCB are rated for 2.4A continuous, meaning they are wide enough (calculated from IPC-2221 trace width tables) to carry this current without exceeding 10°C above ambient rise. This wider copper also acts as a thermal spreader, conducting heat away from the SSOP-24 package pads into the surrounding copper area.

PCB Layout Strategy

The Hercules board is a 2-layer PCB. Unlike the 4-layer boards used for mixed-signal designs, a 2-layer stackup is sufficient here because: (1) component count is minimal, (2) there is no sensitive analog signal path requiring shielding, and (3) high-frequency DDR-class routing constraints don't apply. The design challenge is maximizing copper fill for thermal performance and fitting adequate trace widths in compact board area.

Layer Assignment

Trace Width for Motor Currents

Motor output traces (MOTOR_A+, MOTOR_A−, MOTOR_B+, MOTOR_B−) carry continuous currents up to 2.4A per the PCB rating. IPC-2221 specifies trace width based on maximum temperature rise above ambient. For 2.4A in external copper (1oz = 35µm):

Component Placement Strategy

The Altium pick-and-place file reveals the board coordinates: U1 (TB6612FNG) is centrally placed at (6293, 1425). Decoupling capacitors C1–C4 are all within 0.5–1.5mm of U1's VCC and VBATT+ pins, as required by the app note. J1 and J2 headers are on opposite sides of U1, creating a logical signal-flow topology: inputs arrive on the left (J1), outputs depart to the right (J2).

Drill Statistics

Altium Designer Workflow

The Hercules board was designed in Altium Designer, which uses a tightly integrated schematic-to-PCB flow. This differs from KiCad (used for the STM32 and Mixed-Signal boards) in several key ways relevant to the design process.

Altium vs KiCad: Key Differences

Key Learnings