Compliance voltage is the output voltage range over which a constant current power supply or current source can maintain its specified current accurately. It represents the maximum voltage the source can provide while still regulating the desired current.

If the required voltage exceeds the compliance voltage range, the current source can no longer maintain constant current operation.

How Compliance Voltage Works

A constant current source automatically adjusts its output voltage to maintain a fixed current through the load.

For example:

• A 10 mA current through a 1 kΩ load requires 10V
• If the load resistance increases, the source must increase voltage to maintain the same current

However, the source can only increase voltage up to its compliance limit.

Once this limit is reached:

• Current regulation is lost
• Output current may fall below the target value

Why Compliance Voltage Matters

Compliance voltage is important because it determines:

• The maximum load resistance the current source can drive
• The number of devices that can be connected in series
• Whether current regulation can be maintained

It is especially important in:

• LED drivers
• Current source circuits
• Sensor systems
• Semiconductor testing equipment

Compliance Voltage and Internal Resistance

The compliance voltage is typically lower than the supply voltage because some voltage is lost internally within the regulator or power supply.

The available compliance voltage depends on:

• Input supply voltage
• Internal voltage drops
• Regulator design
• Load current

Example of Compliance Voltage

Consider a power supply with:

• Maximum current: 1.5 mA
• Maximum voltage: 20V

If connected to:

• A 10 kΩ load, the supply reaches compliance at 15V and delivers 1.5 mA
• A 20 kΩ load, the required voltage exceeds the compliance range, so the current cannot reach 1.5 mA

This demonstrates how load resistance affects compliance operation.

Compliance Voltage in LED Circuits

Compliance voltage is very important in constant current LED drivers.

Each LED in a series string has a forward voltage drop, typically:

• Around 1.5V to 3V depending on LED type and colour

The total voltage required is the sum of all LED forward voltages.

For example:

• Five LEDs with 2V forward voltage each require 10V total
• The driver must have at least 10V compliance voltage to maintain the correct current

If the compliance voltage is too low:

• Current regulation fails
• LEDs may dim or operate incorrectly

What Happens Outside the Compliance Range

When the required voltage exceeds the compliance voltage:

• The current source loses regulation
• Output current decreases
• Circuit performance becomes unstable

In some cases, excessive voltage or current stress may damage components.

For reliable operation, designers usually allow additional compliance voltage margin to handle load variations safely.

Compliance Voltage vs Supply Voltage

Compliance voltage is not the same as supply voltage.

• Supply voltage is the input power available
• Compliance voltage is the usable regulated output range of the current source

The compliance voltage is always limited by the power supply design.

People Also Ask

What does compliance voltage mean?

Compliance voltage is the maximum voltage a constant current source can provide while still maintaining its specified output current.

Why is compliance voltage important?

It determines whether a current source can maintain regulation across different load conditions and resistances.

What happens if compliance voltage is exceeded?

If the required voltage exceeds the compliance limit, the current source can no longer maintain constant current operation.

How does compliance voltage affect LEDs?

The compliance voltage must be high enough to overcome the total forward voltage of all LEDs connected in series.

What is the difference between compliance voltage and supply voltage?

Supply voltage is the input voltage to the circuit, while compliance voltage is the regulated output voltage range available from a constant current source.