2.8 LDO Output Voltage

The MCP1781 LDO is available with either two fixed output voltages (3.3V and 5V) or an adjustable output voltage. The output voltage range for the adjustable version is from 1.3V to 5.0V .

The adjustable version of the MCP1781 uses the ADJ pin (pin 5) to get the output voltage feedback for output voltage regulation. This allows the user to set the output voltage of the device with two external resistors. The reference voltage for ADJ is 1.21V typical value.

Figure 2-3 shows the adjustable version of the MCP1781. Resistors R_TOP and R_BOT form the resistor divider network necessary to set the output voltage. With this configuration, the equation for setting VOUT is:

$V_{O U T} = V_{R E F} \times (1 + \frac{R_{T O P}}{R_{B O T}})$

Where:

VOUT = LDO's Output Voltage
V_REF = ADJ Pin Voltage (typically 1.21V)

The allowable resistance value range for resistor R_BOT is from 10 kΩ to 200 kΩ.

Figure 2-4. MCP1781 Adjustable Output Version Typical Application