EN

LM22678, LM22678-Q1

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SNVS585L – SEPTEMBER 2008 – REVISED NOVEMBER 2014

7.3 Feature Description

7.3.1 Precision Enable and UVLO

The precision enable input (EN) is used to control the regulator. The precision feature allows simple sequencing

of multiple power supplies with a resistor divider from another supply. Connecting this pin to ground or to a

voltage less than 1.6 V (typ) will turn off the regulator. The current drain from the input supply, in this state, is 25

µA (typ) at an input voltage of 12 V. The EN input has an internal pullup of about 6 µA. Therefore, this pin can be

left floating or pulled to a voltage greater than 2.2 V (typ) to turn the regulator on. The hysteresis on this input is

about 0.6 V (typ) above the 1.6 V (typ) threshold. When driving the enable input, the voltage must never exceed

the 6 V absolute maximum specification for this pin.

Although an internal pullup is provided on the EN pin, it is good practice to pull the input high when this feature is

not used, especially in noisy environments. This can most easily be done by connecting a resistor between VIN

and the EN pin. The resistor is required because the internal zener diode at the EN pin will conduct for voltages

above about 6 V. The current in this zener must be limited to less than 100 µA. A resistor of 470 k

Ω will limit the

current to a safe value for input voltages as high 42 V. Smaller values of resistor can be used at lower input

voltages.

The LM22678 device also incorporates an input undervoltage lock-out (UVLO) feature. This prevents the

regulator from turning on when the input voltage is not great enough to properly bias the internal circuitry. The

rising threshold is 4.3 V (typ) while the falling threshold is 3.9 V (typ). In some cases, these thresholds may be

too low to provide good system performance. The solution is to use the EN input as an external UVLO to disable

the part when the input voltage falls below a lower boundary. This is often used to prevent excessive battery

discharge or early turn-on during start up. This method is also recommended to prevent abnormal device

operation in applications where the input voltage falls below the minimum of 4.5 V. Figure 10 shows the

connections to implement this method of UVLO. Equation 1 and Equation 2 can be used to determine the correct

resistor values.

(1)

(2)

Where:

Due to the 6 µA pullup, the current in the divider should be much larger than this. A value of 20 k

good first choice. Also, a zener diode may be needed between the EN pin and ground, in order to comply with

the absolute maximum ratings on this pin.

Figure 10. External UVLO Connections

7.3.2 Soft-Start

The soft-start feature allows the regulator to gradually reach steady-state operation, thus reducing start-up

stresses. The internal soft-start feature brings the output voltage up in about 500 µs. This time is fixed and can

not be changed. Soft-start is reset any time the part is shut down or a thermal overload event occurs.

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Product Folder Links: LM22678 LM22678-Q1