LM22678, LM22678-Q1

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SNVS585L – SEPTEMBER 2008 – REVISED NOVEMBER 2014

Feature Description (continued)

In general, hand calculations or simulations can only aid in selecting good power stage components. Good

design practice dictates that load and line transient testing should be done to verify the stability of the application.

Also, Bode plot measurements should be made to determine stability margins. AN-1889 How to Measure the

Loop Transfer Function of Power Supplies (SNVA364) shows how to perform a loop transfer function

measurement with only an oscilloscope and function generator.

7.4 Device Functional Modes

7.4.1 Shutdown Mode

The EN pin provides electrical ON and OFF control for the LM22678 device. When VEN is below 1.6 V, the

device is in shutdown mode. 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. The LM22678 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).

7.4.2 Active Mode

The LM22678 device is in active mode when VEN is above the precision enable threshold and its input voltage is

above its UVLO level. The simplest way to enable the LM22678 is to connect the EN pin to VIN through a

resistor. A resistor of 470 k

Ω will limit the current to a safe value for input voltages as high 42 V.

7.4.3 Current Limit

The LM22678 device has current limiting to prevent the switch current from exceeding safe values during an

accidental overload on the output. This peak current limit is found in the Electrical Characteristics table under the

from Equation 5.

(5)

Where:

L is the value of the power inductor.

When the LM22678 device enters current limit, the output voltage will drop and the peak inductor current will be

load current will not remain constant, but will depend on the severity of the overload and the output voltage.

For very severe overloads "short-circuit", the regulator changes to a low frequency current foldback mode of

operation. The frequency foldback is about 1/5 of the nominal switching frequency. This will occur when the

current limit trips before the minimum on-time has elapsed. This mode of operation is used to prevent inductor

current "run-away", and is associated with very low output voltages when in overload. Equation 6 can be used to

determine what level of output voltage will cause the part to change to low frequency current foldback.

(6)

Where:

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Product Folder Links: LM22678 LM22678-Q1