EN

LM22676

LM22676-Q1

SNVS587K – SEPTEMBER 2008 – REVISED APRIL 2013

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Although an internal pull-up 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, since the internal zener diode, at the EN pin, will conduct for

voltages above about 6V. 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 42V. Smaller values of resistor can be used at lower

input voltages.

The LM22676 also incorporates an input under voltage 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

4.3V (typ.) while the falling threshold is 3.9V (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 4.5V. Figure 13 shows the connections to implement this

method of UVLO. The following equations can be used to determine the correct resistor values:

(1)

where

•

(2)

Due to the 6 µA pull-up, 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 13. External UVLO Connections

Duty-Cycle Limits

Ideally the regulator would control the duty cycle over the full range of zero to one. However due to inherent

delays in the circuitry, there are limits on both the maximum and minimum duty cycles that can be reliably

controlled. This in turn places limits on the maximum and minimum input and output voltages that can be

converted by the LM22676. A minimum on-time is imposed by the regulator in order to correctly measure the

switch current during a current limit event. A minimum off-time is imposed in order the re-charge the bootstrap

capacitor. The following equation can be used to determine the approximate maximum input voltage for a given

output voltage:

where

•

table.

(3)

The worst case occurs at the lowest output voltage. If the input voltage, found in the above equation, is

exceeded, the regulator will skip cycles, effectively lowering the switching frequency. The consequences of this

are higher output voltage ripple and a degradation of the output voltage accuracy.

8

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