EN

LM22677, LM22677-Q1

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SNVS582O – SEPTEMBER 2008 – REVISED AUGUST 2015

Feature Description (continued)

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

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 LM22677 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 (typical) while the falling threshold is 3.9 V (typical). 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 11 shows the connections to

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

values.

(1)

where

•

•

(2)

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 11. 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.

7.3.3 Switching Frequency Adjustment and Synchronization

The LM22677 will operate in three different modes, depending on the condition of the RT/SYNC pin. With the

RT/SYNC pin floating, the regulator will switch at the internally set frequency of 500 kHz (typ). With a resistor in

the range of 25 k

Ω to 200 kΩ, connected from RT/SYNC to ground, the internal switching frequency can be

adjusted from 1 MHz to 200 kHz. Figure 12 shows the typical curve for switching frequency versus the external

resistance connected to the RT/SYNC pin. The accuracy of the switching frequency, in this mode, is slightly

worse than that of the internal oscillator; about ±25% is to be expected. Finally, an external clock can be applied

to the RT/SYNC pin to allow the regulator to synchronize to a system clock or another LM22677. The mode is

connected to the RT/SYNC pin and, after approximately 100 µs, the voltage on the pin is checked against a

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Product Folder Links: LM22677 LM22677-Q1