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LM2678

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SNVS029J – MARCH 2000 – REVISED JULY 2016

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Copyright © 2000–2016, Texas Instruments Incorporated

Feature Description (continued)

7.3.2 Input

The input voltage for the power supply is connected to pin 2. In addition to providing energy to the load the input

voltage also provides bias for the internal circuitry of the LM2678. For ensured performance the input voltage

must be in the range of 8 V to 40 V. For best performance of the power supply the input pin must always be

bypassed with an input capacitor located close to pin 2.

7.3.3 C Boost

A capacitor must be connected from pin 3 to the switch output, pin 1. This capacitor boosts the gate drive to the

high efficiency. The recommended value for C Boost is 0.01 μF.

7.3.4 Ground

This is the ground reference connection for all components in the power supply. In fast-switching, high-current

applications such as those implemented with the LM2678, TI recommends that a broad ground plane be used to

minimize signal coupling throughout the circuit.

7.3.5 Feedback

This is the input to a two-stage high gain amplifier, which drives the PWM controller. It is necessary to connect

pin 6 to the actual output of the power supply to set the DC output voltage. For the fixed output devices (3.3-V, 5-

V and 12-V outputs), a direct wire connection to the output is all that is required as internal gain setting resistors

are provided inside the LM2678. For the adjustable output version two external resistors are required to set the

DC output voltage. For stable operation of the power supply it is important to prevent coupling of any inductor

flux to the feedback input.

7.3.6 ON/OFF

This input provides an electrical ON/OFF control of the power supply. Connecting this pin to ground or to any

voltage less than 0.8 V is completely turn OFF the regulator. The current drain from the input supply when OFF

is only 50 μA. Pin 7 has an internal pullup current source of approximately 20 μA and a protection clamp Zener

diode of 7 V to ground. When electrically driving the ON/OFF pin the high voltage level for the ON condition

should not exceed the 6 V absolute maximum limit. When ON/OFF control is not required pin 7 should be left

open circuited.

7.4 Device Functional Modes

7.4.1 Shutdown Mode

The ON/OFF pin provides electrical ON and OFF control for the LM2678. When the voltage of this pin is lower

than 1.4 V, the device enters shutdown mode. The typical standby current in this mode is 45 μA.

7.4.2 Active Mode

When the voltage of the ON/OFF pin is higher than 1.4 V, the device starts switching and the output voltage rises

until it reaches a normal regulation voltage.