7.3.1 Switch Output

This is the output of a power MOSFET switch connected directly to the input voltage. The switch provides energy to an inductor, an output capacitor, and the load circuitry under control of an internal pulse-width-modulator (PWM). The PWM controller is internally clocked by a fixed 260-kHz oscillator. In a standard step-down application the duty cycle (Time ON/Time OFF) of the power switch is proportional to the ratio of the power supply output voltage to the input voltage. The voltage on pin 1 switches between V_IN (switch ON) and below ground by the voltage drop of the external Schottky diode (switch OFF).

7.3.2 CBoost

A capacitor must be connected from pin 3 to the switch output, pin 1. This capacitor boosts the gate driver to the internal MOSFET above V_IN to fully turn it ON. This minimizes conduction losses in the power switch to maintain high efficiency. The recommended value for CBoost is 0.01 μF.

7.3.3 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 LM2677, TI recommends using a broad ground plane to minimize signal coupling throughout the circuit.

7.3.4 Sync

This input allows control of the switching clock frequency. If left open-circuited the regulator is switched at the internal oscillator frequency, from 225 kHz to 280 kHz. An external clock can be used to force the switching frequency and thereby control the output ripple frequency of the regulator. This capability provides for consistent filtering of the output ripple from system to system as well as precise frequency spectrum positioning of the ripple frequency, which is often desired in communications and radio applications. This external frequency must be greater than the LM2677 internal oscillator frequency, which could be as high as 280 kHz, to prevent an erroneous reset of the internal ramp oscillator and PWM control of the power switch. The ramp oscillator is reset on the positive going edge of the sync input signal. TI recommends ac-coupling the external TTL or CMOS compatible clock (between 0 V and a level greater than 3 V) to the sync input through a 100-pF capacitor and a 1-kΩ resistor to ground at pin 5 as shown in Figure 16.

When the SYNC function is used, current limit frequency foldback is not active. Therefore, the device may not be fully protected against extreme output short-circuit conditions (see Additional Application Information).

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 LM2677. 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 completely turns 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 must not exceed the 6-V absolute maximum limit. When ON/OFF control is not required pin 7 must be left open circuited.

7.3.7 DAP (VSON Package)

The die attach pad (DAP) must be connected to PCB ground plane. For CAD and assembly guidelines, see application note, AN-1187 Leadless Leadframe Package (LLP).

7.4.1 Shutdown Mode

The ON/OFF pin provides electrical ON and OFF control for the LM2677. When the voltage of this pin is lower than 1.4 V, the device is shutdown mode. The typical standby current in this mode is 20 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.