LM2672M-3.3/NOPB

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1. Description

The LM2672M-3.3/NOPB series of regulators are monolithic integrated DC-DC converter built with a LMDMOS process. These regulators provide all the active functions for a step-down (buck) switching regulator, capable of driving a 1-A load current with excellent line and load regulation. These devices are available in fixed output voltages of 3.3 V, 5 V, 12 V, and an adjustable output version. Requiring a minimum number of external components, these regulators are simple to use and include patented internal frequency compensation, fixed frequency oscillator, external shutdown, softstart, and frequency synchronization. The LM2672 series operates at a switching frequency of 260 kHz, thus allowing smaller sized filter components than what is required with lower frequency switching regulators. Because of its very high efficiency (>90%), the copper traces on the printed-circuit board are the only heat sinking required. A family of standard inductors for use with the LM2672 are available from several different manufacturers. This feature greatly simplifies the design of switch-mode power supplies using these advanced ICs. Also included in the datasheet are selector guides for diodes and capacitors designed to work in switch-mode power supplies. Other features include ±1.5%-tolerance on output voltage within specified input voltages and output load conditions, and ±10% on the oscillator frequency. External shutdown is included, featuring typically 50 μA standby current. The output switch includes current limiting, as well as thermal shutdown for full protection under fault conditions.

2. Features

    1. Efficiency up to 96%

    2. Available in 8-Pin SOIC and PDIP Packages

    3. Requires only 5 External Components

    4. 3.3-V, 5-V, 12-V, and Adjustable Output Versions

    5. Adjustable Version Output Voltage Range: 1.21 V to 37 V

    6. ±1.5% Maximum Output Voltage Tolerance Over Line and Load Conditions

    7. Specified 1-A Output Load Current

    8. Wide Input Voltage Range: 8 V to 40 V

    9. 260-kHz Fixed Frequency Internal Oscillator

  10. TTL Shutdown Capability, Low Power Standby Mode

  11. Soft-Start and Frequency Synchronization

  12. Thermal Shutdown and Current Limit Protection

3. Applications

    1. Simple High Efficiency (>90%) Step-Down (Buck) Regulator

    2. Efficient Preregulator for Linear Regulators

4. 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 or time OFF) of the power switch is proportional to the ratio of the power supply output voltage to the input voltage. The voltage on the VSW pin cycles between Vin (switch ON) and below ground by the voltage drop of the external Schottky diode (switch OFF).

5. C Boost

A capacitor must be connected from the CB pin to the VSW pin. This capacitor boosts the gate drive to the internal MOSFET above Vin to fully turn it ON. This minimizes conduction losses in the power switch to maintain high efficiency. The recommended value for C Boost is 0.01 μF.

6. SYNC

This input allows control of the switching clock frequency. If left open-circuited the regulator is switched at the internal oscillator frequency, typically 260 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 LM2672 internal oscillator frequency, which could be as high as 275 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 that the external TTL or CMOS compatible clock (between 0 V and a level greater than 3 V) be AC-coupled to the SYNC pin through a 100-pF capacitor and a 1-kΩ resistor to ground

7. Device Functional Modes

    1. 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 1.4 V completely turns OFF the regulator. The current drain from the input supply when OFF is only 50 μA. The ON/OFF input 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 this pin must be left open.

    2. Shutdown Mode

        The ON/OFF pin provides electrical ON and OFF control for the LM2671. When the voltage of this pin is lower than 1.4 V, the dev ice is shutdown mode. Typical standby current in this mode is 50 μA.

    3. Active mode

        When the voltage on the ON/OFF pin is above 1.4 V, the device starts switching and the output voltage rises until it reaches the normal regulation voltage.

8. Application Information

The LM2672M-3.3/NOPB is a step-down DC-DC regulator. It is typically used to convert a higher DC voltage to a lower DC voltage with a maximum output current of 1 A. The following design procedure can be used to select components for the LM2672. When the output voltage is greater than about 6 V, the duty cycle at the minimum input voltage is greater than about 50%, and the designer must choose the output filter components carefully. When an application designed for these specific operating conditions encounters a current limit fault condition, Large hysteresis may be observed in current limit. This affects the output voltage of device until the load current decreases enough for the current-limiting protection circuit to reset itself. If the output capacitance is large enough, it is possible that as the output tries to recover, the output Capacitor charging current is large enough to repeatedly retrigger the current-limiting circuit before output Completely resolved. This is exacerbated by higher output voltage settings because The output capacitance varies with the square of the output voltage (½ CV2), thus requiring increased charging current. A simple test to determine if this may be the case for a suspect application is to apply a short across the output of the converter and then remove the shorted output condition. With the selection of external components in the appropriate application, the output recovers smoothly. Practical value of having external components It has been found experimentally that COUT = 47 µF, L = 22 µH that works well under these specific operating conditions. Note that even with these components, for the device's ICLIM current limit, the one that minimizes the potential for large current limit hysteresis at maximum load current is ICLIM/2. For example, if the input is 24 V and the output voltage is set to 18 V, then for a desired maximum current of 1.5 A, the switch selected for the current limit must be confirmed to be at least 3 A. In addition to current limiting, the LM267X uses frequency foldback during extreme overcurrent or short circuit conditions. If the cycle-by-cycle inductor current is above the current limit threshold The frequency is automatically reduced to protect the IC. Frequencies below 100 KHz are typical for extreme short circuit conditions.