LM2621

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SNVS033C – MAY 2004 – REVISED MARCH 2005

Because of this type of control scheme, the quiescent current is inherently very low. At light loads the gated

oscillator control scheme offers a much higher efficiency compared to the conventional PWM control scheme.

Figure 14. Typical Step-Up Regulator Waveforms

LOW VOLTAGE START-UP

The LM2621 can start-up from input voltages as low as 1.1V. On start-up, the control circuitry switches the N-

channel MOSFET continuously at 70% duty cycle until the output voltage reaches 2.5V. After this output voltage

is reached, the normal step-up regulator feedback and gated oscillator control scheme take over. Once the

device is in regulation it can operate down to a 0.65V input, since the internal power for the IC can be boot-

SHUTDOWN

The LM2621 features a shutdown mode that reduces the quiescent current to less than a ensured 2.5µA over

temperature. This extends the life of the battery in battery powered applications. During shutdown, all feedback

and control circuitry is turned off. The regulator's output voltage drops to one diode drop below the input voltage.

Entry into the shutdown mode is controlled by the active-low logic input pin EN (Pin 2). When the logic input to

OUTPUT VOLTAGE RIPPLE FREQUENCY

A major component of the output voltage ripple is due to the hysteresis used in the gated oscillator control

scheme. The frequency of this voltage ripple is proportional to the load current. The frequency of this ripple does

not necessitate the use of larger inductors and capacitors however, since the size of these components is

determined by the switching frequency of the oscillator which can be set upto 2MHz using an external resistor.

INTERNAL CURRENT LIMIT AND THERMAL PROTECTION

An internal cycle-by-cycle current limit serves as a protection feature. This is set high enough (2.85A typical,

approximately 4A maximum) so as not to come into effect during normal operating conditions. An internal thermal

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