LM2621

SNVS033C – MAY 2004 – REVISED MARCH 2005

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Figure 2. Functional Diagram

GATED OSCILLATOR CONTROL SCHEME

A unique gated oscillator control scheme enables the LM2621 to have an ultra-low quiescent current and

provides a high efficiency over a wide load range. The switching frequency of the internal oscillator is

programmable using an external resistor and can be set between 300 kHz and 2 MHz.

This control scheme uses a hysteresis window to regulate the output voltage. When the output voltage is below

the upper threshold of the window, the LM2621 switches continuously with a fixed duty cycle of 70% at the

switching frequency selected by the user. During the first part of each switching cycle, the internal N-channel

MOSFET switch is turned on. This causes the current to ramp up in the inductor and store energy. During the

second part of each switching cycle, the MOSFET is turned off. The voltage across the inductor reverses and

forces current through the diode to the output filter capacitor and the load. Thus when the LM2621 switches

continuously, the output voltage starts to ramp up. When the output voltage hits the upper threshold of the

window, the LM2621 stops switching completely. This causes the output voltage to droop because the energy

stored in the output capacitor is depleted by the load. When the output voltage hits the lower threshold of the

hysteresis window, the LM2621 starts switching continuously again causing the output voltage to ramp up

towards the upper threshold. Figure 3 shows the switch voltage and output voltage waveforms.

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 3. Typical Step-Up Regulator Waveforms

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