LM2623

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SNVS188G – MAY 2004 – REVISED DECEMBER 2005

Figure 10. Typical Step-Up Regulator Waveforms

PULSE FREQUENCY MODULATION (PFM)

Pulse Frequency Modulation is typically accomplished by switching continuously until the voltage limit is reached

and skipping cycles after that to just maintain it. This results in a somewhat hysteretic mode of operation. The

coil stores more energy each cycle as the current ramps up to high levels. When the voltage limit is reached, the

system usually overshoots to a higher voltage than required, due to the stored energy in the coil (see Figure 10).

The system will also undershoot somewhat when it starts switching again because it has depleted all the stored

energy in the coil and needs to store more energy to reach equilibrium with the load. Larger output capacitors

and smaller inductors reduce the ripple in these situations. The frequency being filtered, however, is not the basic

switching frequency. It is a lower frequency determined by the load, the input/output voltage and the circuit

parameters. This mode of operation is useful in situations where the load variation is significant. Power managed

computer systems, for instance, may vary from zero to full load while the system is on and this is usually the

preferred regulation mode for such systems.

CYCLE TO CYCLE PFM

When the load doesn't vary over a wide range (like zero to full load), ratio adaptive circuit techniques can be

used to achieve cycle to cycle PFM regulation and lower ripple (or smaller output capacitors). The key to success

here is matching the duty cycle of the circuit closely to what is required by the input to output voltage ratio. This

ratio then needs to be dynamically adjusted for input voltage changes (usually caused by batteries running

down). The chosen ratio should allow most of the energy in each switching cycle to be delivered to the load and

only a small amount to be stored. When the regulation limit is reached, the overshoot will be small and the

system will settle at an equilibrium point where it adjusts the off time in each switching cycle to meet the current

requirements of the load. The off time adjustment is done by exceeding the regulation limit during each switching

cycle and waiting until the voltage drops below the limit again to start the next switching cycle. The current in the

coil never goes to zero like it frequently does in the hysteretic operating mode of circuits with wide load variations

or duty cycles that aren't matched to the input/output voltage ratio. Optimizing the duty cycle for a given set of

input/output voltages conditions can be done by using the circuit values in the Application Notes.

LOW VOLTAGE START-UP

The LM2623 can start-up from voltages as low as 1.1 volts. On start-up, the control circuitry switches the N-

channel MOSFET continuously until the output reaches 3 volts. 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 below .8V input, since the internal power for the IC can be boot-strapped from the output using

the Vdd pin.

Copyright © 2004–2005, Texas Instruments Incorporated

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