LM2623

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SNVS188G – MAY 2004 – REVISED DECEMBER 2005

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.

SHUT DOWN

The LM2623 features a shutdown mode that reduces the quiescent current to less than a guaranteed 2.5uA 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 (pinh- 2). When the logic input to

this pin is pulled below .15Vdd, the device goes into shutdown mode. The logic input to this pin should be above

.7Vdd for the device to work in normal stepup mode.

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

Design Procedure

NON-LINEAR EFFECT

The LM2623 is very similar to the LM2621. The LM2623 is based on the LM2621, except for the fact that the

LM2623 takes advantage of a non-linear effect that allows for the duty cycle to be programmable. The C3

capacitor is used to dump charge on the FREQ pin in order to manipulate the duty cycle of the internal oscillator.

The part is being tricked to behave in a certain manner, in the effort to make this Pulse Frequency Modulated

(PFM) boost switching regulator behave as a Pulse Width Modulated (PWM) boost switching regulator.

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