LM2623

SNVS188G – MAY 2004 – REVISED DECEMBER 2005

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SHUT DOWN

The LM2623 features a shutdown mode that reduces the quiescent current to less than a specified 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.

CHOOSING THE CORRECT C3 CAPACITOR

The C3 capacitor allows for the duty cycle of the internal oscillator to be programmable. Choosing the correct C3

capacitor to get the appropriate duty cycle for a particular application circuit is a trial and error process. The non-

linear effect that C3 produces is dependent on the input voltage and output voltage values. The correct C3

capacitor for particular input and output voltage values cannot be calculated. Choosing the correct C3

capacitance is best done by trial and error, in conjunction with the checking of the inductor peak current to make

sure your not too close to the current limit of the device. As the C3 capacitor value increases, so does the duty

cycle. And conversely as the C3 capacitor value decreases, the duty cycle decreases. An incorrect choice of the

C3 capacitor can result in the part prematurely tripping the current limit and/or double pulsing, which could lead

to the output voltage not being stable.

SETTING THE OUTPUT VOLTAGE

The Vdd supply must be between 3 to 5 volts for the LM2623. This voltage can be bootstrapped from a much

low ripple voltage source (less than 200 millivolts), it may be advisable to use an RC filter to clean it up.

SETTING THE SWITCHING FREQUENCY

the data sheet for choosing the R3 value to achieve the desired switching frequency. A high switching frequency

allows the use of very small surface mount inductors and capacitors and results in a very small solution size. A

switching frequency between 300kHz and 2MHz is recommended.

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