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LM2623AMMX Fiches technique(PDF) 8 Page - Texas Instruments

No de pièce LM2623AMMX
Description  LM2623 General Purpose, Gated Oscillator Based, DC/DC Boost Converter
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Site Internet  http://www.ti.com
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LM2623AMMX Fiches technique(HTML) 8 Page - Texas Instruments

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LM2623
SNVS188G – MAY 2004 – REVISED DECEMBER 2005
www.ti.com
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
protection circuit disables the MOSFET power switch when the junction temperature (TJ) exceeds about 160°C.
The switch is re-enabled when TJ drops below approximately 135°C.
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 output voltage of the step-up regulator can be set by connecting a feedback resistive divider made of RF1
and RF2. The resistor values are selected as follows:
RF1 = RF2 * [(VOUT/ 1.24) −1]
A value of 50k to 100k is suggested for RF2. Then, RF1 can be selected using the above equation.
VDD SUPPLY
The Vdd supply must be between 3 to 5 volts for the LM2623. This voltage can be bootstrapped from a much
lower input voltage by simply connecting the VDD pin to VOUT. In the event that the VDD supply voltage is not a
low ripple voltage source (less than 200 millivolts), it may be advisable to use an RC filter to clean it up.
Excessive ripple on VDD may reduce the efficiency.
SETTING THE SWITCHING FREQUENCY
The switching frequency of the oscillator is selected by choosing an external resistor (R3) connected between VIN
and the FREQ pin. See the graph titled "Frequency vs VIN” in the Typical Performance Characteristics section of
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.
8
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