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LM2623ALD Datasheet(PDF) 8 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
No. de pieza LM2623ALD
Descripción Electrónicos  General Purpose, Gated Oscillator Based, DC/DC Boost Converter
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Fabricante Electrónico  NSC [National Semiconductor (TI)]
Página de inicio  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2623ALD Datasheet(HTML) 8 Page - National Semiconductor (TI)

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Detailed Description (Continued)
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 2). The system will also
undershoot somewhat when it starts switching again be-
cause 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 switch-
ing 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, approxi-
mately 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 (T
J) exceeds about 160˚C. The switch is re-
enabled when T
J drops below approximately 135˚C.
20038815
FIGURE 2. Typical Step-Up Regulator Waveforms
www.national.com
8


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