LP3205F Datasheet, PDF (7/10 Pages) Lowpower Semiconductor inc – 1.5MHz Fixed-Frequency PWM Operation

Preliminary Datasheet LP3205F
Applications Information
The basic LP3205F application circuit is shown in Typical
Application Circuit. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and operating
frequency followed by CIN and COUT.
Inductor selection
The output inductor is selected to limit the ripple
current to some predetermined value. typically 20%~40%
of the full load current at the maximum input voltage.
Large value inductors lower ripple currents. Higher Vin or
V_OUT also increases the ripple current as shown in
equation. A reasonable starting point for setting ripple
current is △I_L=240mA(40% of 800mA).
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720mA rated
Inductor should be enough for most applications
(800mA+120mA). For better efficiency, choose a low
DC-resistance inductor.
C_IN and C_OUT Selection
The input capacitance, C_IN ,is needed to filter the
trapezoidal current at the source of the top MOSFET. To
prevent large ripple voltage, a low ESR input capacitor
Sized for the maximum RMS current should be used.
RMS current is given by:
series resistance(ESR) that is required to minimize
voltage ripple and load step transients, an well as the
amount or bulk capacitance that is necessary to ensure
that the control loop is stable. Loop stability can be
checked by vie sing the load transient response as
described in later section. the output ripple,△V_OUT, is
determined by:
Using ceramic input and output capacitors
Higher values, lower cost ceramic capacitors are now
becoming .Available in smaller case sizes ,their high
ripple current ,high voltage rating and low ESR make
them ideal for switching regulator applications. however
care must be taken when these capacitors are used at the
input and output. When a ceramic capacitor is use at
input and the power is supplied by a wall adapter through
long wires, a load step at the output can induce ringing at
the input ,V_IN, At worst, a sudden inrush of current
through the long wires can potentially cause a voltage
spike at V_IN large enough to damage the part.
Output voltage programming
The output voltage is set by a resistive divider according
to the
Following formula:
The external resistive divider is connected to the output,
allowing Remote voltage sensing as shown in figure3.
This formula has a maximum at V_IN=2V_OUT, where
I_RMS=I_OUT/2.this simple worst-case condition is
commonly.Used for design because even significant
deviations do not offer much relief. Note that ripple
current ratings from capacitor manufacturers are often
based on only 2000 hours of life which makes it advisable
to further derate the Capacitor, or choose a capacitor
rated at a higher temperature Than required. Several
capacitors may also be paralleled to meet size or height
requirements in the design.
Efficiency considerations
The efficiency of a switching regulator is equal to the output
Power divided by the input power times 100%.it is often useful
The selection of C_OUT is determined by the effective
to analyze individual losses to determine what is limiting the
efficiency and which change would produce the most
improvement efficiency can be expressed as :
LP3205F –00 Version. 1.0 Datasheet Feb.-2009
www.lowpowersemi.com
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