參數(shù)資料
型號(hào)： LM3218SE
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING REGULATOR, DSO8
封裝： LTCC-8
文件頁(yè)數(shù)： 4/16頁(yè)
文件大?。?/td> 4709K
代理商： LM3218SE
30050436
FIGURE 3. Typical Operating System Circuit
Circuit Operation
Referring to Figure 1 and Figure 2, the LM3218 operates as
follows: During the first part of each switching cycle, the con-
trol block in the LM3218 turns on the internal PFET (P-
channel MOSFET) switch. This allows current to flow from the
input through the inductor to the output filter capacitor and
load. The inductor limits the current to a ramp with a slope of
around (V
IN - VOUT) / L, by storing energy in a magnetic field.
During the second part of each cycle, the controller turns the
PFET switch off, blocking current flow from the input, and then
turns the NFET (N-channel MOSFET) synchronous rectifier
on. In response, the inductor’s magnetic field collapses, gen-
erating a voltage that forces current from ground through the
synchronous rectifier to the output filter capacitor and load.
As the stored energy is transferred back into the circuit and
depleted, the inductor current ramps down with a slope
around V
OUT / L. The output filter capacitor stores charge
when the inductor current is high, and releases it when low,
smoothing the voltage across the load.
The output voltage is regulated by modulating the PFET
switch on time to control the average current sent to the load.
The effect is identical to sending a duty-cycle modulated rect-
angular wave formed by the power MOSFET switch and
synchronous rectifier to a low-pass filter formed by the induc-
tor and output filter capacitor. The output voltage is equal to
the average voltage at the terminal of the power MOSFET
inverter.
While in operation, the output voltage is regulated by switch-
ing at a constant frequency and then modulating the energy
per cycle to control power to the load. Energy per cycle is set
by modulating the PFET switch on-time pulse width to control
the peak inductor current. This is done by comparing the sig-
nal from the current-sense amplifier with a slope compensat-
ed error signal from the voltage-feedback error amplifier. At
the beginning of each cycle, the clock turns on the PFET
switch, causing the inductor current to ramp up. When the
current sense signal ramps past the error amplifier signal, the
PWM comparator turns off the PFET switch and turns on the
NFET synchronous rectifier, ending the first part of the cycle.
If an increase in load pulls the output down, the error amplifier
output increases, which allows the inductor current to ramp
higher before the comparator turns off the PFET. This in-
creases the average current sent to the output and adjusts for
the increase in the load. Before appearing at the PWM com-
parator, a slope compensation ramp from the oscillator is
subtracted from the error signal for stability of the current
feedback loop. The minimum on time of PFET is 55 ns (typ.)
Shutdown Mode
Setting the EN digital pin low (<0.5V) places the LM3218 in
shutdown mode (0.01 A typ.). During shutdown, the PFET
switch, NFET synchronous rectifier, reference voltage
source, control and bias circuitry of the LM3218 are turned
off. Setting EN high (>1.2V) enables normal operation.
EN should be set low to turn off the LM3218 during power-up
and under-voltage conditions when the power supply is less
than the 2.7V minimum operating voltage. The LM3218 is de-
signed for compact portable applications, such as mobile
phones. In such applications, the system controller deter-
mines power supply sequencing and requirements for small
package size outweigh the additional size required for inclu-
sion of UVLO (Under Voltage Lock-Out) circuitry.
Internal Synchronous Rectification
While in PWM mode, the LM3218 uses an internal NFET as
a synchronous rectifier to reduce rectifier forward voltage
drop and associated power loss. Synchronous rectification
provides a significant improvement in efficiency whenever the
output voltage is relatively low compared to the voltage drop
across an ordinary rectifier diode.
The internal NFET synchronous rectifier is turned on during
the inductor current down slope in the second part of each
cycle. The synchronous rectifier is turned off prior to the next
cycle. The NFET is designed to conduct through its intrinsic
body diode during transient intervals before it turns on, elim-
inating the need for an external diode.
R
DSON(P) Management
The LM3218 has a unique R
DSON(P) management function to
improve efficiency in the low output current region up to 100
mA. When the V
CON voltage is less than 0.40V (typ.), the de-
vice uses only a small part of the PFET to minimize drive loss
of the PFET. When V
CON is greater than 0.42V (typ.), the en-
tire PFET is used to minimize R
DSON(P) loss. This threshold
has about 20 mV (typ.) of hysteresis.
www.national.com
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LM3218
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