LM3208TL/NOPBPB National Semiconductor, LM3208TL/NOPBPB Datasheet - Page 13

• Manufacturer Part Number: LM3208TL/NOPBPB
• Manufacturer: National Semiconductor

Specifications of LM3208TL/NOPBPB

• Lead Free Status / Rohs Status: Supplier Unconfirmed

Operation Description
Circuit Operation
Referring to Figure 1 and Figure 2, the LM3208 operates as
follows. During the first part of each switching cycle, the
control block in the LM3208 turns on the internal PFET
(P-channel MOSFET) switch. This allows current to flow
from the input through the inductor to the output filter capaci-
tor and load. The inductor limits the current to a ramp with a
slope of around (V
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 MOS-
FET) synchronous rectifier on. In response, the inductor’s
magnetic field collapses, generating 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
output filter capacitor stores charge when the inductor cur-
rent 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
rectangular wave formed by the switch and synchronous
rectifier at SW to a low-pass filter formed by the inductor and
output filter capacitor. The output voltage is equal to the
average voltage at the SW pin.
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 con-
trol the peak inductor current. This is done by comparing the
signal from the current-sense amplifier with a slope compen-
sated 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
IN
- V
OUT
) / L, by storing energy in a
(Continued)
FIGURE 3. Typical Operating System Circuit
OUT
/ L. The
13
to ramp higher before the comparator turns off the PFET.
This increases the average current sent to the output and
adjusts for the increase in the load.
Before appearing at the PWM comparator, a slope compen-
sation ramp from the oscillator is subtracted from the error
signal for stability of the current feedback loop. The minimum
on time of PFET is 55ns (typ.)
Shutdown Mode
Setting the EN digital pin low (
shutdown mode (0.01µA typ.). During shutdown, the PFET
switch, NFET synchronous rectifier, reference voltage
source, control and bias circuitry of the LM3208 are turned
off. Setting EN high (
EN should be set low to turn off the LM3208 during power-up
and under voltage conditions when the power supply is less
than the 2.7V minimum operating voltage. The LM3208 is
designed 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 LM3208 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, elimi-
nating the need for an external diode.
R
The LM3208 has a unique R
improve efficiency in the low output current region up to
100mA. When the V
DSON(P)
Management
CON
>
1.2V) enables normal operation.
voltage is less than 0.40V (typ.), the
DSON(P)
<
0.5V) places the LM3208 in
20166336
management function to
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