LM3208TL/NOPBPB National Semiconductor, LM3208TL/NOPBPB Datasheet - Page 14

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

Specifications of LM3208TL/NOPBPB

• Lead Free Status / Rohs Status: Supplier Unconfirmed

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R
device uses only a small part of the PFET to minimize drive
loss of the PFET. When V
the entire PFET is used to minimize R
threshold has about 20mV (typ.) of hysteresis.
V
The output is disabled when V
is enabled when V
has about 25mV (typ.) of hysteresis.
Current Limiting
A current limit feature allows the LM3208 to protect itself and
external components during overload conditions. In PWM
mode, an 1100mA (typ.) cycle-by-cycle current limit is nor-
mally used when V
(typ.) is used when V
sive load pulls the output voltage down to approximately
0.375V, then the device switches to a timed current limit
mode when V
mode the internal PFET switch is turned off after the current
comparator trips and the beginning of the next cycle is
inhibited for 3.5us to force the instantaneous inductor current
to ramp down to a safe value. The synchronous rectifier is off
in timed current limit mode. Timed current limit prevents the
loss of current control seen in some products when the
output voltage is pulled low in serious overload conditions.
Dynamically Adjustable Output
Voltage
The LM3208 features dynamically adjustable output voltage
to eliminate the need for external feedback resistors. The
output can be set from 0.8V to 3.6V by changing the voltage
on the analog V
tions where peak power is needed only when the handset is
far away from the base station or when data is being trans-
mitted. In other instances, the transmitting power can be
reduced. Hence the supply voltage to the PA can be re-
duced, promoting longer battery life. See Setting the Output
Voltage in the Application Information section for further
details. The LM3208 moves into Pulse Skipping mode when
duty cycle is over 92% and the output voltage ripple in-
creases slightly.
Thermal Overload Protection
The LM3208 has a thermal overload protection function that
operates to protect itself from short-term misuse and over-
load conditions. When the junction temperature exceeds
around 150˚C, the device inhibits operation. Both the PFET
and the NFET are turned off in PWM mode. When the
temperature drops below 125˚C, normal operation resumes.
Prolonged operation in thermal overload conditions may
damage the device and is considered bad practice.
Application Information
SETTING THE OUTPUT VOLTAGE
The LM3208 features a pin-controlled variable output volt-
age to eliminate the need for external feedback resistors. It
can be programmed for an output voltage from 0.8V to 3.6V
by setting the voltage on the V
formula:
CON,ON
DSON(P)
CON
Management
CON
is above 0.42V (typ.). In timed current limit
CON
CON
pin. This feature is useful in PA applica-
CON
is above 150mV (typ.). The threshold
is above 0.42V (typ.), and an 800mA
is below 0.40V (typ.). If an exces-
CON
CON
is greater than 0.42V (typ.),
CON
is below 125mV (typ.). It
pin, as in the following
(Continued)
DSON(P)
loss. This
14
When V
will follow proportionally by 2.5 times of V
If V
tion may occur because of insufficient slope compensation. If
V
voltage may not be regulated due to the required on-time
being less than the minimum on-time (55ns). The output
voltage can go lower than 0.8V providing a limited V
is used. Refer to datasheet curve (V
Voltage) for details. This curve is for a typical part and there
could be part-to-part variation for output voltages less than
0.8V over the limited V
is more than 0.15V (typ.), the switches are turned on. When
it is less than 0.125V (typ.), the switches are turned off. This
on/off function has 25mV (typ.) hysteresis. The quiescent
current when (V
ESTIMATION OF MAXIMUM OUTPUT CURRENT
CAPABILITY
Referring to Figure 3, the Inductor peak to peak ripple cur-
rent can be estimated by:
Where, Fsw is switching frequency.
Therefore, maximum output current can be calculated by:
For the worst case calculation, the following parameters
should be used:
F
I
L1 (Lowest inductor value): refer to inductor data-sheet.
Note that inductance will drop with DC bias current and
temperature. The worst case is typically at 85˚C.
For example, V
tance value at 985mA DC bias current and 85˚C), F
1.8MHz , I
The effects of switch, inductor resistance and dead time are
ignored. In real application, the ripple current would be 10%
to 15% higher than ideal case. This should be taken into
account when calculating maximum output current. Special
attention needs to be paid that a delta between maximum
output current capability and the current limit is necessary to
satisfy transient response requirements. In practice, tran-
sient response requirements may not be met for output
current greater than 650mA.
INDUCTOR SELECTION
A 3.3µH inductor with saturation current rating over 1200mA
and low inductance drop at the full DC bias condition is
recommended for almost all applications. The inductor’s DC
resistance should be less than 0.2Ω for good efficiency. For
low dropout voltage, lower DCR inductors are recom-
mended. The lower limit of acceptable inductance is 1.7µH
at 1200mA over the operating temperature range. Full atten-
tion should be paid to this limit, because some small induc-
tors show large inductance drops at high DC bias. These
cannot be used with the LM3208. FDK MIPW3226D3R0M is
an example of an inductor with the lowest acceptable limit
(as of Oct./05).Table 1 suggests some inductors and suppli-
ers.
LIM
SW
CON
CON
(Lowest current limit value): 985mA
(Lowest switching frequency): 1.8MHz
I
IND_PP
voltage is less than 0.32V (V
CON
is over 1.44V (V
LIM
is between 0.32V and 1.44V, the output voltage
= (V
I
= 985mA.
OUT_MAX
I
OUT_MAX
IN
CON
IN
= 4.2V, V
- V
V
= 0V and V
I
OUT
IND_PP
IN
OUT
= 985 – 106 = 876mA
= I
range. When the control pin voltage
OUT
= 2.5 x V
) x V
LIM
OUT
= 212mA
= 3.6V), sub-harmonic oscilla-
- 0.5 x I
OUT
EN
= 3.2V, L1 = 2.0µH (Induc-
CON
= Hi) is around 600µA.
OUT
/ (L1 x F
CON
IND_PP
= 0.8V), the output
Voltage vs Output
CON
SW
.
x V
IN
IN
)
range
SW
=