Datasheet

LM3208
SNVS404B –APRIL 2006–REVISED MARCH 2013 www.ti.com
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, eliminating the need for an external
diode.
R
DSON(P)
Management
The LM3208 has a unique R
DSON(P)
management function to improve efficiency in the low output current region
up to 100mA. When the V
CON
voltage is less than 0.40V (typ.), the device 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 entire PFET is used to minimize
R
DSON(P)
loss. This threshold has about 20mV (typ.) of hysteresis.
V
CON,ON
The output is disabled when V
CON
is below 125mV (typ.). It is enabled when V
CON
is above 150mV (typ.). The
threshold 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 normally used when V
CON
is above 0.42V (typ.), and
an 800mA (typ.) is used when V
CON
is below 0.40V (typ.). If an excessive load pulls the output voltage down to
approximately 0.375V, then the device switches to a timed current limit mode when V
CON
is above 0.42V (typ.).
In timed current limit mode the internal PFET switch is turned off after the current comparator trips and the
beginning of the next cycle is inhibited for 3.5µs 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
CON
pin. This feature is useful in
PA applications where peak power is needed only when the handset is far away from the base station or when
data is being transmitted. In other instances, the transmitting power can be reduced. Hence the supply voltage to
the PA can be reduced, 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 increases slightly.
Thermal Overload Protection
The LM3208 has a thermal overload protection function that operates to protect itself from short-term misuse and
overload 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 voltage 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
CON
pin,
as in the following formula:
V
OUT
= 2.5 x V
CON
(1)
When V
CON
is between 0.32V and 1.44V, the output voltage will follow proportionally by 2.5 times of V
CON
.
14 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LM3208