LM3241
www.ti.com
SNOSB38B –JANUARY 2009–REVISED APRIL 2013
ECO Mode Operation
At very light loads (50 mA to 100 mA), the LM3241 enters ECO mode operation with reduced switching
frequency and supply current to maintain high efficiency. During ECO mode operation, the LM3241 positions the
output voltage slightly higher (+7mV typ.) than the normal output voltage during PWM mode operation, allowing
additional headroom for voltage drop during a load transient from light to heavy load.
ECO Mode at Light Load
High ECO Threshold
Load current increases
Target Output Voltage
Low ECO Threshold
PWM Mode at Heavy Load
Figure 30. Operation in ECO Mode and Transfer to PWM Mode
Shutdown Mode
Setting the EN digital pin low (<0.4V) places the LM3241 in Shutdown mode (0.1 µA typ.). During shutdown, the
PFET switch, the NFET synchronous rectifier, reference voltage source, control and bias circuitry of the LM3241
are turned off. Setting EN high (>1.2V) enables normal operation. EN should be set low to turn off the LM3241
during power-up and undervoltage conditions when the power supply is less than the 2.7V minimum operating
voltage. The LM3241 has an UVLO (Under Voltage Lock Out) comparator to turn the power device off in the
case the input voltage or battery voltage is too low. The typical UVLO threshold is around 2.0V for lock and 2.1V
for release.
Internal Synchronization Rectification
While in PWM mode, the LM3241 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.
With medium and heavy loads, the 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.
Current Limiting
The current limit feature allows the LM3241 to protect itself and external components during overload conditions.
In PWM mode, the cycle-by-cycle current limit is 1450 mA (typ.). If an excessive load pulls the output voltage
down to less than 0.3V (typ.), the NFET synchronous rectifier is disabled, and the current limit is reduced to 530
mA (typ.). Moreover, when the output voltage becomes less than 0.15V (typ.), the switching frequency will
decrease to 3MHz, thereby preventing excess current and thermal stress.
Copyright © 2009–2013, Texas Instruments Incorporated
Submit Documentation Feedback
13
Product Folder Links: LM3241
TI [ TEXAS INSTRUMENTS ]
