Datasheet

LM3243
www.ti.com SNVS782B –OCTOBER 2010–REVISED FEBRUARY 2013
Dynamic Adjustment of Output Voltage
The output voltage of the LM3243 can be dynamically adjusted by changing the voltage on the VCON pin. In RF
PA applications, peak power is required when the handset is far away from the base station. To maximize the
power savings, the LM3243 output should be set just high enough to achieve the desired PA linearity. Hence,
during low-power requirements, reduction of supply voltage to the PA can reduce power consumption from the
PA, making the operation more efficient and promote longer battery life. Please refer to the Setting the Output
Voltage section for further details.
Mode Selection
Table 1 shows the LM3243 parameters for the given modes (PWM or PFM/PWM).
Table 1. Parameters under Different Modes
Parameter/Mode PWM PFM/PWM
MODE pin LOW HIGH
BP pin LOW LOW
Frequency at loads = 75 mA (typ.) 2.7 MHz (typ.) Variable
Frequency at loads = 95 mA (typ.) 2.7 MHz (typ.) 2.7 MHz (typ.)
V
OUT
2.5 x VCON 2.5 x VCON
Max Load Steady State 2.5A (min.) 75 mA (min in PFM) or 2.5A (min. in PWM)
Internal Synchronous Rectification
The LM3243 uses an internal NFET as a synchronous rectifier to reduce rectifier forward voltage drop, thus
increasing efficiency. The reduced forward voltage drop in the internal NFET synchronous rectifier significantly
improves efficiency for low output voltage operation. The NFET is designed to conduct through its intrinsic body
diode during the transient intervals, eliminating the need of an external diode.
Current Limit
The LM3243 current limit feature protects the converter during current overload conditions. Both SW and ACB
pins have positive and negative current limits. The positive and negative current limits bound the SW and ACB
currents in both directions. The SW pin has two positive current limits. The I
LIM,PFET,SteadyState
current limit triggers
the ACB circuit. Once the peak inductor current exceeds I
LIM,PFET,SteadyState
, the ACB circuit starts assisting the
switcher and provides just enough current to keep the inductor current from exceeding I
LIM,PFET,SteadyState
allowing
the switcher to operate at maximum efficiency. Transiently a second current limit I
LIM,PFET,Transient
of 1.9A (typ. or
2.1 max.) limits the maximum peak inductor current possible. The output voltage will fall out of regulation only
after both SW and ACB output pin currents reach their respective current limits of I
LIM,PFET,Transient
and I
LIM,P-ACB
.
Timed Current Limit
If the load or output short circuit pulls the output voltage to 0.3V or lower and the peak inductor current sustains
I
LIM, PFET
Transient more than 10 µs, the LM3243 switches to a timed current limit mode. In this mode, the
internal PFET switch is turned off. After approximately 30 µs, the device will return to the normal operation.
Thermal Overload Protection
The LM3243 IC has a thermal overload protection that protects itself from short-term misuse and overload
conditions. If the junction temperature exceeds 150°C, the LM3243 shuts down. Normal operation resumes after
the temperature drops below 130°C. Prolonged operation in thermal overload condition may damage the device
and is therefore not recommended.
Copyright © 2010–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LM3243