Datasheet

LM3243
SNVS782B –OCTOBER 2010–REVISED FEBRUARY 2013 www.ti.com
PFM Mode
With MODE = HIGH, the LM3243 automatically transitions to from PWM into PFM (Pulse Frequency Modulation)
operation if the average inductor current is less than 75 mA (typ.) and V
IN
− V
OUT
> 0.6V. The switcher regulates
the fixed output voltage by transferring a fixed amount of energy during each cycle and modulating the frequency
to control the total power delivered to the output. The converter switches only as needed to support the demand
of the load current, therefore maximizing efficiency. If the load current should increase during PFM mode to more
than 95 mA (typ.), the part will automatically transition into constant frequency PWM mode. A 20 mA (typ.)
hysteresis window exists between PFM and PWM transitions.
After a transient event, the part temporarily operates in 2.7 MHz (typ.) fixed-frequency PWM mode to quickly
charge or discharge the output. This is true for start-up conditions or if MODE pin is toggled LOW-to-HIGH. Once
the output reaches its target output voltage, and the load is less than 75 mA (typ.), then the part will seamlessly
transition into PFM mode (assuming it is not in forced bypass or auto bypass condition).
Active Current Assist and Analog Bypass (ACB)
The 3GPP time mask requirement for 2G requires high current to be sourced by the LM3243. These high
currents are required for a small time during transients or under a heavy load. Over-rating the switching inductor
for these higher currents would increase the solution size and will not be an optimum solution. So to allow an
optimal inductor size for such a load, an alternate current path is provided from the input supply through the ACB
pin. Once the switcher current limit I
LIM,PFET,SteadyState
is reached, the ACB circuit starts providing the additional
current required to support the load. The ACB circuit also minimizes the dropout voltage by having the analog
bypass FET in parallel with V
OUT
. The LM3243 can provide up to 2.5A (min.) of current in bypass mode with a
4.0A (max.) peak current limit.
Bypass Operation
The Bypass Circuit provides an analog bypass function with very low dropout resistance (R
tot_drop
= 45 mΩ typ).
When BP = LOW the part will be in automatic bypass mode which will automatically determine the amount of
bypass needed to maintain voltage regulation. When the input supply voltage to the LM3243 is lowered to a level
where the commanded duty cycle is higher than what the converter is capable of providing, the part will go into
pulse-skipping mode. The switching frequency will be reduced to maintain a low and well-behaved output voltage
ripple. The analog bypass circuit will allow the converter to stay in regulation until full bypass is reached (100%
duty cycle operation). The converter comes out of full bypass and back into analog bypass regulation mode with
a similar reverse process.
To override the automatic bypass mode, either set VCON > (V
IN
)/(2.5) (but less than V
IN
) or set BP = HIGH for
forced bypass function. Forced bypass function is valid for 2.7V < V
IN
< 5.5V.
Shutdown Mode
To shut down the LM3243 pull the EN pin LOW (<0.5V). In shutdown mode, the current consumption is 0.02 µA
(typ.) and the PFET switch, NFET synchronous rectifier, reference voltage source, control and bias circuit are
turned OFF. To enable LM3243 pull EN HIGH (>1.2V), and the mode of operation will be dependent on the
voltage applied to the MODE pin.
Since the LM3243 does not feature a UVLO (Under Voltage Lock-Out) circuit, the EN pin should be set LOW to
turn off the LM3243 during power-up and during UVLO conditions. For cell-phone applications, the system
controller determines the power supply sequence; thus, it is up to the system controller to ensure proper
sequencing by using all of the available pins and functions properly.
Mode Pin
The MODE pin changes the state of the converter to one of the two allowed modes of operation. Setting the
MODE pin HIGH (>1.2V) sets the device for automatic transition between PFM/PWM mode operation. In this
mode, the converter operates in PFM mode to maintain the output voltage regulation at very light loads and
transitions into PWM mode at loads exceeding 95 mA (typ.). The PWM switching frequency is 2.7 MHz (typ.).
Setting the MODE pin LOW (<0.5V) sets the device for PWM mode operation. The switching operation is in
PWM mode only, and the switching frequency is also 2.7 MHz (typ.).
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