LM3263

SNVS837A – JUNE 2013 – REVISED AUGUST 2013

www.ti.com

PFM Operation

When Auto-PFM mode operation is selected (SMPS_CFG [5] set to 1b), the LM3263 automatically transitions

from PWM operation into PFM operation if the average inductor current is less than 60 mA (min.) and the

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 120 mA (typ.), the part will

automatically transition into PWM mode. A 20 mA (typ.) hysteresis window exists between PFM and PWM

transitions. After a transient event, the part temporarily operates in PWM mode to quickly charge or discharge the

output. This is true for startup conditions or if the mode operation is changed from Forced-PWM to Auto-PFM

mode (SMPS_CFG [5] toggled from 0b to 1b). Once the output reaches its target output voltage, and the load is

less than 60 mA (min.), then the part will seamlessly transition into PFM mode (assuming It is not in forced

bypass condition).

Active Current Assist and Analog Bypass (ACB)

The 3GPP time mask requirement for 2G requires high current to be sourced by the LM3263. 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 is not an optimum solution. Thus, to allow an

optimal inductor size for such a load, an alternate current path is provided from the input supply through the ACB

current required to support the load. The ACB circuit also minimizes the dropout voltage by having the analog

bypass FET in parallel with VSET. The LM3263 can provide up to 2.5A (min.) of current in bypass mode.

Bypass Operation

When SMPS_CFG [4] is set to 0b, 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 LM3263 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 operate the device at the Forced-Bypass mode, set REGISTER_0 to 7Fh or VSET_CTRL to FEh-FFh.

Shutdown Mode

The Shutdown mode is entered whenever the voltage on the VIO pin is 0V. The communications and the

controls are not powered. In this mode, the current consumption is 0.02 µA (typ.).

Low-Power Mode

The Low-Power mode is the initial default state when VIO is applied. In this mode, the DC-DC is disabled and its

SW is tri-state. The current consumption is minimized 0.225 µA (typ.). This mode can be entered by

programming any one of three registers below:

•

Register 00h REGISTER_0 [6:0] to 00h;

•

Register 03h VSET_CTRL[7:0] to 00h or 01h;

•

Register 1Ch PM_TRIG [7:6] to 10b.

Standby Mode

In Standby mode, switching is stopped, and the output power FETs are placed in tri-state. The Standby mode

can be entered by setting PM_TRIG [7:6] and REGISTER_0 or VSET_CTRL registers.

•

Register 00h REGISTER_0 [6:0] to 02h;

•

Register 03h VSET_CTRL [7:0] to 04h or 05h;

•

Register 1Ch PM_TRIG [7:6] to 00b.

14

Submit Documentation Feedback

Copyright © 2013, Texas Instruments Incorporated

Product Folder Links: LM3263