LM3263

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SNVS837 – JUNE 2013

Active Mode

The Active mode is a DC-DC converter operating mode that allows the device to function, process RFFE

commands, and respond to RFFE commands. This mode can be entered by setting register 1Ch PM_TRIG [7:6]

to 00b. Once the device is the Active Mode, the DC-DC converter operating mode and the output voltage can be

programmed by using REGISTER_0 [6:0] and VSET_CTRL[7:0] registers.

Dynamic Adjustment of Output Voltage

The LM3263 can be dynamically programmed the output voltage from 0.4V to 3.6V with 30 mV or 15 mV steps.

REGISTER_0 [6:0] is set to 0Dh to 78h with 30 mV output voltage steps, and VSET_CTRL [7:0] is set to 1Bh to

F0h with 15 mV steps. Although the output voltage can be programmed lower than 0.4V and higher than 3.6V by

setting the registers, the device might suffer from larger output ripple voltage, higher current limit operation, and

decreased linearity.

DC-DC Operating Mode Selection

Programming SMPS_CFG [5] changes the state of the converter to one of the two allowed modes of operation.

SMPS_CFG [5] default is 0b and the device operates in Forced PWM mode (PWM only). Setting the register bit

to 1b 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 120 mA (typ). Setting the register bit to 0b 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). The device

operates in Forced-Bypass mode when SMPS_CFG [4] is set to 1b.

For typical operation mode is set to Auto-PFM and Auto-Bypass modes by setting SMPS_CFG = 20h.

Table 1 shows the LM3263 parameters for the given modes.

Table 1. Parameters under Different Modes of Operation

SMPS_CFG [5] MODE

SMPS_CFG [4] BYPS

Operation Mode

0

0

X

Forced-PWM

1

X

Forced-Bypass

1

0

PFM

1

0

PFM or PWM

1

0

PWM

(1)

doesn't care

Internal Synchronous Rectification

The LM3263 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 LM3263 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

2.1 max.) limits the maximum peak inductor current possible. The output voltage will fall out of regulation only

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

PFET switch is turned off. After approximately 30 µs, the device will return to the normal operation.

Copyright © 2013, Texas Instruments Incorporated

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