L

GND

EN

VIN

C

2.2µF

I

1µH

C

2.2µF

O

VOUT

DC/DC CONVERTER

Undervoltage

Lockout

Thermal

Shutdown

Softstart

Control

Logic

PMOS

Valley

Current

Sense

V

REF

NMOS

Modulator

x2

9

TPS81256

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SLVSAZ9C – JUNE 2012 – REVISED FEBRUARY 2016

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Copyright © 2012–2016, Texas Instruments Incorporated

8 Detailed Description

8.1 Overview

The TPS81256 is a stand-alone, synchronous, step-up converter module. The converter operates at a quasi-

constant 4-MHz frequency pulse width modulation (PWM) at moderate to heavy load currents. At light load

currents, the TPS81256 converter operates in power-save mode with pulse frequency modulation (PFM).

8.2 Functional Block Diagram

8.3 Feature Description

8.3.1 Operation

During PWM operation, the converter uses a novel quasi-constant on-time valley current mode control scheme to

achieve excellent line/load regulation and allows the use of a small ceramic inductor and capacitors. Based on

At the beginning of the switching cycle, the low-side N-MOS switch is turned-on and the inductor current ramps

up to a peak current that is defined by the on-time and the inductance. In the second phase, once the on-timer

has expired, the rectifier is turned-on and the inductor current decays to a preset valley current threshold. Finally,

the switching cycle repeats by setting the on timer again and activating the low-side N-MOS switch.