+

-

V

FB

GND

FB

VOUT

L

VIN

EN

Current

Sensor

Gate

Control

Modulator

Device

Control

VIN

VOUT

RI

VOUT

TPS61260, TPS61261

SLVSA99B – MAY 2011 – REVISED NOVEMBER 2014

www.ti.com

Functional Block Diagrams (continued)

Figure 16. TPS61261

8.3 Feature Description

8.3.1 Controller Circuit

The controlling circuit of the device is based on a current mode topology. The inductor current is regulated by a

fast current regulator loop which is controlled by either a voltage control loop or a reference current. The

controller also uses input and output voltage feedforward. Changes of the input and output voltages are

monitored and immediately change the duty cycle in the modulator to achieve a fast response to those errors.

The voltage error amplifier gets its feedback input from the FB pin. For the adjustable output voltage version, a

resistive voltage divider must be connected to that pin. For the fixed output voltage version, the FB pin must be

connected to the output voltage to directly sense the voltage. Fixed output voltage versions use a trimmed

internal resistive divider. The feedback voltage is compared with the internal reference voltage to generate a

stable and accurate output voltage. The reference current for average output current control is programmed with

a resistor connected between the RI pin and GND.

The programming of the average output current also affects the maximum switch current in the main switch

which basically is the input current. The lower the average output current is programmed, the lower the maximum

input current. Now, maximum input power is controlled as well as the maximum peak current to achieve safe and

stable operation under all possible conditions. Smaller inductors with lower saturation current ratings can be

used, when lower average output currents are programmed.

8.3.2 Synchronous Boost Operation

The device uses 3 internal N-channel MOSFETs to maintain synchronous power conversion at all possible

operating conditions. This enables the device to keep high efficiency over a wide input voltage and output power

range. Using 2 rectifying switches also enables the device to control the output voltage and current during startup

conditions when the input voltage is higher than the output voltage. During startup, the rectifying switch works in

a linear mode until the output voltage is near the input voltage. Once in regulation, operating with the input

voltage greater than the output voltage may cause either the output voltage or current to exceed its regulation

value. Although this operating point is not recommended, the device will not be damaged by this as long as

absolute maximum ratings are not violated.

10

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