Datasheet

TPS61260, TPS61261

SLVSA99A –MAY 2011–REVISED FEBRUARY 2013

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DETAILED DESCRIPTION

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

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 input voltage

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

Though this operating point is not recommended, the device will not be damaged by this as long as absolute

maximum ratings are not violated.

As opposed to a standard boost converter, the implemented 3 switch topology enables the output to be

disconnected from the input during device shutdown when disabled. Current does not flow from output to input or

from input to output.

Power Save Mode

At normal load conditions with continuous inductor current, the device operates at a quasi fixed frequency. If the

load gets lower, the inductor current decreases and becomes discontinuous. If this happens and the load is

further decreased, the device lowers the switching frequency and turns off parts of the control to minimize

internal power consumption. The output voltage is controlled by a low power comparator at a level about 1%

higher than the nominal output voltage. If the output voltage reaches the nominal value or drops below it, the

device control is turned on again to handle the new load condition. The boundary between power save mode and

PWM mode is when the inductor current becomes discontinuous.

Accurate average output current regulation requires continuous inductor current. This means that there is no

power save mode during current regulation.

Device Enable

The device is put into operation, when EN is set high. It is put into a shutdown mode, when EN is set to GND. In

shutdown mode, the regulator stops switching, all internal control circuitry is switched off, and the load is

disconnected from the input. This means that the output voltage can drop below the input voltage during

shutdown.