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TPS61260DRVR Datasheet(HTML) 10 Page - Texas Instruments
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– MAY 2011
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 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
can 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. At adjustable output voltages, a resistive voltage divider must be
connected to that pin. At fixed output voltages, FB 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 will be
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 RI 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 will be. Now, maximum input power can be controlled as well as the maximum peak current to
achieve a safe and stable operation under all possible conditions. Since switch current and inductor current have
the same level smaller inductors can be used when lower average output currents are programmed.
Synchronous Boost Operation and Down Conversion Mode
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 at conditions
when the input voltage is higher than the actual output voltage. This for example happens at output short circuit
conditions, during startup or if the supply voltage is just higher than the regulated output voltage. In this down
conversion mode the rectifying switch works in a linear mode.
In difference to a standard boost converter technology the implemented 3 switch topology enables the output to
be disconnected from the input during device shutdown.
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 gets 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.
Accurate average output current regulation requires continuous inductor current. This means that there will be no
power save mode during current regulation.
Dynamic Current Limit
To protect the device and the application the inductor current is limited internally on the IC. At nominal operating
conditions, this current limit is constant at the programmed value. If the supply voltage at VIN drops to values
close to the undervoltage lockout threshold, the device stops operating. This can happen when the input power
source becomes weak. Increasing output impedance, when the batteries are almost discharged, or an additional
heavy pulse load is connected to the battery can cause such VIN voltage drops. If the voltage at VIN recovers
the device starts operating again. This way the average input current is reduced if the output impedance of the
power source is causing the voltage drop, allowing the system to stay in operation at a decreased output power.
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. During start-up of the converter, the duty cycle and the peak current are limited in order to avoid high
peak currents flowing from the input.
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