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DETAILED DESCRIPTION

CONTROLLER CIRCUIT

Synchronous Operation

Down Regulation

Power Save Mode

Device Enable

TPS61200

TPS61201

TPS61202

SLVS577B – MARCH 2007 – REVISED FEBRUARY 2008

The controlling circuit of the device is based on an average current mode topology. The average inductor current

is regulated by a fast current regulator loop which is controlled by a voltage control loop. The controller also uses

input and output voltage feedforward. Changes of input and output voltage are monitored and can 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 adjustable output voltages, a resistive voltage divider must be

connected to that pin. For 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 is compared

with the internal reference voltage to generate a stable and accurate output voltage.

The controller circuit also senses the average input current as well as the peak input current. Thus, the maximum

input power is controlled as well as the maximum peak current to achieve a safe and stable operation under all

possible conditions. To protect the device from overheating, an internal temperature sensor is implemented.

The device uses three 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.

To avoid ground shift problems due to the high currents in the switches, two separate ground pins GND and

PGND are used. The reference for all control functions is the GND pin. The power switches are connected to

PGND. Both grounds must be connected on the PCB at only one point ideally close to the GND pin. Due to the

3-switch topology, the load is always disconnected from the input during shutdown of the converter.

A boost converter only regulates output voltages which are higher than the input voltage. This device operates

differently. For example, it is able to regulate 3 V at the output with two fresh alkaline cells at the input having a

total cell voltage of 3.2 V. Another example is powering white LEDs with a forward voltage of 3.6 V from a fully

charged Li-Ion cell with an output voltage of 4.2 V. To control these applications properly, a Down Conversion

mode is implemented.

If the input voltage reaches or exceeds the output voltage, the converter automatically changes to a Down

Conversion mode. In this mode, the control circuit changes the behavior of the two rectifying switches. While

continuing switching it sets the voltage drop across the rectifying switches as high as needed to regulate the

output voltage. This means the power losses in the converter increase. This must be taken into account for

thermal consideration.

The Power Save (PS) pin can be used to select different operation modes. To enable Power Save mode the PS

pin must be set low. Power Save mode is used to improve efficiency at light load. If Power Save mode is

enabled, the converter stops operating if the average inductor current decreases below about 300 mA and the

output voltage is at or above its nominal value. If the output voltage decreases below its nominal value, the

device ramps up the output voltage again by starting operation using a programmed average inductor current

higher than required by the current load condition. Operation can last for one or several pulses. The converter

stops operating once the conditions for stopping operation are met again.

The Power Save mode can be disabled by programming high at the PS pin. In Down Conversion mode, Power

Save mode is always enabled and the device cannot be forced into fixed frequency operation at light loads. The

PS input supports standard logic thresholds.

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 including the low-battery comparator is

switched off, and the load is disconnected from the input. This also 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 drawn from the battery.

Copyright © 2007–2008, Texas Instruments Incorporated

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Product Folder Link(s): TPS61200 TPS61201 TPS61202