I

L

t

200mA

(typ.)

ContinuousCurrentOperation

DiscontinuousCurrentOperation

200mA

(typ.)

TPS61222-EP

www.ti.com

SLVSBI2 – SEPTEMBER 2012

DETAILED DESCRIPTION

OPERATION

The TPS61222 is a high performance, high efficient switching boost converter. To achieve high efficiency the

power stage is realized as a synchronous boost topology. For the power switching two actively controlled low

CONTROLLER CIRCUIT

The device is controlled by a hysteretic current mode controller. This controller regulates the output voltage by

keeping the inductor ripple current constant in the range of 200 mA and adjusting the offset of this inductor

current depending on the output load. In case the required average input current is lower than the average

inductor current defined by this constant ripple the inductor current gets discontinuous to keep the efficiency high

at low load conditions.

Figure 9. Hysteretic Current Operation

To regulate the output voltage, the voltage error amplifier compares this feedback voltage to the internal voltage

reference and adjusts the required offset of the inductor current accordingly. At fixed output voltage versions an

internal feedback network is used to program the output voltage, at adjustable versions an external resistor

divider needs to be connected.

The self oscillating hysteretic current mode architecture is inherently stable and allows fast response to load

variations. It also allows using inductors and capacitors over a wide value range.

Device Enable and Shutdown Mode

The device is enabled when EN is set high and shut down when EN is low. During shutdown, the converter stops

switching and all internal control circuitry is turned off. In this case the input voltage is connected to the output

through the back-gate diode of the rectifying MOSFET. This means that there always will be voltage at the output

which can be as high as the input voltage or lower depending on the load.

Startup

After the EN pin is tied high, the device starts to operate. In case the input voltage is not high enough to supply

the control circuit properly a startup oscillator starts to operate the switches. During this phase the switching

frequency is controlled by the oscillator and the maximum switch current is limited. As soon as the device has

built up the output voltage to about 1.8V, high enough for supplying the control circuit, the device switches to its

normal hysteretic current mode operation. The startup time depends on input voltage and load current.

Operation at Output Overload

If in normal boost operation the inductor current reaches the internal switch current limit threshold the main

switch is turned off to stop further increase of the input current.

In this case the output voltage will decrease since the device can not provide sufficient power to maintain the set

output voltage.

If the output voltage drops below the input voltage the backgate diode of the rectifying switch gets forward biased

and current starts flow through it. This diode cannot be turned off, so the current finally is only limited by the

remaining DC resistances. As soon as the overload condition is removed, the converter resumes providing the

set output voltage.

Copyright © 2012, Texas Instruments Incorporated

Submit Documentation Feedback

9

Product Folder Links :TPS61222-EP