TPS61220, TPS61221, TPS61222

SLVS776B – JANUARY 2009 – REVISED NOVEMBER 2014

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Feature Description (continued)

10.3.2 Device Enable And Shutdown Mode

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

stops switching and all internal control circuitry is turned off. During shutdown, the input voltage is connected to

the output through the back-gate diode of the rectifying MOSFET. This means that voltage is always present at

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

10.3.3 Startup

After the EN pin is tied high, the device begins to operate. If the input voltage is not high enough to supply the

control circuit properly, a startup oscillator operates the switches. During this phase, the switching frequency is

controlled by the oscillator, and the maximum switch current is limited. When the device has built up the output

voltage to approximately 1.8V, high enough to supply the control circuit, the device switches to its normal

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

10.3.4 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

because the device cannot 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 becomes forward

biased, and current starts to 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.

10.3.5 Undervoltage Lockout

An undervoltage lockout function stops the operation of the converter if the input voltage drops below the typical

undervoltage lockout threshold. This function is implemented in order to prevent converter malfunction.

10.3.6 Overvoltage Protection

If, for any reason, the output voltage is not fed back properly to the input of the voltage amplifier, control of the

output voltage is lost. Therefore an overvoltage protection is implemented to avoid the output voltage exceeding

critical values for the device and possibly for the system it is supplying. For this protection, the TPS6122x output

voltage is also monitored internally. If it reaches the internally programmed threshold of 6.5 V, typically the

voltage amplifier regulates (limits) the output voltage to this value.

If the TPS6122x is used to drive LEDs, this feature protects the circuit if the LED fails.

10.3.7 Overtemperature Protection

The device has a built-in temperature sensor which monitors the internal IC junction temperature. If the

temperature exceeds the programmed threshold (see electrical characteristics table), the device stops operating.

As soon as the IC temperature has decreased below the programmed threshold, it starts operating again. To

prevent unstable operation close to the region of overtemperature threshold, a built-in hysteresis is implemented.

10.4 Device Functional Modes

•

Enabled or disabled

•

Continuous or discontinuous current operation

•

Protective mechanisms

–

Output Overload

–

Undervoltage

–

Overvoltage

–

Overtemperature

12

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