I

L

t

200mA

(typ.)

ContinuousCurrentOperation

DiscontinuousCurrentOperation

200mA

(typ.)

TPS61097A-33

SLVSCF2A – JANUARY 2014 – REVISED DECEMBER 2014

www.ti.com

8.3 Feature Description

8.3.1

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. If the required average input current is lower than the average inductor

current defined by this constant ripple the inductor current goes discontinuous to keep the efficiency high at low

load conditions.

Figure 16. 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.

8.3.2 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.

8.3.3 Bypass Switch

The TPS61097A-33 contains a P-channel MOSFET (Bypass Switch) in parallel with the synchronous rectifying

low impedance connection from the input voltage (at the L pin) to the load (VOUT). The Bypass Switch is not

impacted by Undervoltage lockout, Overvoltage or Thermal shutdown.

8.3.4 Startup

After the EN pin is tied high, the device starts to operate. If 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 1.8 V, 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.

8.3.5 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. Because this diode cannot be turned off, the load current is only limited by the

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

normal operation and enters the appropriate soft start mode depending on the operating conditions.

8.3.6 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 malfunctioning of the converter.

The undervoltage lockout function has no control of the Bypass Switch. If the Bypass Switch is enabled

10

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