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DETAILED DESCRIPTION

CONTROLLER CIRCUIT

Synchronous Rectifier

Device Enable

Undervoltage Lockout

Soft Start and Short Circuit Protection

TPS61070, TPS61071

TPS61072, TPS61073

SLVS510A – JULY 2006 – REVISED JANUARY 2007

The controller circuit of the device is based on a fixed frequency multiple feedforward controller topology. Input

voltage, output voltage, and voltage drop on the NMOS switch are monitored and forwarded to the regulator. So,

changes in the operating conditions of the converter directly affect the duty cycle and must not take the indirect

and slow way through the control loop and the error amplifier. The control loop, determined by the error

amplifier, only has to handle small signal errors. The input for it is the feedback voltage on the FB pin. It is

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

The peak current of the NMOS switch is also sensed to limit the maximum current flowing through the switch

and the inductor. The typical peak-current limit is set to 600 mA. An internal temperature sensor prevents the

device from getting overheated in case of excessive power dissipation.

The device integrates an N-channel and a P-channel MOSFET transistor to realize a synchronous rectifier.

conversion efficiency reaches values above 90%. A special circuit is applied to disconnect the load from the

input during shutdown of the converter. In conventional synchronous rectifier circuits, the backgate diode of the

high-side PMOS is forward biased in shutdown and allows current flowing from the battery to the output.

However, this device uses a special circuit which takes the cathode of the backgate diode of the high-side

PMOS and disconnects it from the source when the regulator is not enabled (EN = low).

The benefit of this feature for the system design engineer is that the battery is not depleted during shutdown of

the converter. No additional components must be added to the design to make sure that the battery is

disconnected from the output of the converter.

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 isolated

from the input (as described in the Synchronous Rectifier Section). 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.

An undervoltage lockout function prevents the device from operating if the supply voltage on VBAT is lower than

approximately 0.8 V. When in operation and the battery is being discharged, the device automatically enters the

shutdown mode if the voltage on VBAT drops below approximately 0.8 V. This undervoltage lockout function is

implemented in order to prevent the malfunctioning of the converter.

When the device enables, the internal start-up cycle starts with the first step, the precharge phase. During

precharge, the rectifying switch is turned on until the output capacitor is charged to a value close to the input

voltage. The rectifying switch is current limited during this phase. The current limit increases with the output

voltage. This circuit also limits the output current under short-circuit conditions at the output. Figure 22 shows

the typical precharge current vs output voltage for specific input voltages:

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