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DETAILED DESCRIPTION

Controller Circuit

Synchronous Rectifier

Device Enable

Undervoltage Lockout

Softstart

TPS61030

TPS61031, TPS61032

SLUS534D – SEPTEMBER 2002 – REVISED APRIL 2004

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 or, at fixed output

voltage versions, the voltage on the internal resistor divider. 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 4000 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.

Because the commonly used discrete Schottky rectifier is replaced with a low RDS(ON) PMOS switch, the power

conversion efficiency reaches 96%. To avoid ground shift due to the high currents in the NMOS switch, two

separate ground pins are used. The reference for all control functions is the GND pin. The source of the NMOS

switch is connected to PGND. Both grounds must be connected on the PCB at only one point close to the GND

pin. 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. This device however 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 have to 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 including the low-battery comparator 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 device start-up if the supply voltage on VBAT is lower than

approximately 1.6 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 1.6 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 current is limited in that phase. This also limits the output current under short-circuit

conditions at the output. After charging the output capacitor to the input voltage the device starts switching. Until

the output voltage is reached, the boost switch current limit is set to 40% of its nominal value to avoid high peak

currents at the battery during startup. When the output voltage is reached, the regulator takes control and the

switch current limit is set back to 100%.

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