TPS61090, TPS61091, TPS61092
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SLVS484C – JUNE 2003 – REVISED DECEMBER 2014
9.3 Feature Description
9.3.1 Synchronous Rectifier
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.
9.3.2 Controller Circuit
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 2200 mA.
An internal temperature sensor prevents the device from getting overheated in case of excessive power
dissipation.
9.3.3 Device Enable
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.
9.3.4 Undervoltage Lockout
An undervoltage lockout function prevents device start-up if the supply voltage on VBAT is lower than typically
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.
9.3.5 Softstart
When the device enables the internal startup 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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