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TPS61070
TPS61071
SLVS510 鈥?JUNE 2004
DETAILED DESCRIPTION
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 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 overheating due to excessive power dissipation.
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 r
DS(on)
PMOS switch, the power
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.
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.
Undervoltage Lockout
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.
Soft Start
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. 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. If the input voltage is below 1.8 V, the device works with a fixed duty cycle of 70% until the output
voltage reaches 1.8 V. Then the duty cycle is set depending on the input output voltage ratio. Until the output
voltage reaches its nominal value, the boost switch current limit is set to 50% of its nominal value to avoid
high-peak currents at the battery during start-up. As soon as the output voltage is reached, the regulator takes
control, and the switch current limit is set back to 100%.
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