SLVS451D – SEPTEMBER 2003 – REVISED FEBRUARY 2006
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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 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 1500 mA. An internal temperature sensor prevents the
device from getting overheated in case of 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 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.
Down Regulation
In general, a boost converter only regulates output voltages which are higher than the input voltage. This device
operates differently. For example, it is able to regulate 3.0 V at the output with two fresh alkaline cells at the
input having a total cell voltage of 3.2 V. Another example is powering white LEDs with a forward voltage of 3.6
V from a fully charged Li-Ion cell with an output voltage of 4.2 V. To control these applications properly, a down
conversion mode is implemented.
If the input voltage reaches or exceeds the output voltage, the converter changes to the conversion mode. In
this mode, the control circuit changes the behavior of the rectifying PMOS. It sets the voltage drop across the
PMOS as high as needed to regulate the output voltage. This means the power losses in the converter increase.
This has to be taken into account for thermal consideration. The down conversion mode is automatically turned
off as soon as the input voltage falls about 50 mV below the output voltage. For proper operation in down
conversion mode the output voltage should not be programmed below 50% of the maximum input voltage which
can be applied.
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.
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