SMB119 Datasheet, PDF (22/34 Pages) Summit Microelectronics, Inc. – Seven-Channel Programmable DC-DC Power Manager

SMB119
APPLICATIONS INFORMATION (CONTINUED)
BUCK CONVERTERS
The SMB119 has three synchronous buck converters
with integrated p-channel MOSFETS and a driver for an
external NFET, see Figure 14. Each channel has an
output voltage range from the input supply to
approximately 0.5V.
VIN
SW
Vo
PWM
DRVL
COMP
FB
COMP
Figure 14 – Buck channel with internal PFET.
Buck Channel Asynchronous Operation
The Buck converters use either a constant frequency or
variable frequency current mode control technique.
During the fixed frequency PWM mode of operation, the
converter switches at a fixed frequency and modulates
the duty cycle to attain the correct output voltage. This
can lead to “charge shuttling” under light load
conditions were the charge transferred to the output
capacitor during the on time of the PFET is discharged
to ground during the on time of the NFET. This mode of
operation is desirable in situations requiring low voltage
ripple, the ability to sink current, or a known switching
frequency for all loads.
During the PFM mode of operation the converter
operates asynchronously where the NFET is held off
and the body diode of the FET is used as a “catch”
diode; preventing the voltage on the switch node from
falling below ground by more than a diode drop. It is
desirable to operate asynchronously under light load so
that charge shuttling does not occur. The asynchronous
operation allows the converter to only switch when the
voltage falls below the error amplifier reference voltage.
While it is advantageous to operate asynchronously for
light load currents, it is less efficient for moderate loads
where the power loss across the forward voltage drop
of the diode leads to decreased efficiency. To increase
the efficiency for these moderate load conditions an
external schottky diode can be placed in parallel with
the body diode of the FET.
To maximize the converter efficiency for both light and
heavy loads the Buck converters automatically switch
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from PFM to PWM mode. The PWM to PFM crossover
is accomplished by observing the voltage on the COMP
pin, the voltage on the COMP pin is directly
proportional to the load current. When the voltage on
the COMP pin falls below a programmable reference,
the converter operates in PFM. The NFET driver will
stay in the off state until the voltage on the COMP pin
rises above the PFM to PWM crossover voltage.
Each channel has an over current protection
mechanism. When a channel reaches its current limit,
the output voltage will be reduced as the load rises.
This is accomplished by clamping the COMP node to
one of four programmable settings. The over-current
level can be programmed to four different levels by
clamping the error amplifier's output voltage to a
programmable voltage.
All current limits and PFM to PWM crossover currents
are calculated by the GUI interface.
The output of all Buck converters is determined by the
portion of the switching period for which the inductor
voltage is at the converter supply voltage; this
percentage is referred to as the duty cycle. For a Buck
channel operating synchronously, duty cycle and the
output voltage are related by equation 1 below:
Equation 1: Vo = D * Vin
Each Buck converter can operate up to 100% duty
cycle allowing the output to equal the input. The
minimum voltage is determined by the minimum duty
cycle listed in the electrical specifications section. For a
Buck converter operating in PFM mode the duty cycle
is essentially 0% implying that the output can go to
ground.
Each converter has a separate VIN input used to power
the converter. This supply attaches to the source of the
integrated PFET. It is important to connect an input (or
Bulk) as close to the VIN pin as possible. For
information on the type of capacitor to use, refer to the
component selection section.