SMB118 SUMMIT Microelectronics, Inc:, SMB118 Datasheet - Page 26

• Manufacturer Part Number: SMB118
• Description: Six-channel Programmable Dc-dc Power Managers With Battery Charger
• Manufacturer: SUMMIT Microelectronics, Inc:

BUCK CONVERTERS
The SMB118 and SMB218 have three synchronous
buck converters with integrated p-channel MOSFETS
and a driver for an external NFET, see Figure 10. Each
channel has an output voltage range from the input
supply to approximately 0.5V.
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 delivered 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 FET 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.
Summit Microelectronics, Inc
Figure 10 – Buck channel with internal PFET.
APPLICATIONS INFORMATION (CONTINUED)
COMP
PWM
DRVL
COMP
FB
VIN
SW
Vo
2107 2.4 5/5/2008
To maximize the converter efficiency for both light and
heavy loads the buck converters automatically switch
from PFM to PWM mode for higher loads. 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 drivers
will stay in this state until the voltage on the COMP pins
rise above the programmable PFM to PWM crossover
voltage.
Each
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:
Each buck converter can operate up to 100% duty
cycle allowing the output to equal the input.
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.
Boost Controller
The SMB118 and SMB218 have one asynchronous
current mode Boost converter with over-current
protection and either a PWM or PFM mode of
operation.
As a current mode boost controller a sense resistor
must be added, externally, in series with the source of
the N-channel MOSFET, see Figure 11. The over-
channel
Vo
=
has
D
*
an
Vin
Preliminary Specification
over
SMB118/218
current
protection
The
26