SMB218 View Datasheet(PDF) - Summit Microelectronics
Part Name
Description
Manufacturer
SMB218 Datasheet PDF : 37 Pages
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PLEASE NOTE: The SMB118 has entered End-of-Life.
SMB118/218
APPLICATIONS INFORMATION (CONTINUED)
current circuitry is identical to that descried for the buck
converter, the current limit is displayed in the GUI.
The PWM to PFM crossover current is identical to the
circuitry used for the buck converter, we monitor the
voltage on the COMP node and when the voltage is
below a programmable reference the NFET is held off.
The boost converter has a fixed PWM option, when
enabled the boost channel will switch every cycle
keeping the ripple voltage low. Care must be taken in
selecting the PWM option on the boost channel, as this
converter does not have the ability to shuttle charge. As
a result, the load must be sufficient to deplete the
deposited charge every cycle or else the output voltage
will rise above the output set point.
All boost controller drivers are powered from the
VBATT supply pin. Therefore, without voltage on the
VBATT input the boost converters will not function.
The output of all boost controllers are determined by
the portion of the switching period for which the
inductor voltage is at ground, this percentage is
referred to as the duty cycle. For a boost controller
where the inductor current does not go to 0A during the
cycle (CCM) the relation between the duty cycle and
the output voltage is determined by equation 2 below:
Equation 2: Vo
=
1
1
−
D
* Vin
The maximum duty cycle the boost converter can
achieve is determined by the max duty cycle specified
in the electrical specification section of the datasheet.
VIN
BOOST OR BUCK CONTROLLER
The SMB118 and SMB218 have one voltage mode
output that can be configured as either a boost or a
buck controller; see Figures 12 and 13.
VIN
Boost
PCHSEQ
Sequencing VIN
PWM
Dx RVH
DRVL
COMP
COMPA
COMPB
Vout
Figure 12 –Buck or boost channel configured as
boost.
Buck
VIN
PCHSEQ
Sequencing VIN
PWM
DRVH
DRVL
COMP
COMPA
COMPB
Vout
Boost
PCHSEQ
Sequencing
PWM
VDDP
DRVL
COMPENSATION
CSH
CSL
FB
RCS
COMP
Figure 11: Boost Channel
Vout
Figure 13 – Buck or boost channel configured as
buck.
A type three-compensation network is used for this
voltage mode controller to provide optimal transient
response. Both configurations can operate in PWM or
PFM mode. In addition, when configured a buck the
output is allowed to reach a 100% duty cycle and
operate in a low dropout mode.
This is not to be confused with a Buck-Boost converter
that can act as a buck if the output is below the input or
a Boost if the output is below the input.
Summit Microelectronics, Inc
2107 3.0 10/15/2008
27
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