SMB119N View Datasheet(PDF) - Summit Microelectronics
Part Name
Description
MFG CO.
'SMB119N' PDF : 34 Pages View PDF
SMB119
APPLICATIONS INFORMATION (CONTINUED)
BOOST CONTROLLERS
The SMB119 has two asynchronous current mode
Boost converters with over-current protection and either
a PWM or PFM mode of operation.
When configured as a current mode Boost, a sense
resistor must be added, externally, in series with the
source of the N-channel MOSFET, see Figure 15. The
over-current circuitry is identical to that descried for the
Buck converter, and 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.
The driver supplies for the boost converters are
powered from the VDDP supply pin. Therefore, without
voltage on the VDDP input the Boost converters will not
function.
The output of all Boost channels is 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 converter, when the
inductor current does not go to zero Amperes during
the cycle (CCM), the relation between the duty cycle
and the output voltage is determined by Equation 2:
Equation 2: Vo
=
1
1
−
D
* Vin
The maximum duty cycle the boost converter can
achieve is determined by the max duty cycle spec in
the electrical specification section of the datasheet.
BOOST OR BUCK CONTROLLER
The SMB119 has one voltage mode output that can be
configured as either a Boost or a Buck converter, but
not both; see Figures 16 and 17. When configured as a
Buck the output voltage can only be less than or equal
to the input voltage. When a hardware modification is
preformed, the output can function as a Boost, whose
output voltage is greater than the input voltage. As a
voltage mode converter, this channel has no inherent
over current protection and requires a type three-
compensation network. Since the FETs are external for
this channel, the output current capabilities can be
scaled by choosing larger components.
VIN
Boost
PCHSEQ
Sequencing
PWM
VDDP
DRVL
COMPENSATION
CSH
CSL
FB
RCS
COMP
Vout
Figure 15: Boost Converter
VIN
Boost
PCHSEQ
Sequencing VIN
PWM
Dx RVH
DRVL
COMP
COMPA
COMPB
Vout
Figure 16: Buck or Boost configures as boost
Buck
VIN
PCHSEQ
Sequencing VIN
PWM
DRVH
DRVL
Vout
COMP
COMPA
COMPB
Figure 17: Buck or Boost configures as buck
Summit Microelectronics, Inc
23
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