SMB218 View Datasheet(PDF) - Summit Microelectronics

Part Name

Description

Manufacturer

SMB218

Six-Channel Programmable DC-DC Power Managers with Battery Charger

Summit Microelectronics 

PLEASE NOTE: The SMB118 has entered End-of-Life.

SMB118/218

APPLICATIONS INFORMATION (CONTINUED)

MOSFETS

When selecting the appropriate FET to use attention

must be paid to the gate to source rating, input

capacitance, and maximum power dissipation.

Most FETs are specified by an on resistance (RDSON)

for a given gate to source voltage (VGS). It is essential

to ensure that the FETs used will always have a VGS

voltage grater then the minimum value shown on the

datasheet. It is worth noting that the specified VGS

voltage must not be confused with the threshold voltage

of the FET.

The input capacitance must be chosen such that the

rise and fall times specified in the datasheet do not

exceed ~5% of the switching period.

To ensure the maximum load current will not exceed

the power rating of the FET, the power dissipation of

each FET must be determined. It is important to look at

each FET individually and then add the power

dissipation of complementary FETs after the power

dissipation over one cycle has been determined. The

Power dissipation can be approximated as follows:

Equation 6: P ~ RDSON * I L 2 *TON

Where TON is the on time of the primary switch. TON can

be calculated as follows:

Equations 7, 8, 9:

Buck − NFET : (1− VO ) *T

VIN

Buck − PFET : VO *T

VIN

Boost : (VO −VIN ) *T

VO

Compensation:

Summit provides a design tool to called Summit Power

Designer” that will automatically calculate the

compensation values for a design or allow the system

to be customized for a particular application. The power

designer software can be found at

http://www.summitmicro.com/prod_select/xls/SummitPo

werDesigner_Install.zip.

Summit Microelectronics, Inc

2107 3.0 10/15/2008

29

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