SP6125 Datasheet, PDF(7/12 Page) Sipex Corporation – High-Voltage, Step Down Controller in TSOT6

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SP6125 Datasheet, PDF (7/12 Pages) Sipex Corporation – High-Voltage, Step Down Controller in TSOT6

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Power MOSFET Selection

Select the Power MOSFET for Voltage rating

BVDSS, On resistance RDS(ON), and thermal

resistance Rthja. BVDSS should be about twice

as high as VIN in order to guard against

switching transients. Recommended MOSFET

voltage rating for VIN of 5V, 12V and 24V is

12V, 30V and 40V respectively. RDS(ON), must

be selected such that when operating at peak

current and junction temperature the

Overcurrent threshold of the SP6125 is not

exceeded. Allowing 50% for temperature

coefficient of RDS(ON) and 15% for inductor

current ripple, the following expression can be

used:

RDS

(ON

)

â¤

ï£¬ï£«

ï£ 1.5

300mV

Ã1.15 Ã Iout

ï£·ï£¶

ï£¸

Within this constraint, selecting MOSFETs with

lower RDS(ON) will reduce conduction losses at

the expense of increased switching losses. As

a rule of thumb select the highest RDS(ON)

MOSFET that meets the above criteria.

Switching losses can be assumed to roughly

equal the conduction losses. A simplified

expression for conduction losses is given by:

Pcond

Iout Ã

RDS

(ON

)

ï£¬ï£«

ï£

Vout

Vin

ï£·ï£¶

ï£¸

MOSFETâs junction temperature can be

estimated from:

T = (2 Ã Pc Ã Rthja) + Tambient

Schottky Rectifier selection

Select the Schottky for Voltage rating VR,

Forward voltage Vf, and thermal resistance

Rthja. Voltage rating should be selected using

the same guidelines outlined for MOSFET

voltage selection. For a low duty cycle

application such as the circuit shown on first

page, the Schottky is conducting most of the

time and its conduction losses are the largest

component of losses in the converter.

Conduction losses can be estimated from:

Iout

ï£¬ï£ï£«1 â

Vout

Vin

ï£·ï£¶

ï£¸

General Overview

where:

Vf is diode forward voltage at IOUT

Schottkyâs AC losses due to its switching

capacitance are negligible.

Inductor Selection

Select the Inductor for inductance L and

saturation current Isat. Select an inductor with

Isat higher than the programmed overcurrent.

Calculate inductance from:

(Vin

Vout

)Ã

ï£¬ï£«

ï£

Vout

Vin

ï£·ï£¶

ï£¸

ï£¬ï£¬ï£ï£«

ï£·ï£·ï£¸ï£¶

ï£¬ï£¬ï£ï£«

Irip

ï£·ï£·ï£¸ï£¶

where:

VIN is converter input voltage

VOUT is converter output voltage

f is switching frequency

IRIP is inductor peak-to-peak current ripple

(nominally set to 30% of IOUT)

Keep in mind that a higher IRIP results in a

smaller inductor which has the advantages of

small size, low DC equivalent resistance DCR,

high saturation current Isat and allows the use

of a lower output capacitance to meet a given

step load transient. A higher Irip, however,

increases the output voltage ripple and

increases the current at which converter enters

Discontinuous Conduction Mode. The output

current at which converter enters DCM is Â½ of

IRIP. Note that a negative current step load that

drives the converter into DCM will result in a

large output voltage transient. Therefore the

lowest current for a step load should be larger

than Â½ of IRIP.

Output Capacitor Selection

Select the output capacitor for voltage rating,

capacitance and Equivalent Series Resistance

(ESR). Nominally the voltage rating is selected

to be twice as large as the output voltage.

Select the capacitance to satisfy the

specification for output voltage overshoot or

undershoot caused by current step load. A

steady-state output current IOUT corresponds to

inductor stored energy of Â½ L IOUT2. A sudden

decrease in IOUT forces the energy surplus in L

to be absorbed by COUT.

Jan28-08 RevG

SP6125: TSOT-6 PFET Buck Controller

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