SP6126EK1-L datasheet(7/14 Pages) SIPEX

Part #	SP6126EK1-L

Description	High-Voltage, Step Down Controller in TSOT6

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Manufacturer	SIPEX [Sipex Corporation]

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7 / 14 page

Mar 29-07 RevD

SP6126: TSOT-6 PFET Buck Controller

 2007 Sipex Corporation

7

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) has to

be selected such that when operating at peak

current

and

junction

temperature

the

Overcurrent threshold of the SP6126 is not

exceeded.

Allowing

50%

for

temperature

coefficient of RDS(ON) and 15% for inductor

current ripple, the following expression can be

used:













×

≤

Iout

mV

ON

RDS

15

.

1

5

.

1

300

)

(

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:













×

=

Vin

Vout

ON

RDS

Iout

Pcond

)

(

MOSFET’s

junction

temperature

can

be

estimated from:

(

) Tambient

Rthja

Pc

T

+

×

= 2

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:













−

×

=

Vin

Vout

Iout

Vf

Pc

1

where:

Vf is diode forward voltage at IOUT

Schottky’s 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:

(

)













×













×













×

−

=

Irip

f

Vin

Vout

Vin

L

1

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 ½ 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 ½ 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/undershoot caused by current step

load.

A

steady-state

output

current

IOUT

corresponds to inductor stored energy of ½ L IOUT

2.

GENERAL OVERVIEW

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