SP6127 datasheet(7/12 Pages) EXAR

7 / 12 page

Jan28-08 RevE

SP6127 TSOT-6 PFET Buck Controller

 2007 Exar Corporation

Page 7

______________________________________________________ GENERAL OVERVIEW

MOSFET Gate Drive

The P-channel drive is derived through an

internal regulator that generates VIN-5V. This pin

(VDR) must be connected to VIN with a 0.1µF

decoupling capacitor. The gate drive circuit

swings between VIN and VIN-5 and employs

powerful drivers for efficient switching of the P-

channel MOSFET.

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. The 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

SP6127 is not exceeded. Allowing 50% for

temperature coefficient of RDS(ON) and 15% for

inductor current ripple, the following expression

can be used:













≤

Iout

RDS

)

(

Within this constraint, selecting MOSFETs with

lower RDS(ON) will reduce conduction losses at

the expense of increased switching losses. As a

rule

thumb,

select

the

highest

RDS(ON)

MOSFET that meets the above criteria. Switching

losses can be assumed to roughly equal to the

conduction losses. A simplified expression for

conduction losses is given by:













Vin

Vout

RDS

Iout

Pcond

)

(

The MOSFET’s junction temperature can be

estimated from:

(

) Tambient

Rthja

= 2

Schottky Rectifier selection

Select the Schottky Diode for Voltage rating VR,

Forward voltage Vf, and thermal resistance

RTHJA. The 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 diode 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

where:

VF is diode forward voltage at IOUT

The Schottky diode’s AC losses due to its

switching capacitance are negligible.

Inductor Selection

Select

the

Inductor

for

inductance

and

saturation current ISAT. Select an inductor with

ISAT higher than the programmed overcurrent.

Calculate inductance from:

(

)





































−

Irip

Vin

Vout

Vin

where:

VIN is converter input voltage

VOUT is converter output voltage

f is switching frequency

IRIP

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.

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