SC2453_05 datasheet(14/22 Pages) SEMTECH

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SC2453

POWER MANAGEMENT

waveform. Assuming that the output current ripple is

small, the following formula can be used to estimate the

RMS value of the ripple current in the input capacitor.

Let the duty ratio and output current of Channel 1 and

Channel 2 be D

1, D2 and Io1, Io2, respectively.

If D

1<0.5 and D2<0.5, then:

Cin

≈

If D

1>0.5 and (D1-0.5) < D2<0.5, then:

Cin

(

)

)(

(

−

≈

If D

1>0.5 and D2 < (D1-0.5) < 0.5, then:

Cin

(

)

(

−

≈

If D

1>0.5 and D2 > 0.5, then:

Cin

(

)

)(

(

−

≈

Choosing Power MOSFET’s

Main considerations in selecting the MOSFET’s are power

dissipation, cost and packaging. Switching losses and

conduction losses of the MOSFET’s are directly related

to the total gate charge (C

g) and channel on-resistance

ds(on)). In order to judge the performance of MOSFET’s,

the product of the total gate charge and on-resistance is

used as a figure of merit (FOM). Transistors with the same

FOM follow the same curve in Figure 5.

The closer the curve is to the origin, the lower is the FOM.

This means lower switching loss or lower conduction loss

or both. It may be difficult to find MOSFET’s with both

low C

g and low Rds(on. Usually a trade-off between Rds(on

and C

g has to be made.

FOM:100*10^{-12}

FOM:200*10^{-12}

FOM:500*10^{-12}

On-resistance (mOhm)

Cg 100 Rds

()

Cg 200 Rds

()

Cg 500 Rds

()

Rds

Figure 5. Figure of Merit curves.

MOSFET selection also depends on applications. In many

applications, either switching loss or conduction loss

dominates for a particular MOSFET. For synchronous buck

converters with high input to output voltage ratios, the

top MOSFET is hard switched but conducts with very low

duty cycle. The bottom switch conducts at high duty cycle

but switches at near zero voltage. For such applications,

MOSFET’s with low C

g are used for the top switch and

MOSFET’s with low R

ds(on) are used for the bottom switch.

MOSFET power dissipation consists of

a) conduction loss due to the channel resistance R

ds(on),

b) switching loss due to the switch rise time t

r and fall

time t

f, and

c) the gate loss due to the gate resistance R

Top Switch:

The RMS value of the top switch current is calculated

as:

)

(

rms

The conduction losses are then:

tc = IQ1,rms

ds(on)

ds(on) varies with temperature and gate-source voltage.

Curves showing R

ds(on) variations can be found in

manufacturers’ data sheet. From the Si4860 datasheet,

ds(on) is less than 8mW when Vgs is greater than 10V.

However R

ds(on) increases by 50% as the junction

temperature increases from 25oC to 110oC.

The switching losses can be estimated using the simple

formula

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