LT3758 datasheet(14/20 Pages) LINER

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LTC3872-1

38721f

For more information www.linear.com/LTC3872-1

is 11.5A. With a maximum sense voltage of about 140mV,

the sense resistor value would be 10m

Ω, and the power

dissipated in this resistor would be 514mW at maximum

output current. Assuming an efficiency of 90%, this

sense resistor power dissipation represents 1.3% of the

overall input power. In other words, for this application,

the use of VDS sensing would increase the efficiency by

approximately 1.3%.

For more details regarding the various terms in these

equations, please refer to the section Boost Converter:

Power MOSFET Selection.

3. The losses in the inductor are simply the DC input cur-

rentsquaredtimesthewindingresistance.Expressingthis

loss as a function of the output current yields:

PR(WINDING) =

IO(MAX)

1– DMAX









•RW

4. Losses in the boost diode. The power dissipation in the

boost diode is:

PDIODE = IO(MAX) • VD

The boost diode can be a major source of power loss in

a boost converter. For the 3.3V input, 5V output at 7A ex-

ample given above, a Schottky diode with a 0.4V forward

voltage would dissipate 2.8W, which represents 7% of the

input power. Diode losses can become significant at low

output voltages where the forward voltage is a significant

percentage of the output voltage.

5. Other losses, including CIN and CO ESR dissipation and

inductor core losses, generally account for less than 2%

of the total additional loss.

Checking Transient Response

The regulator loop response can be verified by looking at

theloadtransientresponse.Switchingregulatorsgenerally

take several cycles to respond to an instantaneous step

in resistive load current. When the load step occurs, VO

immediately shifts by an amount equal to (

DILOAD)(ESR),

and then CO begins to charge or discharge (depending on

the direction of the load step) as shown in Figure 7. The

regulator feedback loop acts on the resulting error amp

output signal to return VO to its steady-state value. During

this recovery time, VO can be monitored for overshoot or

ringing that would indicate a stability problem.

A second, more severe transient can occur when con-

necting loads with large (>1µF) supply bypass capacitors.

The discharged bypass capacitors are effectively put in

parallel with CO, causing a nearly instantaneous drop in

VO. No regulator can deliver enough current to prevent

this problem if the load switch resistance is low and it is

driven quickly. The only solution is to limit the rise time

of the switch drive in order to limit the inrush current

di/dt to the load.

Boost Converter Design Example

Thedesignexamplegivenherewillbeforthecircuitshown

on the front page. The input voltage is 3.3V, and the output

is 5V at a maximum load current of 2A.

1. The duty cycle is:

D =

VO + VD – VIN

VO + VD







 =

5+ 0.4 – 3.3

5+ 0.4

= 38.9%

2. An inductor ripple current of 40% of the maximum load

current is chosen, so the peak input current (which is also

the minimum saturation current) is:

IIN(PEAK) = 1+









•

IO(MAX)

1– DMAX

= 1.2 •

1– 0.39

= 3.9A

The inductor ripple current is:

∆IL = c•

IO(MAX)

1–DMAX

= 0.4•

1– 0.39

=1.3A

And so the inductor value is:

L =

VIN(MIN)

∆IL • f

•DMAX =

3.3V

1.3A •550kHz

•0.39 =1.8µH

The component chosen is a 2.2µH inductor made by

Sumida (part number CEP125-H 1ROMH).

applicaTions inForMaTion

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