LTC3872 datasheet(13/20 Pages) LINER

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LTC3872

3872fa

Power MOSFET Selection.

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

rent squared times the winding resistance. Expressing this

loss as a function of the output current yields:

R WINDING

OMAX

MAX

()

–

•

⎛

⎝⎜

⎞

⎠⎟

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

the load transient response. Switching regulators generally

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 (

ΔILOAD)(ESR),

and then CO begins to charge or discharge (depending on

the direction of the load step) as shown in Figure 6. 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

The design example given here will be for the circuit shown

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:

⎛

⎝⎜

⎞

⎠⎟

–. – .

50 4 3 3

50 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:

IN PEAK

OMAX

MAX

()

•

–

.•

–

⎛

⎝⎜

⎞

⎠⎟

The inductor ripple current is:

Δ=

OMAX

MAX

χ •

–

.•

–.

()

10 39

And so the inductor value is:

AkHz

IN MIN

MAX

()

•

.•

•.

1 3

550

039 1..8

μH

The component chosen is a 2.2μH inductor made by

Sumida (part number CEP125-H 1ROMH).

APPLICATIO S I FOR ATIO

Figure 6. Load Transient Response for a 3.3V Input,

5V Output Boost Converter Application, 0.1A to 1A Step

500mA/DIV

VOUT

200mV/DIV

AC COUPLED

μs/DIV

3872 F06

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