LTC3402 datasheet(20/32 Pages) LINER

20 / 32 page

LTC1871X

1871xf

For more information www.linear.com/LTC1871X

APPLICATIONS INFORMATION

then off, a packet of gate charge QG is transferred from

INTVCC to ground. The resulting dQ/dt is a current that

must be supplied to the INTVCC capacitor through the

VIN pin by an external supply. If the IC is operating in

CCM:

IQ(TOT) ≈ IQ = f • QG

PIC = VIN • (IQ + f • QG)

2. Power MOSFET switching and conduction losses. The

technique of using the voltage drop across the power

MOSFET to close the current feedback loop was chosen

because of the increased efficiency that results from

not having a sense resistor. The losses in the power

MOSFET are equal to:

PFET =

IO(MAX)

1– DMAX

⎛

⎝⎜

⎞

⎠⎟

• RDS(ON) •DMAX •ρT

k • VO1.85 •

IO(MAX)

1– DMAX

(

)

• CRSS • f

The I2R power savings that result from not having a

discrete sense resistor can be calculated almost by

inspection.

PR(SENSE) =

IO(MAX)

1–DMAX

⎛

⎝⎜

⎞

⎠⎟

•RSENSE •DMAX

To understand the magnitude of the improvement with

this VDS sensing technique, consider the 3.3V input,

5V output power supply shown in Figure 1. The maxi-

mum load current is 7A (10A peak) and the duty cycle

is 39%. Assuming a ripple current of 40%, the peak

inductor current is 13.8A and the average is 11.5A.

With a maximum sense voltage of about 140mV, the

sense resistor value would be 10mΩ, and the power

dissipated in this resistor would be 514mW at maxi-

mum 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 ap-

plication, 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-

rent squared times the winding resistance. Expressing

this 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 example given above, a Schottky diode with a 0.4V

forwardvoltagewoulddissipate2.8W,whichrepresents

7% of the input power. Diode losses can become signifi-

cant 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 (

∆ILOAD)(ESR),

and then CO begins to charge or discharge (depending on

the direction of the load step) as shown in Figure 12. 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.

Figure 12. Load Transient Response for a 3.3V Input,

5V Output Boost Converter Application, 0.7A to 7A Step

IOUT

2V/DIV

VOUT (AC)

100mV/DIV

100µs/DIV

1871x F12

VIN = 3.3V

VOUT = 5V

MODE/SYNC = INTVCC

(PULSE-SKIP MODE)

LTC3402 датащи(PDF) 20 Page - Linear Technology

LTC3402 датащи(HTML) 20 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

датащи скачать

ссылки URL