ADP1828YRQZ-R7 datasheet(19/32 Pages) AD

19 / 32 page

ADP1828

Rev. 0 | Page 19 of 32

During a load step transient on the output, the output capacitor

supplies the load until the control loop has a chance to ramp the

inductor current. This initial output voltage deviation, due to a

change in load, is dependent on the output capacitor charac-

teristics. Again, usually the capacitor ESR dominates this

response, and the ΔVOUT in Equation 6 can be used with the

load step current value for ΔIL.

SELECTING THE MOSFETS

The choice of MOSFET directly affects the dc-to-dc converter

performance. The MOSFET must have low on resistance to

reduce I2R losses and low gate charge to reduce transition losses.

In addition, the MOSFET must have low thermal resistance to

ensure that the power dissipated in the MOSFET does not result

in excessive MOSFET die temperature.

The high-side MOSFET carries the load current during on-time

and usually carries most of the transition losses of the converter.

Typically, the lower the MOSFET’s on resistance, the higher the

gate charge and vice versa. Therefore, it is important to choose a

high-side MOSFET that balances the two losses. The conduction

loss of the high-side MOSFET is determined by the equation

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

≅

OUT

DSON

LOAD

)

(

(7)

where:

PC is the conduction power loss.

RDSON is the MOSFET on resistance.

The gate charging loss is approximated by the equation

P ≅

(8)

where:

PG is the gate charging loss power.

VPV is the gate driver supply voltage.

QG is the MOSFET total gate charge.

fSW is the converter switching frequency.

The high-side MOSFET transition loss is approximated by the

equation

)

(

LOAD

(9)

where:

PT is the high-side MOSFET switching loss power.

tR is the MOSFET rise time.

tF is the MOSFET fall time.

The total power dissipation of the high-side MOSFET is the

sum of all the previous losses, or

≅

(10)

where PHS is the total high-side MOSFET power loss.

The conduction losses may need an adjustment to account

for the MOSFET RDSON variation with temperature. Note that

MOSFET RDSON increases with increasing temperature. The

MOSFET data sheet should list the thermal resistance of the

package, θJA, along with a normalized curve of the temperature

coefficient of the RDSON. For the power dissipation estimated in

Equation 10, calculate the MOSFET junction temperature rise

over the ambient temperature of interest:

TJ = TA + θJAPD

(11)

Then, calculate the new RDSON from the temperature coefficient

curve and the RDSON specification at 25°C. An alternate method

to calculate the MOSFET RDSON at a second temperature, TJ, is

RDSON @ TJ = RDSON @ 25°C (1 + TC(TJ − 25°C))

(12)

where TC is the temperature coefficient of the MOSFET’s RDSON,

and its typical value is 0.004/°C.

Then the conduction losses can be recalculated and the proce-

dure iterated until the junction temperature calculations are

relatively consistent.

The synchronous rectifier, or low-side MOSFET, carries the

inductor current when the high-side MOSFET is off. The low-

side MOSFET transition loss is small and can be neglected in

the calculation. For high input voltage and low output voltage,

the low-side MOSFET carries the current most of the time.

Therefore, to achieve high efficiency, it is critical to optimize

the low-side MOSFET for low on resistance. In cases where the

power loss exceeds the MOSFET rating or lower resistance is

required than is available in a single MOSFET, connect multiple

low-side MOSFETs in parallel. The equation for low-side MOSFET

power loss is

⎥

⎦

⎤

⎢

⎣

⎡

−

≅

OUT

DSON

LOAD

)

(

(13)

where:

PLS is the total low-side MOSFET power loss.

RDSON is the total on resistance of the low-side MOSFET(s).

Check the gate charge losses of the synchronous rectifier using

Equation 8 to be sure it is reasonable. If multiple low-side

MOSFETs are used in parallel, then use the parallel combina-

tion of the on resistances for determining RDSON to solve this

equation.

ADP1828YRQZ-R7 数据表(PDF) 19 Page - Analog Devices

ADP1828YRQZ-R7 数据表(HTML) 19 Page - Analog Devices

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

数据表下载

链接网址

ADP1828YRQZ-R7 数据表(PDF) 19 Page - Analog Devices

ADP1828YRQZ-R7 数据表(HTML) 19 Page - Analog Devices

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

数据表 下载

链接网址

数据表下载