ADP1821ARQZ-R7 datasheet(13/24 Pages) AD

13 / 24 page

ADP1821

Rev. B | Page 13 of 24

In the case of output capacitors where the impedance of the

ESR and ESL are small at the switching frequency, for instance,

where the output capacitor is a bank of parallel MLCC capaci-

tors, the capacitive impedance dominates and the ripple

equation reduces to

OUT

≅

(7)

Make sure that the ripple current rating of the output capacitors

is greater than the maximum inductor ripple current.

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 character-

istics. 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 carries all the transition losses of the converter. Typically,

the lower the MOSFET 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

(8)

where:

PC = conduction power loss.

RDSON = MOSFET on resistance.

The gate charging loss is approximated by the equation

PVCC

P ≅

(9)

where:

PG = gate charging loss power.

VPVCC = gate driver supply voltage.

QG = MOSFET total gate charge.

fSW = converter switching frequency.

The high-side MOSFET transition loss is approximated by the

equation

(

)

LOAD

(10)

where:

PT = high-side MOSFET switching loss power.

tR = MOSFET rise time.

tF = MOSFET fall time.

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

sum of all the previous losses, or

≅

(11)

where PD 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. A 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 above, calculate

the MOSFET junction temperature rise over the ambient

temperature of interest.

TJ = TA + θJAPD

(12)

Then calculate the new RDSON from the temperature coeffi-

cient curve and the RDSON spec at 25°C. A typical value of the

temperature coefficient (TC) of the RDSON is 0.004/°C, so an

alternate method to calculate the MOSFET RDSON at a second

temperature, TJ, is

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

(13)

Then the conduction losses can be recalculated and the pro-

cedure iterated once or twice 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

(14)

where:

PLS is the low-side MOSFET on resistance.

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

Check the gate charge losses of the synchronous rectifier

using the PG equation (Equation 9) to be sure it is reasonable.

If multiple low-side MOSFETs are used in parallel, then use

the parallel combination of the on resistances for determining

RDSON to solve this equation.

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