ADP1823ACPZ-R7 datasheet(17/32 Pages) AD

17 / 32 page

ADP1823

Rev. A | Page 17 of 32

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 cap is a bank of parallel MLCC capacitors, 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 characteristics.

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

(RDSON) to reduce I2R losses and low gate-charge to reduce

switching losses. In addition, the MOSFET must have low

thermal resistance to ensure that the power dissipated in the

MOSFET does not result in overheating.

The power switch, or high-side MOSFET, carries the load

current during the PWM on-time, carries the transition loss of

the switching behavior, and requires gate charge drive to switch.

Typically, the smaller the MOSFET RDSON, the higher the gate

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

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

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

OUT

DSON

≅

(8)

where:

PC = conduction power loss.

RDSON = MOSFET on resistance.

The gate charge losses are dissipated by the ADP1823 regulator

and gate drivers and affect the efficiency of the system. The gate

charge loss is approximated by the equation

P ≅

(9)

where:

PG = gate charge power.

QG = MOSFET total gate charge.

fSW = converter switching frequency.

Making the conduction losses balance the gate charge losses

usually yields the most efficient choice.

Furthermore, the high-side MOSFET transition loss is

approximated by the equation

(

)

≅

(10)

where tR and tF are the rise and fall times of the selected

MOSFET as stated in the MOSFET data sheet.

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

sum of the previous losses:

(11)

where PD is the total high-side MOSFET power loss. This

dissipation heats the high-side MOSFET.

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

above, calculate the MOSFET junction temperature rise over

the ambient temperature of interest:

(12)

Then calculate the new RDSON from the temperature coefficient

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

)]

(

[

−

DSON

(13)

Then the conduction losses can be recalculated and the

procedure 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. For high

input voltage and low output voltage, the low-side MOSFET

carries the current most of the time, and therefore, to achieve

high efficiency it is critical to optimize the low-side MOSFET

for small 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

(14)

where:

PLS is the low-side MOSFET on resistance.

RDSON is the parallel combination of the resistances of the low-

side MOSFETs.

Check the gate charge losses of the synchronous rectifier(s)

using the PG equation (Equation 9) to be sure they are

reasonable.

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