ADP1876ACPZ-R7 datasheet(20/24 Pages) AD

20 / 24 page

ADP1876

Data Sheet

Rev. A | Page 20 of 24

Usually, the impedance is dominated by ESR, such as in

electrolytic or polymer capacitors, at the switching frequency, as

stated in the maximum ESR rating on the capacitor data sheet;

therefore, output ripple reduces to

ESR

OUT

∆

≅

∆

Electrolytic capacitors also have significant ESL, on the order

of 5 nH to 20 nH, depending on type, size, and geometry. PCB

traces contribute some ESR and ESL, as well. However, using

the maximum ESR rating from the capacitor data sheet often

provides enough margin such that measuring the ESL is not

usually required.

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 capacitors, the

capacitive impedance dominates and the output capacitance

equation reduces to

OUT

∆

≅

Ensure that the ripple current rating of the output capacitors is

greater than the maximum inductor ripple current.

For example, during a load step transient on the output, when

the load is suddenly increased, the output capacitor supplies the

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

current. This initial output voltage deviation results in a voltage

droop or undershoot. The output capacitance (assuming 0 Ω

ESR) that is required to satisfy the voltage droop requirement

can be approximated by

DROOP

STEP

OUT

∆

≅

where:

∆ISTEP is the step load.

∆VDROOP is the voltage droop at the output.

When a load is suddenly removed from the output, the energy

stored in the inductor rushes into the capacitor, causing the

output to overshoot. The output capacitance required to satisfy

the output overshoot requirement can be approximated by

)

(

OUT

OVERSHOOT

OUT

STEP

OUT

−

∆

≅

where:

∆VOVERSHOOT is the overshoot voltage during the step load.

Select the largest output capacitance given by any of the

previous three equations.

MOSFET SELECTION

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

performance. A MOSFET with low on resistance reduces I2R

losses, and a low gate charge reduces transition losses. A MOSFET

that has low thermal resistance ensures that the power dissipated

in the MOSFET does not result in excessive MOSFET die tem-

perature.

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 on resistance of the MOSFET, 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

)

(

where:

RDSON is the MOSFET on resistance.

The gate charging loss is approximated by the equation

PG ≅ VPV × QG × fSW

where

VPV is the gate driver supply voltage.

QG is the MOSFET total gate charge.

Note that the gate charging power loss is not dissipated in the

MOSFET but rather in the ADP1876 internal drivers. This

power loss must be considered when calculating the overall

power efficiency.

The high-side MOSFET transition loss is approximated by the

equation

)

(

LOAD

≅

where:

PT is the high-side MOSFET switching loss power.

tR is the rise time in charging the high-side MOSFET.

tF is the fall time in discharging the high-side MOSFET.

tR and tF can be estimated by the following equations:

RISE

DRIVER

GSW

≅

FALL

DRIVER

GSW

≅

where:

QGSW is the gate charge of the MOSFET during switching and is

given in the MOSFET data sheet.

IDRIVER_RISE and IDRIVER_FALL are the driver current output by the

ADP1876 internal gate drivers.

If QGSW is not given in the data sheet, it can be approximated by

GSW

≅

where QGD and QGS are the gate-to-drain and gate-to-source

charges given in the MOSFET data sheet.

IDRIVER_RISE and IDRIVER_FALL can be estimated by

GATE

SOURCE

RISE

DRIVER

−

≅

ADP1876ACPZ-R7 Datenblatt(PDF) 20 Page - Analog Devices

ADP1876ACPZ-R7 Datenblatt(HTML) 20 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

Datenblatt Download

Link URL