ADP1879ACPZ-1.0-R7 datasheet(27/40 Pages) AD

27 / 40 page

Data Sheet

ADP1878/ADP1879

Rev. A | Page 27 of 40

The amount of loss through the body diode of the low-side

MOSFET during the anti overlap state is given by the following

expression:

where:

tBODY(LOSS) is the body conduction time (refer to Figure 84 for

dead time periods).

tSW is the period per switching cycle.

VF is the forward drop of the body diode during conduction.

(Refer to the selected external MOSFET data sheet for more

information about the VF parameter.)

Figure 84. Body Diode Conduction Time vs. Low Voltage Input (VREG)

Inductor Loss

During normal conduction mode, further power loss is caused

by the conduction of current through the inductor windings,

which have dc resistance (DCR). Typically, larger sized inductors

have smaller DCR values.

The inductor core loss is a result of the eddy currents generated

within the core material. These eddy currents are induced by the

changing flux, which is produced by the current flowing through

the windings. The amount of inductor core loss depends on the

core material, the flux swing, the frequency, and the core volume.

Ferrite inductors have the lowest core losses, whereas powdered iron

inductors have higher core losses. It is recommended to use shielded

ferrite core material type inductors with the ADP1878/ADP1879

for a high current, dc-to-dc switching application to achieve

minimal loss and negligible electromagnetic interference (EMI).

INPUT CAPACITOR SELECTION

The goal in selecting an input capacitor is to reduce or minimize

input voltage ripple and to reduce the high frequency source

impedance, which is essential for achieving predictable loop

stability and transient performance.

The problem with using bulk capacitors, other than their physical

geometries, is their large equivalent series resistance (ESR) and

large equivalent series inductance (ESL). Aluminum electrolytic

capacitors have such high ESR that they cause undesired input

voltage ripple magnitudes and are generally not effective at high

switching frequencies.

If bulk electrolytic capacitors are used, it is recommended to use

multilayered ceramic capacitors (MLCC) in parallel due to their

low ESR values. This dramatically reduces the input voltage ripple

amplitude as long as the MLCCs are mounted directly across the

drain of the high-side MOSFET and the source terminal of the

low-side MOSFET (see the Layout Considerations section).

Improper placement and mounting of these MLCCs may cancel

their effectiveness due to stray inductance and an increase in

trace impedance.

The maximum input voltage ripple and maximum input capacitor

rms current occur at the end of the duration of 1 − D while the

high-side MOSFET is in the off state. The input capacitor rms

current reaches its maximum at time D. When calculating the

maximum input voltage ripple, account for the ESR of the input

capacitor as follows:

VMAX,RIPPLE = VRIPP + (ILOAD,MAX × ESR)

where:

VRIPP is usually 1% of the minimum voltage input.

ILOAD,MAX is the maximum load current.

ESR is the equivalent series resistance rating of the input capacitor.

Inserting VMAX,RIPPLE into the charge balance equation to

calculate the minimum input capacitor requirement gives

where D = 50%.

THERMAL CONSIDERATIONS

The ADP1878/ADP1879 are used for dc-to-dc, step down, high

current applications that have an on-board controller, an on-board

LDO, and on-board MOSFET drivers. Because applications may

require up to 20 A of load current and be subjected to high ambient

temperature, the selection of external high- and low-side MOSFETs

must be associated with careful thermal consideration to not

exceed the maximum allowable junction temperature of 125°C.

To avoid permanent or irreparable damage, if the junction temper-

ature reaches or exceeds 155°C, the part enters thermal shutdown,

turning off both external MOSFETs, and is not reenabled until

the junction temperature cools to 140°C (see the On-Board Low

Dropout (LDO) Regulator section).

In addition, it is important to consider the thermal impedance

of the package. Because the ADP1878/ADP1879 employ an

on-board LDO, the ac current (fxCxV) consumed by the internal

drivers to drive the external MOSFETs, adds another element of

2.7

5.5

4.8

4.1

3.4

VREG (V)

+125°C

+25°C

–40°C

1MHz

300kHz

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