MAX17021 datasheet(44/48 Pages) MAXIM

44 / 48 page

For most applications, nontantalum chemistries (ceram-

ic, aluminum, or OS-CON) are preferred due to their

resistance to inrush surge currents typical of systems

with a mechanical switch or connector in series with the

input. If the Quick-PWM controller is operated as the

second stage of a two-stage power-conversion system,

tantalum input capacitors are acceptable. In either con-

figuration, choose an input capacitor that exhibits less

than +10°C temperature rise at the RMS input current

for optimal circuit longevity.

Power-MOSFET Selection

Most of the following MOSFET guidelines focus on the

challenge of obtaining high load-current capability

when using high-voltage (> 20V) AC adapters. Low-

current applications usually require less attention.

The high-side MOSFET (NH) must be able to dissipate

the resistive losses plus the switching losses at both

VIN(MIN) and VIN(MAX). Calculate both these sums.

Ideally, the losses at VIN(MIN) should be approximately

equal to losses at VIN(MAX), with lower losses in

between. If the losses at VIN(MIN) are significantly high-

er than the losses at VIN(MAX), consider increasing the

size of NH (reducing RDS(ON) but with higher CGATE).

Conversely, if the losses at VIN(MAX) are significantly

higher than the losses at VIN(MIN), consider reducing

the size of NH (increasing RDS(ON) to lower CGATE). If

VIN does not vary over a wide range, the minimum

power dissipation occurs where the resistive losses

equal the switching losses.

Choose a low-side MOSFET that has the lowest possible

on-resistance (RDS(ON)), comes in a moderate-sized

package (i.e., one or two 8-pin SOs, DPAK, or D2PAK),

and is reasonably priced. Make sure that the DL_ gate

driver can supply sufficient current to support the gate

charge and the current injected into the parasitic gate-

to-drain capacitor caused by the high-side MOSFET

turning on; otherwise, cross-conduction problems might

occur (see the

MOSFET Gate Drivers section).

MOSFET Power Dissipation

Worst-case conduction losses occur at the duty factor

extremes. For the high-side MOSFET (NH), the worst-

case power dissipation due to resistance occurs at the

minimum input voltage:

where

ηTOTAL is the total number of phases.

Generally, a small high-side MOSFET is desired to

reduce switching losses at high input voltages.

However, the RDS(ON) required to stay within package

power dissipation often limits how small the MOSFET

can be. Again, the optimum occurs when the switching

losses equal the conduction (RDS(ON)) losses. High-

side switching losses do not usually become an issue

until the input is greater than approximately 15V.

Calculating the power dissipation in high-side MOSFET

(NH) due to switching losses is difficult since it must

allow for difficult quantifying factors that influence the

turn-on and turn-off times. These factors include the

internal gate resistance, gate charge, threshold volt-

age, source inductance, and PCB layout characteris-

tics. The following switching-loss calculation provides

only a very rough estimate and is no substitute for

breadboard evaluation, preferably including verification

using a thermocouple mounted on NH:

where COSS is the NH MOSFET’s output capacitance,

QG(SW) is the charge needed to turn on the NH

MOSFET, and IGATE is the peak gate-drive source/sink

current (2.2A typ).

Switching losses in the high-side MOSFET can become

an insidious heat problem when maximum AC adapter

voltages are applied due to the squared term in the C x

VIN2 x ƒSW switching-loss equation. If the high-side

MOSFET chosen for adequate RDS(ON) at low-battery

voltages becomes extraordinarily hot when biased from

VIN(MAX), consider choosing another MOSFET with

lower parasitic capacitance.

For the low-side MOSFET (NL), the worst-case power

dissipation always occurs at maximum input voltage:

The worst case for MOSFET power dissipation occurs

under heavy overloads that are greater than

ILOAD(MAX) but are not quite high enough to exceed

PD N

sistive

OUT

IN MAX

(Re

)

()

⎛

⎝

⎜

⎞

⎠

⎟

⎡

⎣

⎢1-

⎢⎢

⎤

⎦

⎥

⎛

⎝⎜

⎞

⎠⎟

LOAD

TOTAL

DS ON

()

PD N Switching

IN MAX LOAD SW

TOTAL

()

⎛

⎝⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

GSW

GATE

OSS IN SW

()

PD N

sistive

OUT

LOAD

TOTA

(Re

)

⎛

⎝⎜

⎞

⎠⎟ η

DS ON

⎛

⎝⎜

⎞

⎠⎟

()

Dual-Phase, Quick-PWM Controllers for

IMVP-6+/IMVP-6.5 CPU Core Power Supplies

______________________________________________________________________________________

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