Step-Down Controllers with Prebias Startup,

Lossless Sensing, Synchronization, and OVP

______________________________________________________________________________________

21

MOSFET Selection

The MAX8543/MAX8544 drive two or four external,

logic-level, n-channel MOSFETs as the circuit switch

elements. The key selection parameters are:

be at least 20% higher than the input supply rail at

the high-side MOSFET’s drain.

For a 3.3V input application, choose a MOSFET with a

≤ 4.5V.

For a good compromise between efficiency and cost,

choose the high-side MOSFET (N1, N2) that has conduc-

tion losses equal to the switching loss at nominal input

voltage and output current. The selected low-side

current-limit-setting condition above. Ensure that the low-

side MOSFET does not spuriously turn on due to dV/dt

caused by the high-side MOSFET turning on as this

would result in shoot-through current and degrade the

higher immunity to dV/dt. For high-current applications, it

is often preferable to parallel two MOSFETs rather than to

use a single large MOSFET.

For proper thermal-management design, the power dis-

sipation must be calculated at the desired maximum

operating junction temperature, maximum output current,

and worst-case input voltage (for the low-side MOSFET,

and low-side MOSFETs have different loss components

due to the circuit operation. The low-side MOSFET oper-

ates as a zero-voltage switch; therefore, major losses

the dead time between high-side and low-side switching

The high-side MOSFET operates as a duty-cycle control

switch and has the following major losses: the channel-

MOSFET does not have body-diode conduction loss

because the diode never conducts current:

capability determined by:

on-resistance (1

resistance of the MOSFET (

≈0.5Ω to 3Ω):

In addition to the losses above, allow about 20% more for

additional losses due to MOSFET output capacitances

and low-side MOSFET body-diode reverse-recovery

charge dissipated in the high-side MOSFET, but it is not

well defined in the MOSFET data sheet. Refer to the

MOSFET data sheet for thermal resistance specifications

to calculate the PC board area needed to maintain the

desired maximum operating junction temperature with the

above calculated power dissipations.

To reduce EMI caused by switching noise, add a 0.1µF

ceramic capacitor from the high-side switch drain to

the low-side switch source or add resistors in series

with DH and DL to slow down the switching transitions.

However, adding series resistors increases the power

dissipation of the MOSFET, so be sure this does not

overheat the MOSFET.

PQ

V

f

R

RR

HSDR

G

GS

S

GATE

GATE

DS ON HS

=×

×

×

+

()(

)

I

V

RR

GATE

VL

DS ON HS

GATE

≅

×

+

05

.

()(

)

PV

I

QQ

I

f

HSSW

IN

LOAD

GS

GD

GATE

S

=×

×

+

×

P

V

V

IR

HSCC

OUT

IN

LOAD

DS ON

=×

×

()

2

PI

V

t

f

LSDC

LOAD

F

DT

S

=×

×

×

×

2

P

V

V

IR

LSCC

OUT

IN

LOAD

DS ON

=−

⎛

⎝⎜

⎞

⎠⎟

××

1

2

()