11

FN9030.8

March 10, 2006

switch realizes most of the switching losses when the converter

is sinking current (see the equations below).

These equations assume linear voltage-current transitions

and do not adequately model power loss due the reverse-

recovery of the upper and lower MOSFET’s body diode. The

gate-charge losses are dissipated by the ISL6522 and do not

heat the MOSFETs. However, large gate-charge increases

MOSFET switching losses. Ensure that both MOSFETs are

within their maximum junction temperature at high ambient

temperature by calculating the temperature rise according to

package thermal-resistance specifications. A separate

heatsink may be necessary depending upon MOSFET

power, package type, ambient temperature and air flow.

Standard-gate MOSFETs are normally recommended for

use with the ISL6522. However, logic-level gate MOSFETs

can be used under special circumstances. The input voltage,

upper gate drive level, and the MOSFETs absolute gate-to-

source voltage rating determine whether logic-level

MOSFETs are appropriate.

Figure 9 shows the upper gate drive (BOOT pin) supplied by

develops a floating supply voltage referenced to the PHASE

turns on. A logic-level MOSFET can only be used for Q1 if

the MOSFETs absolute gate-to-source voltage rating

logic-level MOSFET can be used if its absolute gate-to-

source voltage rating exceeds the maximum voltage applied

to PVCC.

Figure 10 shows the upper gate drive supplied by a direct

less. The peak upper gate-to-source voltage is approximately

for the bias, the gate-to-source voltage of Q1 is 7V. A logic-level

MOSFET is a good choice for Q1 and a logic-level MOSFET

can be used for Q2 if its absolute gate-to-source voltage rating

Schottky Selection

Rectifier D2 is a clamp that catches the negative inductor

swing during the dead time between turning off the lower

MOSFET and turning on the upper MOSFET. The diode must

be a Schottky type to prevent the lossy parasitic MOSFET

body diode from conducting. It is acceptable to omit the diode

and let the body diode of the lower MOSFET clamp the

negative inductor swing, but efficiency will drop one or two

percent as a result. The diode's rated reverse breakdown

voltage must be greater than the maximum input voltage.

Losses while Sourcing Current

Losses while Sinking Current

P

LOWER

Io

2

r

DS ON

()

×

1D

–

()

×

1

2

--- Io

⋅

V

IN

×

t

SW

F

S

×

×

+

=

Io

2

r

DS ON

()

×

D

×

1

2

--- Io

⋅

V

IN

×

t

SW

F

S

×

×

+

=

+12V

PGND

ISL6522

GND

LGATE

UGATE

PHASE

BOOT

VCC

+5V OR +12V

FIGURE 9. UPPER GATE DRIVE - BOOTSTRAP OPTION

NOTE:

NOTE:

Q1

Q2

PVCC

+5V

OR +12V

D2

+

-

+

-

+12V

PGND

LGATE

UGATE

PHASE

BOOT

VCC

+5V OR LESS

NOTE:

NOTE:

Q1

Q2

PVCC

+5V

OR +12V

D2

ISL6522

GND

+

-

ISL6522