FN4307 Rev.1.00

Page 9 of 10

September 1997

HIP6007

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

develops a floating supply voltage referenced to the PHASE

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

MOSFET’s absolute gate-to-source voltage rating exceeds the

Figure 10 shows the upper gate drive supplied by a direct

connection to VCC. This option should only be used in

converter systems where the main input voltage is +5VDC or

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 is a good choice for Q1 under these conditions.

Schottky Selection

Rectifier D2 conducts when the upper MOSFET Q1 is off. The

diode should be a Schottky type for low power losses. The

power dissipation in the schottky rectifier is approximated by:

In addition to power dissipation, package selection and heatsink

requirements are the main design tradeoffs in choosing the

schottky rectifier. Since the three factors are interrelated, the

selection process is an iterative procedure. The maximum

junction temperature of the rectifier must remain below the

package thermal resistance specification and the schottky

power dissipation equation (shown above), the junction

temperature of the rectifier can be estimated. Be sure to use the

available airflow and ambient temperature to determine the

junction temperature rise.

FIGURE 9. UPPER GATE DRIVE - BOOTSTRAP OPTION

+12V

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GND

UGATE

PHASE

BOOT

VCC

+5V OR +12V

Q1

D2

NOTE:

+

-

+

-

+

-

+12V

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GND

UGATE

PHASE

BOOT

VCC

+5V OR LESS

NOTE:

Q1

D2