AN-9052

APPLICATION NOTE

© 2008 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.0 • 11/10/08

2

start-up phase can be defined by the time constants Rbs, Rvs,

and Cbs. When the load is connected and forms the charge

path in the bootstrap circuit, the initial charging time is

defined by Cbs and the relationship between Rbs, Rvs, and

the load impedance RL. Most designs, the value of Cbs is

picked with some margins, which leads to longer star-up

time. If node X is left floating for a long time, Vbs will

decrease due to leakage current, and consequently the gate

drive IC will go into UVLO condition. Controlling the low

side switch properly, the bootstrap capacitor can be

recharged and maintain the voltage level needed by the gate

drive IC. The gate drive IC FAN7085 (block diagram shown

in Fig.2) has a built-in recharge switch that will charge the

bootstrap capacitor regardless of the application. If Q1 is

turned off and FAN7085 is used as a gate drive IC in the

bootstrap circuit shown in Figure 1, the internal recharge

switch of the FAN7085 will activate to provide the path to

charge the bootstrap capacitor (Cbs) fully. Let's look at a

specific application of the FAN7085, which is shown in Fig-

ure 3. In the event that both of the switch (S1 and S2) are

turned off, the internal recharge switch of the FAN7085 will

provide the path to charge the bootstrap capacitor. If the cur-

rent level passing through the recharge switch is higher than

the leakage current, bootstrap capacitor will charge through

the recharge path. The voltage level on the VS pin of the

FAN7085 at a given current level when the recharge switch

is turned on is defined on the datasheet.

Figure 2. Internal block diagram of FAN7085

Figure 3. Application example of FAN7085

2. Selection of Bootstrap

Components

2.1 Selection of Bootstrap Capacitor

The maximum allowable voltage drop across the bootstrap

capacitor to ensure enough gate-source voltage is highly

dependent to the internal undervoltage shutdown level of the

gate drive IC, and the voltage level at the source connection

of Q1 (node X)

Where:

If = static forward voltage drop of Dbs

Vop= minimum gate-source voltage level required that will

prevent Q1 to go into a high dissipation mode

X = MOSFET source connection

greater than UVLO voltage level, then equation 2 can be

denoted as follows

gate drive IC.

The total charge (Qbs) required by the bootstrap capacitor

can be calculated by summing the Q1 gate charge, charge

required for the level shifter in the gate drive IC, and leakage

charges resulting from leakage current.

Where:

Qg.= gate charge of Q1

The total leakage current is the summation of all of the indi-

vidual component's leakage currents

Where,

VCC

Under Voltage

Reset VCC to GND

Logic

Pulse

Filter

Level Shifter

ON

Level Shifter

OFF

Delay

Under

Voltage Reset

VB to VS

Pulse Filter

Flip Flop

Brake before

make

RESET -

IN-

GND

VS

HO

VB

VCC

IN-

GND

RESET-

VB

HO

NC

VS

C2

C1

R1

R2

R3

R4

C3

5V

VCC

VS

C4

Load

GND

From LS Driver

S1

S2

D3

D4

D5

FAN7085

,

–

–

–

=

2

()

V

BSDrop

,

V

CC

V

f

–

V

BSUVMax

,

–

V

X

–

=

3

()

Q

BS

Q

g

I

LK

I

QBS

+

() T

ON

×

Q

LS

++

=

4

()

,

,

,

,

+++

=

5

()