Maximum Input Voltage at VIN pin computed

using the Power Dissipation Limit

The maximum input voltage that the FC9911 can withstand

without damage if the regulator is drawing about 2.8mA will

depend on the ambient temperature. If we consider an ambi-

ent temperature of 40°C, the power dissipation in the pack-

age cannot exceed:

P

O

= 850mW

The above equation is based on package power dissipation

limits as given in the Absolute Maximum Limits section of

this datasheet.

To dissipate a maximum power of 850mW in the package,

the maximum input voltage cannot exceed:

V

= 296V

Since the maximum voltage is far greater than the actual

input voltage (24V), power dissipation will not be a problem

for this design.

For this design, at 24V input, the increase in the junction

temperature of the IC (over the ambient) will be

Δθ = V

where θ

the 16-Lead SOIC package of the FC9911.

Minimum Input Voltage at VIN pin

The minimum input voltage at which the converter will start

and stop depends on the minimum voltage drop required for

the linear regulator. The internal linear regulator will regu-

late the voltage at the VDD pin when V

250V. However, when V

will still function as long as V

voltage lockout. Thus, the converter might be able to start

at input voltages lower than 9.0V. The start/stop voltages at

the VIN pin can be determined using the minimum voltage

drop across the linear regulator as a function of the current

drawn. This data is shown in Fig. 3 for different junction tem-

peratures.

Fig. 3. Graph of the Input Current vs Mini-

mum Voltage Drop Across Linear Regulator

for Different Junction Temperatures

Assume a maximum junction temperature of 85°C (this give a

reasonable temperature rise of 45°C at an ambient tempera-

ture of 40°C). At 2.86mA input current, the minimum voltage

drop from Fig. 3 can be approximately estimated to be V

DROP

= 0.75V. However, before the IC starts switching the current

.t

n

e

r

r

u

c

e

v

ir

d

E

T

A

G

e

h

t

s

u

n

i

m

t

n

e

r

r

u

c

l

a

t

o

t

e

h

t

e

b

ll

i

w

n

w

a

r

d

In this case, that current is I

level, the voltage drop is approximately V

the start/stop V

VIN

= 7.2V + 0.4V

= 7.60V

VIN

= 7.2V - 0.5V + 0.75V

= 7.45V

Note:

In some cases, if the GATE drive draws too much current,

VIN

control IC will oscillate between ON and OFF if the input

voltage is between the start and stop voltages. In these

circumstances, it is recommended that the input voltage

be kept higher than VIN

Minimum drop in Linear Regulator (V)

0 2

0.4

0.6

0.8

1

1 2

1.4

1 6

1.8

2

2.2

2.4

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

-40°C

25°C

85°C

125°C

FC9911

2012. 10. 31

9/13

Revision No : 0