9

Applications Information

SC2308A

Duty Cycle

The duty cycle is the ratio of the switch on-time to the

switching period. For a boost converter, the duty cycle in

continuous-conduction mode (CCM) is:

CESAT

D

OUT

IN

D

OUT

V

V

V

V

V

V

D

()

where V

rectifying diode forward voltage.

Setting the Output Voltage

The converter output voltage is set with an external resis-

tive divider. The center tap of the divider is tied to the FB

pin ( Figure 3).

FB

2

SC2308A

Figure 3. R

The expression for V

2

1

OUT

R

R

1

220

.

1

V

(2)

R

follows:

1

220

.

1

V

R

R

OUT

2

1

(3)

Using large resistors for the FB voltage divider reduces

power consumption.

Minimum Off-Time Limitation

There is also a 00ns minimum switch off-time, which limits

the maximum duty cycle. This determines the maximum

attainable output voltage for a given V

(), the maximum output voltage for a boost converter

can be derived:

D

MAX

CESAT

MAX

IN

OUT

V

D

1

V

D

V

V

(4)

where D

Example: Determine the highest output voltage that can

be achieved from a 3V input using the SC2308A as a boost

regulator. Assuming V

0.87:

V

6

.

20

5

.

0

87

.

0

1

3

.

0

87

.

0

3

V

OUT

The transient headroom requirement further reduces the

maximum achievable output voltage to less than 20V.

Maximum Output Current

In a boost converter, the inductor is connected to the

input. The inductor DC current is the regulator input

current. When the power switch is turned on, the inductor

current flows into the switch. When the power switch is

off, the inductor current flows through the rectifying

diode to the output. The output current is the average

diode current. The diode current waveform is trapezoidal

with a pulse width of (–D)T

available from a boost converter, therefore, depends on

the converter operating duty cycle. The power switch

current in the SC2308A is internally limited to at least

2.2A. This is also the maximum peak inductor or the peak

input current. If the inductor ripple current is low, then

the maximum regulator input current will be very close to

the switch current limit I

Switch Current

Diode Current

ON

ON

OFF

ON

ON

OFF

ON

OFF

(1-D)T

S

DT

S

Inductor

Current

0

0

Figure 4. Current Waveforms in a Boost Converter

Switch Current

Diode Current

ON

ON

OFF

ON

ON

OFF

ON

OFF

(1-D)T

S

DT

S

Inductor

Current

0

0

Figure 4. Current Waveforms in a Boost Converter