鈭?/div>
V
SW
(
)
DC
MAX
is the maximum duty cycle of each driver with
respect to the entire cycle which consists of two periods
(Q1A on and Q1B on). So the effective duty cycle is
2 鈥?DC
MAX
. The controller, in general, determines maxi-
mum duty cycle. A 44% maximum duty cycle is a guaran-
teed value for this part.
Some Common Turns Ratios
V
IN
5
鹵10%
5
鹵10%
5
鹵10%
V
OUT
12
15
3.3
N
3.6
4.4
1.1
Remember to add sufficient margin in the turns ratio to
account for IR drops in the transformer windings, worst-
case diode forward drop (V
F
) and switch-on voltage (V
SW
).
There are a number of ways to choose the inductance
value for L
O
. We suggest as a starting point that L
O
be
selected such that the converter is continuous at
I
OUT(MAX)
/4. If your minimum I
OUT
is higher than this, or
you are operating at low currents such that the IC and
components can handle higher peak currents, then use a
higher number.
Continuous operation occurs when the current in the
inductor never goes to zero. Discontinuous operation
occurs when the inductor current drops to zero before the
start of the next cycle and can occur with small inductors
and light loads. There is nothing inherently bad about
discontinuous operation, however, the converter control
and operation is somewhat different. The inductor is
smaller for discontinuous operation but the peak currents
in the switch, the transformer, the diodes, inductor and
capacitor will be higher. But for low power situations these
may not present a big constraint.
V
OUT
11