Datasheet

8
LT1316
APPLICATIONS INFORMATION
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Discontinuous Mode Operation
A boost converter with a high V
OUT
:V
IN
ratio operates with
a high duty cycle in continuous mode. For duty cycles
exceeding the LT1316’s guaranteed minimum specifica-
tion of 0.73, the circuit will need to be designed for
discontinuous operation. Additionally, very low peak cur-
rent limiting below 50mA may necessitate operating in this
mode unless high inductance values are acceptable. When
operating in discontinuous mode, a different equation
governs available output power. For each switch cycle, the
inductor current ramps down to zero, completely releas-
ing the stored energy. Energy stored in the inductor at any
time is equal to 1/2 LI
2
. Because this energy is released
each cycle, the equation for maximum power out is:
P
OUT(MAX)
= 1/2L(I
PEAK
2
)f
Where f =
+ t
OFF
1

I
PEAK
(L)
V
IN
– V
SAT
) )
When designing for very low peak currents (<50mA), the
inductor size needs to be large enough so that on-time is
a least 1µs. On-time can be calculated by the equation:
On-Time =
I
PEAK
• L
(V
IN
– V
SAT
)
) )
where V
SAT
= 0.2V.
Also, at these low current levels, current overshoot due to
power transistor turn-off delay will be a significant portion
of peak current. Increasing inductor size will keep this to
a minimum.
Design Example 1
Requirements: V
IN
= 2V, V
OUT
= 5V and I
LOAD
= 10mA.
1. Find duty cycle
DC =
V
OUT
– V
IN
+ V
D

V
OUT
– V
SAT
+ V
D
) )
=
= 0.654
5 – 2 + 0.4
5 – 0.2 + 0.4
) )
Because duty cycle is less than the LT1316 minimum
specification (0.73), the circuit can be designed for
continuous operation.
2.
I
PEAK
=
2(I
OUT
)
1 – DC
= = 58mA
2(10mA)
1 – 0.654
3. Find L
L =
V
OUT
– V
IN
+ V
D

0.4(I
PEAK
)
) )
=
= 293µH
5 – 2 + 0.4
0.4(58mA)
) )
t
OFF
2µs
4. Find R
SET
resistor
Overshoot = 300ns
V
IN

L
) )
=
= 1.8mA
2
330µH
) )
Find R
SET
from Figure 3 for 58mA – 1.8mA = 56.2mA
R
SET
≈ 47k
Design Example 2
Requirements: V
IN
= 3.3V, V
OUT
= 28V and I
LOAD
= 5mA.
1. Find duty cycle:
DC =
V
OUT
– V
IN
+ V
D

V
OUT
– V
SAT
+ V
D
) )
=
= 0.89
28 – 3.3 + 0.4
28 – 0.2 + 0.4
) )
Because duty cycle exceeds LT1316 minimum specifi-
cation of 73%, the circuit must be designed for discon-
tinuous operation.
2. Find P
OUT(MAX)
Multiply P
OUT
by 1.4 to give a safe operating margin
P
OUT(MAX)
= P
OUT
(1.4) = (5mA)(28V)(1.4) = 0.196W
3. Set the on-time to the data sheet minimum of 3.4µs and
find L
L =
(t
ON
2
)(V
IN
– V
SAT
)
2

2P
OUT(MAX)
(t
ON
+ t
OFF
)
=
= 52µH
(3.4µs
2
)(3.3 – 0.2)
2

2(0.196W)(3.4µs + 2µs)