Datasheet

7
LT1316
APPLICATIONS INFORMATION
WUU
U
then adding in the amount of overshoot that will occur due
to turn-off delay of the power transistor. This turn-off
delay is approximately 300ns.
Peak switch current = DC current limit from graph +
V
IN
/L(turn-off delay)
Example:
Set peak switch current to 100mA for: V
IN
= 2V,
L = 33µH
Overshoot = V
IN
/L(turn-off delay) = (2/33µH)(300ns)
= 18.2mA
Refer to R
SET
graph and locate
(100mA – 18.2mA) ≈ 82mA
R
SET
≈ 33k
Calculating Duty Cycle
For a boost converter running in continuous conduction
mode, duty cycle is constrained by V
IN
and V
OUT
according
to the equation:
DC =
V
OUT
– V
IN
+ V
D

V
OUT
– V
SAT
+ V
D
where V
D
= diode voltage drop ≈ 0.4V and V
SAT
= switch
saturation voltage ≈ 0.2V.
If the duty cycle exceeds the LT1316’s minimum specified
duty cycle of 0.73, the converter cannot operate in con-
tinuous conduction mode and must be designed for
discontinuous mode operation.
Inductor Selection and Peak Current Limit for
Continuous Conduction Mode
Peak current and inductance determine available output
power. Both must be chosen properly. If peak current or
inductance is increased, output power increases. Once
output power or current and duty cycle are known, peak
current can be set by the following equation, assuming
continuous mode operation:
I
PEAK
=
2(I
OUT
) 
1 – DC
(1)
Inductance can now be calculated using the peak current:
L = (t
OFF
)
V
OUT
– V
IN
+ V
D

0.4(I
PEAK
)
(2)
where t
OFF
= 2µs and V
D
= 0.4V.
As a result of equations 1 and 2, ripple current during
switching will be 40% of the peak current (see Figure 2).
Using these equations at the specified I
OUT
, the part is
delivering approximately 60% of its maximum output
power. In other words, the part is operating on a 40%
reserve. This is a safe margin to use and can be decreased
if input voltage and output current are tightly controlled.
For some applications, this recommended inductor size
may be too large. Inductance can be reduced but available
output power will decrease. Also, ripple current during
switching will increase and may cause discontinuous
operation. Discontinuous operation occurs when
inductor current ramps down to zero at the end of each
switch cycle (see Figure 4). Shown in Figure 5 is minimum
inductance vs peak current for the part to remain in
continuous mode.
Figure 5. Minimum Inductance vs Peak Current
for Continuous Mode Operation
PEAK CURRENT (mA)
10
10
MINIMUM INDUCTANCE FOR 
CONTINOUS MODE OPERATION (µH)
100
1000
100 1000
1316 F05
5V TO 18V
5V TO 12V
2V TO 5V
2µs/DIV
Figure 4. Discontinuous Mode Operation
1316 F04
SW PIN
5V/DIV
0mA
INDUCTOR
CURRENT
100mA/DIV