LT1108_15 Datasheet, PDF (7/12 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1108
APPLICATI S I FOR ATIO
Energy required from the inductor is
P_{L =} 315mW _{= 16.6µJ}
(07)
f_OSC 19kHz
Picking an inductor value of 100µH with 0.2Ω DCR results
in a peak switch current of
I_PEAK
=
2V
1.0Ω

^{1 –}
–1.0Ω × 36µs_
e _{100µH }
=
605mA
(08)
Substituting I_PEAK into Equation 04 results in
( ) ( ) ^E_L
=
1
2
100µH
6.605A ² = 18.3µJ
(09)
Since 18.3µJ > 16.6µJ, the 100µH inductor will work. This
trial-and-error approach can be used to select the optimum
inductor. Keep in mind the switch current maximum rating
of 1.5A. If the calculated peak current exceeds this, an
external power transistor can be used.
A resistor can be added in series with the I_LIM pin to invoke
switch current limit. The resistor should be picked so the
calculated I_PEAK at minimum V_IN is equal to the Maximum
Switch Current (from Typical Performance Characteristic
curves). Then, as V_IN increases, switch current is held
constant, resulting in increasing efficiency.
Step-Down Converter
The step-down case (Figure 2) differs from the step-up in
that the inductor current flows through the load during both
the charge and discharge periods of the inductor. Current
through the switch should be limited to ~650mA in this
mode. Higher current can be obtained by using an external
switch (see Figure 3). The I_LIM pin is the key to successful
operation over varying inputs.
After establishing output voltage, output current and input
voltage range, peak switch current can be calculated by the
formula:
I_PEAK
=
2 I _OUT
DC


V_OUT + V _D
V_IN – V _SW + V
D


(10)
where DC = duty cycle (0.60)
V_SW = switch drop in step-down mode
V_D = diode drop (0.5V for a 1N5818)
I_OUT = output current
V_OUT = output voltage
V_IN = minimum input voltage
V_SW is actually a function of switch current which is in turn
a function of V_IN, L, time, and V_OUT. To simplify, 1.5V can
be used for V_SW as a very conservative value.
Once I_PEAK is known, inductor value can be derived from
L=
V_{IN MIN}
− V_SW
I_PEAK
− V_OUT
× t_ON
(11)
where t_ON = switch-ON time (36µs).
Next, the current limit resistor R_LIM is selected to give I_PEAK
from the R_LIM Step-Down Mode curve. The addition of this
resistor keeps maximum switch current constant as the
input voltage is increased.
As an example, suppose 5V at 300mA is to be generated
from a 12V to 24V input. Recalling Equation (10),
( ) ^{2 300mA}
I_PEAK = _0.60

¹²
5
–
+ 0.5
1.5 + 0.5


=
500mA
(12)
Next, inductor value is calculated using Equation (11)
L
=
12 – 1.5 –
500mA
5
36µs
=
396µH
(13)
Use the next lowest standard value (330µH).
Then pick R_LIM from the curve. For I_PEAK = 500mA,
R_LIM = 220Ω.
Positive-to-Negative Converter
Figure 4 shows hookup for positive-to-negative conver-
sion. All of the output power must come from the inductor.
In this case,
P_L = (V_OUT+ V_D)(I_OUT)
(14)
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