LT1111_03 Datasheet, PDF (8/16 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1111
APPLICATI S I FOR ATIO
Picking an inductor value of 47µH with 0.2Ω DCR results
in a peak switch current of:
 –1.0Ω •7µs 
I _PEAK
=
4.5V
1.0Ω
 _{1 –}
e
47µH


=
623mA.
(8)


Substituting I_PEAK into Equation 4 results in:
( ) ( ) ^EL
=
1
2
47µH
0.623A ² = 9.1µJ
(9)
Since 9.1µJ > 6.7µJ, the 47µH inductor will work. This
trial-and-error approach can be used to select the opti-
mum inductor. Keep in mind the switch current maximum
rating of 1.5A. If the calculated peak current exceeds this,
consider using the LT1110. The 70% duty cycle of the
LT1110 allows more energy per cycle to be stored in the
inductor, resulting in more output power.
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 − V_OUT
I_PEAK
• t_ON
(11)
where t_ON = switch-on time (7µs).
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.
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.
As an example, suppose 5V at 300mA is to be generated
from a 12V to 24V input. Recalling Equation (10),
( ) ^{2 300mA}
I_PEAK = _0.50

¹²
5
–
+ 0.5
1.5 + 0.5



=
600mA
(12)
Inductor Selection — Step-Down Converter
Next, inductor value is calculated using Equation (11):
The step-down case (Figure 5) 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 6). 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
=
2I _OUT
DC



V
V
IN
OUT ^{+ V} D
– V _SW + V
D



(10)
where DC = duty cycle (0.50)
V_SW = switch drop in step-down mode
L = ^{12 – 1.5 – 5} 7µs = 64µH.
(13)
600mA
Use the next lowest standard value (56µH).
Then pick R_LIM from the curve. For I_PEAK = 600mA, R_LIM
= 56Ω.
Inductor Selection — Positive-to-Negative Converter
Figure 7 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)
In this mode the switch is arranged in common collector
or step-down mode. The switch drop can be modeled as
a 0.75V source in series with a 0.65Ω resistor. When the
1111fd
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