LT1110 Datasheet, PDF (8/16 Pages) Linear Technology – Micropower DC-DC Converter Adjustable and Fixed 5V, 12V

LT1110
APPLICATI S I FOR ATIO
Picking an inductor value of 47µH with 0.2Ω DCR results
in a peak switch current of
I_PEAK
=
4.5V
1.0W

1 −

−1.0W•10ms
e _47mH



=
862mA.
(08)
Substituting I_PEAK into Equation 05 results in
( ) ( ) ^E_L
=
1
2
47µH
0.862A ² = 17.5µJ.
(09)
Since 17.5µJ > 13.7µJ, 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,
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 such
that 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.
Inductor Selection — Step-Down Converter
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 ~800mA
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 _OUT +
V_IN – V _SW
V_D
+V
D


(10)
where DC = duty cycle (0.69)
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 (10µs).
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 250mA is to be generated
from a 9V to 18V input. Recalling Equation (10),
( ) ^{2 250mA}
I_PEAK = _0.69

 ⁹
5 + 0.5
– 1.5 + 0.5



=
498mA
.
(12)
Next, inductor value is calculated using Equation (11)
L = ^{9 – 1.5 – 5} • 10µs = 50µH.
(13)
498mA
Use the next lowest standard value (47µH).
Then pick R_LIM from the curve. For I_PEAK = 500mA,
R_LIM = 82Ω.
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
8