LT1107 Datasheet, PDF (8/12 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1107
APPLICATI S I FOR ATIO
where DC = duty cycle (0.50 in step-down mode)
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:
( ) ^{VIN MIN − VSW − VOUT}
L=
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 Maximum Switch Current vs R_LIM curve.
The addition of this resistor keeps maximum switch cur-
rent 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 = _0.50

¹²
5 + 0.5
− 1.5 + 0.5



=
600mA
(12)
Next, inductor value is calculated using Equation (11):
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 4 shows hookup for positive-to-negative conver-
sion. All of the output power must come from the inductor.
In this case,
( )( ) ^PL ^{= V}OUT ^{+ V}D ^IOUT
(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
switch closes, current in the inductor builds according to:
( ) ^IL
t
=
V_L
R′

^1−

e
−R′t
L



(15)
where R′ = 0.65Ω + DCR_L
V_L = V_IN – 0.75V
As an example, suppose –5V at 50mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
( )( ) ^PL ^{= −5V + 0.5V 50mA = 275mW}
(16 )
Energy required from the inductor is:
P_{L =} 275mW _{= 4.4µJ}
(17)
f_OSC 63kHz
Picking an inductor value of 100µH with 0.2Ω DCR results
in a peak switch current of:
( ) ^{4.5V − 0.75V}  ^{−0.85Ω × 9µs}
( ) ^{IPEAK = 0.65Ω + 0.2Ω 1− e 100µH }
= 325mA
(18)
Substituting I_PEAK into Equation (04) results in:
( ) ( ) ^EL
=
1
2
100µH
0.325A ² = 5.28µJ
(19)
Since 5.28µJ > 3.82µJ, the 100µH inductor will work.
With this relatively small input range, R_LIM is not usually
necessary and the I_LIM pin can be tied directly to V_IN. As in
the step-down case, peak switch current should be limited
to ~650mA.
Step-Up (Boost Mode) Operation
A step-up DC/DC converter delivers an output voltage
higher than the input voltage. Step-up converters are not
short-circuit protected since there is a DC path from input
to output.
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