Datasheet

8
LT1107
1107fa
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:
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),
Energy required from the inductor is:
Picking an inductor value of 100µH with 0.2Ω DCR results
in a peak switch current of:
Substituting I
PEAK
into Equation (04) results in:
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.
P
f
mW
kHz
J
L
OSC
==
275
63
44 17.()µ
It
V
R
e
L
L
Rt
L
()
=
′
−








−
′
115()
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:
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):
Next, inductor value is calculated using Equation (11):
L
mA
sH=
−−
=
12 1 5 5
600
764 13
.
()µµ
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,
P (14)
L
=+
()
()
VVI
OUT D OUT
L
VVV
I
t
IN MIN
SW OUT
PEAK
ON
=
−−
×
()
()11
I
mA
mA
PEAK
=
()
+
−+








=
2 300
050
505
12 15 05
600 12
.
.
..
()
P = 275mW (16)
L
=− +
()
()
50550VVmA.
I
VV
e
mA
PEAK
s
H
=
−
()
+
()
−








=
−
45 075
065 02
1
325 18
085 9
100
..
..
()
.•
ΩΩ
Ωµ
µ
EHAJ
L
=
()( )
=
1
2
100 0 325 5 28 19
2
µµ.. ()
U
S
A
O
PP
L
IC
AT
I
WU
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I FOR ATIO