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LT1108CN8-5#PBF

P1-P3P4-P6P7-P9P10-P12
7
LT1108
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Energy required from the inductor is
P
f
mW
kHz
J
L
OSC
==
315
19
16 6 07.()µ
Picking an inductor value of 100µH with 0.2Ω DCR results
in a peak switch current of
I
V
emA
PEAK
s
H
=
=
×
2
10
1 605 08
10 36
100
.
–()
–.
Ω
Ωµ
µ
Substituting I
PEAK
into Equation 04 results in
EHAJ
L
=
()( )
=
1
2
100 6 605 18 3 09
2
µµ.. ()
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
I
DC
VV
VV V
PEAK
OUT OUT D
IN SW D
=
+
+
2
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
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),
Next, inductor value is calculated using Equation (11)
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)
8
LT1108
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
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
It
V
R
e
L
L
Rt
L
()
=
'
–()
–'
115
where: R' = 0.65Ω + DCR
L
V
L
= V
IN
– 0.75V
As an example, suppose –5V at 100mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
P
L
= (–5V+ 0.5V)(100mA) = 550mW. (16)
Energy required from the inductor is
P
f
mW
kHz
J
L
OSC
==
550
19
28 9 17.()µ
Picking an inductor value of 220µH with 0.3Ω DCR results
in a peak switch current of
I
VV
e
mA
PEAK
s
H
=
()
+
()
=
×
45 075
65 0 3
1
568
095 36
220
.–.
..
–
–.
0 ΩΩ
Ωµ
µ
Substituting I
PEAK
into Equation (04) results in
EHAJ
L
=
()( )
=
1
2
220 0 568 35 5 19
2
µµ.. ()
Since 35.5µJ > 28.9µJ, the 220µH inductor will work.
Finally, R
LIM
should be selected by looking at the Switch
Current vs R
LIM
curve. In this example, R
LIM
= 150Ω.
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.
The usual step-up configuration for the LT1108 is shown in
Figure 1. The LT1108 first pulls SW1 low causing V
IN
–
V
CESAT
to appear across L1. A current then builds up in L1.
At the end of the switch-ON time the current in L1 is
(18)
*Expression 20 neglects the effect of switch and coil resistance. This is taken into account in the
"Inductor Selection" section.
Immediately after switch turn-off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches V
OUT
+ V
D
, the
inductor current flows through D1 into C1, increasing V
OUT
.
This action is repeated as needed by the LT1108 to keep V
FB
at the internal reference voltage of 1.245V. R1 and R2 set
the output voltage according to the formula
STEP-DOWN (BUCK MODE) OPERATION
A step-down DC/DC converter converts a higher voltage to
a lower voltage. The usual hookup for an LT1108 based
step-down converter is shown in Figure 2.
When the switch turns on, SW2 pulls up to V
IN
– V
SW
. This
puts a voltage across L1 equal to V
IN
– V
SW
– V
OUT
, causing
a current to build up in L1. At the end of the switch- ON time,
the current in L1 is equal to
Figure 1. Step-Up Mode Hookup
L1
LT1108 • F01
GND SW2
SW1
LIM
I
IN
V
D1
R3
LT1108
+
V
OUT
R2
R1
C1
V
IN
FB
9
LT1108
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AT
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U
I FOR ATIO
HIGHER CURRENT STEP-DOWN OPERATION
Output current can be increased by using a discrete PNP
pass transistor as shown in Figure 3. R1 serves as a
current limit sense. When the voltage drop across R1
equals 0.5V
BE
, the switch turns off. As shown, switch
current is limited to 2A. Inductor value can be calculated
based on formulas in the Inductor Selection Step-Down
Converter section with the following conservative expres-
sion for V
SW
:
R2 provides a current path to turn off Q1. R3 provides base
drive to Q1. R4 and R5 set output voltage. A PMOS FET can
be used in place of Q1 when V
IN
is between 10V and 20V.
Figure 2. Step-Down Mode Hookup
LT1108 • F02
GND
SW2
SW1
I
LIM
R3
100Ω
FB
V
OUT
+
C2
+
C1
D1
1N5818
V
IN
R2
R1
L1
LT1108
V
IN
LT1108 • F03
D1
1N5821
+
+
V
OUT
V
IN
30V
MAX
L1
R1
0.15Ω
R2
100Ω
Q1
ZETEX ZTX749
R3
330Ω
R4
R5
C1
LT1108
GND
SW2
SW1
V
IN
I
L
FB
C2
R6
100Ω
V
OUT
= 1.245V 1 +
R4
R5
()
INVERTING CONFIGURATIONS
The LT1108 can be configured as a positive-to-negative
converter (Figure 4), or a negative-to-positive converter
(Figure 5). In Figure 4, the arrangement is very similar to a
step-down, except that the high side of the feedback is
referred to ground. This level shifts the output negative. As
in the step-down mode, D1 must be a Schottky diode,
andV
OUT
should be less than 6.2V. More negative output
voltages can be accommodated as in the prior section.
In Figure 5, the input is negative while the output is positive.
In this configuration, the magnitude of the input voltage can
be higher or lower than the output voltage. A level shift,
I
V
VV
L
t
PEAK
IN
SW OUT
ON
=
−−
()22
When the switch turns off, the SW2 pin falls rapidly and
actually goes below ground. D1 turns on when SW2
reaches 0.4V below ground.
D1 MUST BE A SCHOTTKY
DIODE
. The voltage at SW2 must never be allowed to go
below –0.5V. A silicon diode such as the 1N4933 will allow
SW2 to go to –0.8V, causing potentially destructive power
dissipation inside the LT1108. Output voltage is deter-
mined by
V
R
R
V
OUT
=+
()
1
2
1
1 245 23.()
R3 programs switch current limit. This is especially impor-
tant in applications where the input varies over a wide
range. Without R3, the switch stays on for a fixed time each
cycle. Under certain conditions the current in L1 can build
up to excessive levels, exceeding the switch rating and/or
saturating the inductor. The 100Ω resistor programs the
switch to turn off when the current reaches approximately
700mA. When using the LT1108 in step-down mode,
output voltage should be limited to 6.2V or less. Higher
output voltages can be accommodated by inserting a
1N5818 diode in series with the SW2 pin (anode connected
to SW2).
Figure 3. Q1 Permits Higher Current Switching
The LT1108 Functions as Controller
P1-P3P4-P6P7-P9P10-P12

LT1108CN8-5#PBF

Mfr. #:
Buy LT1108CN8-5#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
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