LT1073CS8 Linear Technology, LT1073CS8 Datasheet - Page 7

• Manufacturer Part Number: LT1073CS8
• Description: IC DC/DC CONVERTER ADJSTBL 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost), Invertingr

Specifications of LT1073CS8

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adjustable
• Current - Output: 400mA
• Frequency - Switching: 19kHz
• Voltage - Input: 1 ~ 30 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Power - Output: 500mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
saturation current ratings in the 300mA to 1A range
(depending on application) are adequate. Lastly, the in-
ductor must have sufficiently low DC resistance so that
excessive power is not lost as heat in the windings. An
additional consideration is electro-magnetic interference
(EMI). Toroid and pot core type inductors are recom-
mended in applications where EMI must be kept to a
minimum; for example, where there are sensitive analog
circuitry or transducers nearby. Rod core types are a less
expensive choice where EMI is not a problem.
Specifying a proper inductor for an application requires
first establishing minimum and maximum input voltage,
output voltage and output current. In a step-up converter,
the inductive events add to the input voltage to produce the
output voltage. Power required from the inductor is deter-
mined by:
where V
Maximum power in the inductor is
where
At maximum V
to exceed the maximum switch current shown in Figure 2.
Some input/output voltage combinations will cause con-
tinuous
needed between I
current under control. See the “Using the I
for details.
NOTE 1: i.e., inductor current does not go to zero when the switch is off.
P
R = Switch equivalent resistance (1 maximum)
added to the DC resistance of the inductor and t
time of the switch.
P
i
PEAK
L
L
= (V
1
E f
D
2
1
L OSC
mode operation. In these cases a resistor is
is the diode drop (0.5V for a 1N5818 Schottky).
L i
OUT
•
V
PEAK
R
+ V
IN
IN
and ON time, i
LIM
2
D
•
U
1–
– V
f
(Pin 1) and V
OSC
e
IN
–
)(I
U
Rt
L
OUT
ON
PEAK
)
IN
W
should not be allowed
(Pin 2) to keep switch
LIM
Pin” section
U
ON
= ON
Capacitor Selection
Selecting the right output capacitor is almost as important
as selecting the right inductor. A poor choice for a filter
capacitor can result in poor efficiency and/or high output
ripple. Ordinary aluminum electrolytics, while inexpensive
and readily available, may have unacceptably poor equiva-
lent series resistance (ESR) and ESL (inductance). There
are low-ESR aluminum capacitors on the market specifi-
cally designed for switch-mode DC/DC converters which
work much better than general purpose units. Tantalum
capacitors provide still better performance at more ex-
pense. We recommend OS-CON capacitors from Sanyo
Corporation (San Diego, CA). These units are physically
quite small and have extremely low ESR. To illustrate,
Figures 3, 4, and 5 show the output voltage of an LT1073
based converter with three 100 F capacitors. The peak
switch current is 500mA in all cases. Figure 3 shows a
Sprague 501D aluminum capacitor. V
150mV when the switch turns off, followed by a drop in
voltage as the inductor dumps into the capacitor. This
works out to be an ESR of over 300m . Figure 4 shows the
same circuit, but with a Sprague 150D tantalum capacitor
replacing the aluminum unit. Output jump is now about
30mV, corresponding to an ESR of 60m . Figure 5 shows
the circuit with an OS-CON unit. ESR is now only 30m .
In very low power applications where every microampere
is important, leakage current of the capacitor must be
considered. The OS-CON units do have leakage current in
the 5 A to 10 A range. If the load is also in the
Figure 2. Maximum Switch Current vs Input Voltage
1200
1000
800
600
400
200
0
0
1
2
V
IN
(V)
3
OUT
4
1073 F02
jumps by over
LT1073
5
7