Datasheet

LTM8020
8
8020fd
For most applications, the design process is straight
forward, summarized as follows:
1. Look at Table 1 and find the row that has the desired
input range and output voltage.
2. Apply the C
IN
, C
OUT
, R
ADJ
and BIAS connection indicated
on that row.
While these component combinations have been tested for
proper operation, it is incumbent upon the user to verify
proper operation over the intended system’s line, load and
environmental conditions.
If an output voltage other than those listed in Table 1 is
desired, use the equation R
ADJ
= 623.75/(V
OUT
– 1.25),
where R
ADJ
is in kΩ. As a starting point, use values for
C
IN
and C
OUT
that correspond to the input voltage and
output voltage that most closely matches the intended
application, and verify proper operation over the system’s
line, load and environmental conditions.
Capacitor Selection Considerations
The C
IN
and C
OUT
capacitor values in Table 1 are the
minimum recommended values for the associated oper-
ating conditions. Applying capacitor values below those
indicated in Table 1 is not recommended, and may result
in undesirable operation. An input system bulk capacitor
is assumed. Using larger values is generally acceptable,
and can yield improved dynamic response, if it is neces-
sary. Again, it is incumbent upon the user to verify proper
operation over the intended system’s line, load and envi-
ronmental conditions.
Ceramic capacitors are small, robust and have very low
ESR. However, not all ceramic capacitors are suitable.
X5R and X7R types are stable over temperature and ap-
plied voltage and give dependable service. Other types,
including Y5V and Z5U have very large temperature and
voltage coefficients of capacitance. In an application cir-
cuit they may have only a small fraction of their nominal
capacitance resulting in much higher output voltage ripple
than expected.
Ceramic capacitors are also piezoelectric. The LTM8020’s
switching frequency depends on the load current, and
at light loads it can excite a ceramic capacitor at audio
APPLICATIONS INFORMATION
frequencies, generating audible noise. Since the LTM8020
operates at a lower current limit during Burst Mode opera-
tion, the noise is typically very quiet to a casual ear.
If this audible noise is unacceptable, use a high performance
electrolytic capacitor at the output. The input capacitor can
be a parallel combination of a 2.2μF ceramic capacitor and
a low cost electrolytic capacitor.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8020. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8020 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possi-
bly exceeding the device’s rating. This situation is easily
avoided; see the Hot-Plugging Safely section.
Shorted Input Protection
Care needs to be taken in systems where the output will be
held high when the input to the LTM8020 is absent. This
may occur in battery charging applications or in battery
backup systems where a battery or some other supply
is diode ORed with the LTM8020’s output. If the V
IN
pin
is allowed to float and the SHDN pin is held high (either
by a logic signal or because it is tied to V
IN
), then the
LTM8020’s internal circuitry will pull its quiescent current
from its output. This is fine if your system can tolerate a
few milliamps in this state. If you ground the SHDN pin,
this quiescent current will drop to essentially zero. How-
ever, if the V
IN
pin is grounded while the output is held
high, then parasitic diodes inside the LTM8020 can pull
large currents from the output through the internal power
switch, possibly damaging the device. Figure 2 shows a
circuit that will run only when the input voltage is present
and that protects against a shorted or reversed input.
Figure 2. Diode D1 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output, as Well as Protecting the
LTM8020 from a Reversed Input
V
IN
LTM8020
SHDN
V
IN
8020 F02
GND
D1
100k
1M