LTM8021
9
8021fd
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. Using larger values is generally
acceptable, and can yield improved dynamic response or
fault recovery, if it is necessary. Again, it is incumbent
upon the user to verify proper operation over the intended
system’s line, load and environmental 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. At light loads,
the LTM8021 skips switching cycles in order to maintain
regulation. The resulting bursts of current can excite
a ceramic capacitor at audio frequencies, generating
audible noise.
If this audible noise is unacceptable, use a high performance
electrolytic capacitor at the output. This output capacitor
can be a parallel combination of a 1µF ceramic capacitor
and a low cost electrolytic capacitor.
A final precaution regarding ceramic capacitors con-
cerns the maximum input voltage rating of the LTM8021.
A ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8021 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.
Minimum Input Voltage
The LTM8021 is a step-down converter, so a minimum
amount of headroom is required to keep the output in
regulation. For most applications at full load, the input
must be about 1.5V above the desired output. In addition,
it takes more input voltage to turn on than is required for
continuous operation. This is shown in Figure 1.
applicaTions inForMaTion
Figure 1. The LTM8021 Requires More Voltage to Start Than to Run
LOAD CURRENT (A)
0.001
2.0
INPUT VOLTAGE (V)
3.0
4.0
5.0
0.01
0.1
6.0
2.5
3.5
4.5
5.5
1
8021 F01
V
OUT
= 3.3V
TO START
TO RUN
LOAD CURRENT (A)
0.001
2
INPUT VOLTAGE (V)
3
4
6
5
0.01
0.1
8
7
1
V
OUT
= 5V
RUN/SS
ENABLED
RUN/SS
ENABLED
TO START
TO RUN