LTM8022
9
8022fd
For most applications, the design process is straight
forward, summarized as follows:
1. In Table 1, fi nd the row that has the desired input voltage
range and output voltage.
2. Apply the recommended C
IN
, C
OUT
, R
ADJ
and R
T
values.
3. Connect BIAS as indicated.
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 the desired output voltage is not listed in Table 1, set the
output by applying an R
ADJ
resistor whose value is given by
the equation R
ADJ
= 394.21/(V
OUT
– 0.79), where R
ADJ
is
in kΩ and V
OUT
is in volts. Verify the LTM8022’s operation
over the system’s intended 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. Using larger values is generally
acceptable, and can yield improved dynamic response, 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 applied voltage
and give dependable service. Other types, including Y5V and
Z5U, have very large temperature and voltage coeffi cients
of capacitance. In an application circuit 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. In Burst Mode
operation, the LTM8022’s switching frequency depends
on the load current, and can excite a ceramic capacitor
at audio frequencies, generating audible noise. Since the
LTM8022 operates at a lower current limit during Burst
Mode operation, the noise is typically very quiet to a
casual ear.
If this audible noise is unacceptable, use a high per-
formance 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 fi nal precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8022. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8022 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the device’s rating. This situation is easily
avoided; see the Hot-Plugging Safely section.
Frequency Selection
The LTM8022 uses a constant-frequency PWM architecture
that can be programmed to switch from 200kHz to 2.4MHz
by using a resistor tied from the R
T
pin to ground. Table 2
provides a list of R
T
resistor values and their resultant
frequencies.
Table 2. Switching Frequency vs R
T
Value
SWITCHING FREQUENCY (MHz)
R
T
VALUE (kΩ)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
187
121
88.7
68.1
56.2
46.4
40.2
34
29.4
23.7
19.1
16.2
13.3
11.5
9.76
8.66
Operating Frequency Tradeoffs
It is recommended that the user apply the optimal R
T
value given in Table 1 for the input and output operating
condition. System level or other considerations, however,
may necessitate another operating frequency. While the
LTM8022 is fl exible enough to accommodate a wide range
APPLICATIONS INFORMATION
