ltm8027 Linear Technology Corporation, ltm8027 Datasheet - Page 11

• Manufacturer Part Number: ltm8027
• Description: 60v, 4a Dc/dc ?module Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
For most applications, the design process is straight
forward, summarized as follows:
1. Look at Table 2 and fi nd the row that has the desired
2. Apply the recommended C
3. Connect the BIAS pins as indicated.
While these component and connection 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.
Capacitor Selection Considerations
The C
minimum recommended values for the associated oper-
ating conditions. Applying capacitor values below those
indicated in Table 2 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 ap-
plied voltage and give dependable service. Other types,
including Y5V and Z5U have very large temperature and
voltage coeffi cients 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 LTM8027’s
switching frequency depends on the load current, and
at light loads it can excite a ceramic capacitor at audio
frequencies, generating audible noise.
A fi nal precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8027. A
ceramic input capacitor combined with trace or cable
input range and output voltage.
IN
and C
OUT
capacitor values in Table 2 are the
IN
, C
OUT
, R
ADJ
and R
T
values.
inductance forms a high Q (under damped) tank circuit.
If the LTM8027 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.
Input Power Requirements
The LTM8027 is biased using an internal linear regula-
tor to generate operational voltages from the V
Virtually all of the circuitry in the LTM8027 is biased via
this internal linear regulator output (INTV
internally decoupled with a low ESR capacitor to GND.
The V
is ample voltage on the V
approximately 1V of dropout, and will follow the V
with voltages below the dropout threshold.
The LTM8027 has a typical start-up requirement of V
7.5V. This assures that the onboard regulator has ample
headroom to bring INTV
The INTV
the BIAS pin above 8.5V allows use of externally derived
power for the IC. This effectively shuts down the internal
linear regulator and reduces power dissipation within the
LTM8027. Using the onboard regulator for start-up, then
deriving power for V
mizes conversion effi ciencies and is common practice. If
V
LTM8027 can continue to operate with V
BIAS Power
The internal circuitry of the LTM8027 is powered by the
INTV
tioned internal linear regulator or the BIAS1 and BIAS2
pins, if it is greater than 8.5V. Since the internal linear
regulator is by nature dissipative, deriving INTV
an external source through the BIAS pins reduces the
power lost within the LTM8027 and can increase overall
system effi ciency.
CC
is maintained above 6.5V using an external source, the
CC
CC
bus, which is derived either from the afore men-
regulator generates an 8V output provided there
CC
regulator can only source current, so forcing
CC
from the converter output maxi-
CC
IN
above its UVLO threshold.
pin. The V
LTM8027
CC
IN
CC
regulator has
as low as 4V.
). This pin is
CC
11
IN
IN
from
pin.
IN
8027f
pin
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