LTM8008HVPBF LINER [Linear Technology], LTM8008HVPBF Datasheet - Page 12

• Manufacturer Part Number: LTM8008HVPBF
• Description: 72VIN, 6 Output DC/DC SEPIC ?Module Regulator
• Manufacturer: LINER [Linear Technology]

LTM8008
X7R. The Z5U and Y5V dielectrics are good for providing
high capacitances in a small package, but they tend to
have strong voltage and temperature coefficients. When
used with a 5V regulator, a 16V 10μF Y5V capacitor can
exhibit an effective value as low as 1μF to 2μF for the DC
bias voltage applied and over the operating temperature
range. The X5R and X7R dielectrics result in more stable
characteristics and are more suitable for use as the output
capacitor. The X7R type has better stability across tem-
perature, while the X5R is less expensive and is available in
higher values. Care still must be exercised when using X5R
and X7R capacitors; the X5R and X7R codes only specify
operating temperature range and maximum capacitance
change over temperature. Capacitance change due to DC
bias with X5R and X7R capacitors is better than Y5V and
Z5U capacitors, but can still be significant enough to drop
capacitor values below appropriate levels. Capacitor DC
bias characteristics tend to improve as component case
size increases, but expected capacitance at operating
voltage should be verified.
Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates
voltage across its terminals due to mechanical stress,
similar to the way a piezoelectric accelerometer or micro-
phone works. For a ceramic capacitor, the stress can be
induced by vibrations in the system or thermal transients.
The resulting voltages produced can cause appreciable
amounts of noise, especially when a ceramic capacitor is
used for noise bypassing.
Post Regulator Thermal Considerations
The power handling capability of the post regulators will
be limited by the maximum rated junction temperature
(150°C). The power dissipated by each post regulator is
approximately the output current multiplied by the input/
output voltage differential: (I
applicaTions inForMaTion
12
OUT
) • (SPV – V
OUT
).
The post regulators have internal thermal limiting de-
signed to protect the device during overload conditions.
For continuous normal conditions, the maximum junction
temperature rating of 150°C must not be exceeded. It is
important to give careful consideration to all sources of
thermal resistance from junction-to-ambient. Additional
heat sources mounted nearby must also be considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
Protection Features
The linear post regulators incorporate several protection
features. In addition to the normal protection features
associated with monolithic regulators, such as current
limiting and thermal limiting, the devices are protected
against reverse input voltages, reverse output voltages
and reverse voltages from output to input. Current limit
protection and thermal overload protection are intended
to protect the device against current overload conditions
at the output of the device. For normal operation, the
junction temperature should not exceed 150°C.
When any of the V
of the LTM8008, input current to the corresponding post
regulator will typically drop to less than 2μA. The output
of the linear post regulators can be pulled below ground
without damaging the device. If the input is left open-cir-
cuit or grounded, the output can be pulled below ground
by 20V. The outputs will act like a large resistor, typically
500k or higher, limiting current flow to less than 100μA.
In the case of a short circuit, the output will source the
short-circuit current of the device and will protect itself
by thermal limiting.
OUT
pins are forced above the SPV pin
8008fa