ltm8008 Linear Technology Corporation, ltm8008 Datasheet - Page 10

• Manufacturer Part Number: ltm8008
• Description: Ltm8008 - 72vin, 6 Output Dc/dc Sepic ?module Regulator
• Manufacturer: Linear Technology Corporation

LTM8008
SS pin, initiating a soft-start operation. The soft-start in-
terval is set by the soft-start capacitor selection according
to the equation:
Thermal Lockout
If LTM8008 internal temperature reaches 165°C (typical),
the part will go into thermal lockout. The power switch
will be turned off. A soft-start operation will be triggered.
The part will be enabled again when the die temperature
has dropped by 5°C (nominal).
Loop Compensation
Loop compensation determines the stability and transient
performance. The LTM8008 uses current mode control to
regulate the output which simplifies loop compensation.
The optimum values depend on the converter topology, the
component values and the operating conditions (including
the input voltage, load current, etc.). To compensate the
feedback loop of the LTM8008, a series resistor-capacitor
network is usually connected from the V
Figure 2 shows the typical V
most applications, the capacitor C
of 1nF to 47nF , and the resistor R
of 2.5k to 50k. A small capacitor C
parallel with the R
V
through the internal error amplifier. The parallel capacitor
usually ranges in value from 10pF to 100pF . A practical
approach to design the compensation network is to start
with one of the circuits in this data sheet that is similar
to your application, and tune the compensation network
to optimize the performance. Stability should then be
applicaTions inForMaTion
10
C
T
voltage ripple induced from the output voltage ripple
SS
= C
Figure 2. A Typical Compensation Network
SS
•
1.25V
10µA
C
compensation network to attenuate the
C
C2
C
R
compensation network. For
C
C
C1
C
C1
C2
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should be in the range
V
should be in the range
C
is often connected in
C
pin to GND.
checked across all operating conditions, including load
current, input voltage and temperature.
Board Layout
The high speed operation of the LTM8008 demands careful
attention to board layout and component placement. The
GND pads of the package are the primary heat path of the
device, and are important for thermal management. There-
fore, it is crucial to achieve a good electrical and thermal
contact between the GND pads and the ground plane of
the board. For the LTM8008 to deliver its full output power,
it is imperative that a good thermal path be provided to
dissipate the heat generated within the package.
It is recommended that multiple vias in the printed circuit
board be used to conduct heat away from the LTM8008
and into a copper plane with as much area as possible. To
prevent radiation and high frequency resonance problems,
proper layout of the components connected to the LTM8008
is essential, especially the power paths with higher di/dt. The
high di/dt loop should be kept as tight as possible to reduce
inductive ringing. In the SEPIC configuration, the high
di/dt loop contains the power MOSFET, sense resistor,
output capacitor, Schottky diode and the coupling capacitor.
Keep the circuit path among these components as short
as possible.
The LTM8008 is a switching power supply, so care must
be taken to minimize EMI and ensure proper operation.
Even with the high level of integration, you may fail to
achieve specified operation with a haphazard or poor
layout. See Figure 3 for a suggested layout. Ensure that
the grounding and heat-sinking are acceptable. Here are
additional tips to follow:
1. Place the L1, R
2. Place the C
3. Place the C
sible to their respective pins.
as possible to their respective pins. If more than one
capacitor is required in parallel, place as many of the
electrically paralleled capacitors as close as possible
to their respective pin.
currents follow a path as short as possible.
IN
IN
, C
and C
INTVCC
T
and V
OUT
, SPV and C
capacitors such that their ground
C
components as close as pos-
OUT
capacitors as close
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