LTM8028
9
8028fb
For more information www.linear.com/LTM8028
operaTion
Current generation FPGA and ASIC processors place
stringent demands on the power supplies that power the
core, I/O and transceiver channels. Power supplies that
power these processors have demanding output voltage
specifications, especially at low voltages, where they
require tight tolerances, small transient response excur
-
sions, low noise and high bandwidth to achieve the lowest
bit-error rates. This can be accomplished with some high
performance linear regulators, but this can be inefficient
for high current and step-down ratios.
The LTM8028 is a 5A high efficiency, UltraFast transient
response linear regulator. It integrates a buck regulator with
a high performance linear regulator, providing a precisely
regulated output voltage digitally programmable from 0.8V
to 1.8V. As shown in the Block Diagram, the LTM8028
contains a current mode controller, power switches,
power inductor, linear regulator, and a modest amount
of capacitance. To achieve high efficiency, the integrated
buck regulator is automatically controlled (Input-Output
Control on the Block Diagram) to produce the optimal
voltage headroom to balance efficiency, tight regulation
and transient response at the linear regulator output.
Figure 1 is a composite graph of the LTM8028’s power
loss compared to the theoretical power loss of a traditional
linear regulator. Note that the power loss (left hand Y axis)
is plotted on the log scale. For 1.2V
OUT
at 5A and 24V
IN
the LTM8028 only loses 4W, while the traditional linear
regulator theoretically dissipates over 110W.
The LTM8028 switching buck converter utilizes fixed-
frequency, forced continuous current mode control to
regulate its output voltage. This means that the switching
regulator will stay in fixed frequency operation even as the
LTM8028 output current falls to zero. The LTM8028 has
an analog control pin, I
MAX
, to set the maximum allow-
able current output of the LTM8028. The analog control
range of I
MAX
is from 0V to 1.5V. The RUN pin functions
as a precision shutdown pin. When the voltage at the RUN
pin is lower than 1.55V, switching is terminated. Below
this threshold, the RUN pin sinks 5.5µA. This current can
be used with a resistor between RUN and V
IN
to set the
hysteresis. During start-up, the SS pin is held low until the
part is enabled, after which the capacitor at the soft-start
pin is charged with an 11μA current source. The switching
frequency is determined by a resistor at the RT pin. The
LTM8028 may also be synchronized to an external clock
through the use of the SYNC pin.
The output linear regulator supplies up to 5A of output
current with a typical dropout voltage of 85mV. Its high
bandwidth provides UltraFast transient response using low
ESR ceramic output capacitors, saving bulk capacitance,
PCB area and cost. The output voltage for the LTM8028
is digitally selectable in 50mV increments over a 0.8V to
1.8V range, and analog margining function allows the user
to adjust system output voltage over a continuous ±10%
range. It also features a remote sense pin for accurate
regulation at the load, and a PGOOD circuit that indicates
whether the output is in or out of regulation or if an internal
fault has occurred.
The LTM8028 is equipped with a thermal shutdown to
protect the device during momentary overload conditions.
It is set above the 125°C absolute maximum internal tem
-
perature rating to avoid interfering with normal specified
operation, so internal device temperatures will exceed
the absolute maximum rating when the overtemperature
protection is active. So, continuous or repeated activation
of the thermal shutdown may impair device reliability.
During thermal shutdown, all switching is terminated
and
the SS pin is driven low.
INPUT VOLTAGE (V)
0
0
POWER LOSS (W)
TEMPERATURE RISE (°C)
10
100
30
40
50
35
45
55
10 20
30 40
TRADITIONAL LINEAR
REGULATOR POWER LOSS
TEMPERATURE RISE
POWER LOSS
Figure 1. This Graph Shows the Full Load Power Loss and
Temperature Rise of the LTM8028 over a Range of Input
Voltages. Compare These Numbers to a Traditional Linear
Regulator Powering the Same Load an Operating Condition.
Note the Log Scale for Power Loss.
