ltm8031 Linear Technology Corporation, ltm8031 Datasheet - Page 11

• Manufacturer Part Number: ltm8031
• Description: Ultralow Noise Emc 36v, 1a Dc/dc ?module Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
than 56V. If BIAS power is applied from a remote or noisy
voltage source, it may be necessary to apply a decoupling
capacitor locally to the LTM8031.
Load Sharing
Two or more LTM8031s may be paralleled to produce higher
currents. This may, however, alter the EMI performance of
the LTM8031s. To do this, tie the V
pins of all the paralleled LTM8031s together. To ensure that
paralleled modules start up together, the RUN/SS pins may
be tied together, as well. Synchronize the LTM8031s to an
external clock to eliminate beat frequencies, if required.
If the RUN/SS pins are not tied together, make sure that
the same valued soft-start capacitors are used for each
module. An example of two LTM8031 modules confi gured
for load sharing is given in the Typical Applications sec-
tion. For 2A applications also see the LTM8032, 2A EMC
DC/DC μModule regulator
Burst Mode Operation
To enhance effi ciency at light loads, the LTM8031 auto-
matically switches to Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst Mode
operation, the LTM8031 delivers single cycle bursts of
current to the output capacitor followed by sleep periods
where the output power is delivered to the load by the output
capacitor. In addition, V
reduced to typically 20μA and 50μA respectively during
the sleep time. As the load current decreases towards a
no-load condition, the percentage of time that the LTM8031
operates in sleep mode increases and the average input
current is greatly reduced, resulting in higher effi ciency.
Burst Mode operation is enabled by tying SYNC to GND. To
disable Burst Mode operation, tie SYNC to a stable voltage
above 0.7V. Do not leave the SYNC pin fl oating.
Minimum Input Voltage
The LTM8031 is a step-down converter, so a minimum
amount of headroom is required to keep the output in regu-
lation. In addition, the input voltage required to turn on is
higher than that required to run, and depends upon whether
the RUN/SS is used. As shown in the Typical Performance
IN
and BIAS quiescent currents are
IN
, ADJ, V
OUT
and SHARE
Characteristics section, it takes only about 3.6V
LTM8031 to run a 3.3V output at light load. If RUN/SS is
pulled up to V
is enabled via the RUN/SS pin, the minimum voltage to
start at light loads is lower, about 4.4V. Similar curves for
2.5V
the Typical Performance Characteristics section.
Soft-Start
The RUN/SS pin can be used to soft-start the LTM8031,
reducing the maximum input current during start-up. The
RUN/SS pin is driven through an external RC network to
create a voltage ramp at this pin. Figure 1 shows the start-
up and shutdown waveforms with the soft-start circuit. By
choosing an appropriate RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply at least 20μA when
the RUN/SS pin reaches 2.5V.
Synchronization
The internal oscillator of the LTM8031 can be synchro-
nized by applying an external 250kHz to 2MHz clock to
the SYNC pin. Do not leave this pin fl oating. The resistor
tied from the RT pin to ground should be chosen such
that the LTM8031 oscillates 20% lower than the intended
synchronization frequency (see the Frequency Selection
section). The LTM8031 will not enter Burst Mode operation
while synchronized to an external clock, but will instead
skip pulses to maintain regulation.
0.22μF
RUN
15k
OUT
Figure 1. To Soft-Start the LTM8031, Add a Resistor
and Capacitor to the RUN/SS Pin
, 5V
RUN/SS
OUT
GND
IN
, it takes 5.7V
and 8V
OUT
operation are also provided in
IN
to start. If the LTM8031
2ms/DIV
LTM8031
IN
8031 F01
11
for the
I
0.5A/DIV
V
2V/DIV
V
2V/DIV
L
RUN/SS
OUT
8031fa