LTM8032 Datasheet, PDF (11/20 Pages) Linear Technology – Ultralow Noise EMC Compliant 36V, 2A DC/DC μModule

LTM8032
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 LTM8032.
Load Sharing
Two or more LTM8032s may be paralleled to produce higher
currents. This may, however, alter the EMI performance of
the LTM8032s. To do this, tie the V_IN, ADJ, V_OUT and SHARE
pins of all the paralleled LTM8032s together. To ensure that
paralleled modules start up together, the RUN/SS pins may
be tied together, as well. Synchronize the LTM8032s 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 LTM8032 modules configured for load
sharing is given in the Typical Applications section.
Burst Mode Operation
To enhance efficiency at light loads, the LTM8032 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 LTM8032 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_IN and BIAS quiescent currents are
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 LTM8032
operates in sleep mode increases and the average input
current is greatly reduced, resulting in higher efficiency.
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 floating.
Minimum Input Voltage
The LTM8032 is a step-down converter, so a minimum
amount of headroom is required to keep the output in
regulation. 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 Figure 1, it
takes only about 3.6V_IN for the LTM8032 to run a 3.3V
output at light load. If RUN/SS is pulled up to V_IN, it takes
6.0
V_OUT = 3.3V
5.5
5.0
4.5
4.0
3.5
3.0
0
TO RUN
TO START
RUN/SS ENABLED
500
1000
1500
LOAD CURRENT (mA)
2000
8032 F01a
7.5
V_OUT = 5V
7.0
6.5
6.0
5.5
5.0
0
TO RUN
TO START
RUN/SS ENABLED
500
1000
1500
LOAD CURRENT (mA)
2000
8032 F01b
Figure 1. The LTM8032 Needs More Voltage to Start Than Run
5.5V_IN to start. If the LTM8032 is enabled via the RUN/SS
pin, the minimum voltage to start at light loads is lower,
about 4.5V. A similar curve for 5V_OUT operation is also
provided in Figure 1.
Soft-Start
The RUN/SS pin can be used to soft-start the LTM8032,
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 2 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.
8032f
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