ltm8033 Linear Technology Corporation, ltm8033 Datasheet - Page 14

• Manufacturer Part Number: ltm8033
• Description: Ultralow Noise Emc 36vin, 3a Dc/dc
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTM8033
no-load condition, the percentage of time that the LTM8033
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 LTM8033 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
BIAS power whether RUN/SS is used. If BIAS is available
before V
may be reduced. As shown in the Typical Performance
Characteristics section, the minimum input voltage to
run a 3.3V output at light load is only about 3.6V, but, if
RUN/SS is pulled up to V
LTM8033 is enabled with the RUN/SS pin, the minimum
voltage to start at light loads is lower, about 4.2V. Similar
curves detailing this behavior of the LTM8033 for other
outputs are also included in the Typical Performance
Characteristics section.
14
OUT
ramps up, the minimum V
Figure 2. To Soft-Start the LTM8033, Add a Resistor and Capacitor to the RUN/SS Pin
IN
, it takes 5.6V
RUN
15k
0.22μF
RUN/SS
GND
IN
IN
voltage to start
to start. If the
INDUCTOR
INTERNAL
CURRENT
V
1A/DIV
RUN/SS
2V/DIV
2V/DIV
V
OUT
Soft-Start
The RUN/SS pin can be used to soft-start the LTM8033,
reducing the maximum input current during start-up. The
RUN/SS pin is driven through an external RC filter to cre-
ate 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.
Frequency Foldback
The LTM8033 is equipped with frequency foldback which
acts to reduce the thermal and energy stress on the internal
power elements during a short-circuit or output overload
condition. If the LTM8033 detects that the output has fallen
out of regulation, the switching frequency is reduced as a
function of how far the output is below the target voltage.
This in turn limits the amount of energy that can be delivered
to the load under fault. During the start-up time, frequency
foldback is also active to limit the energy delivered to the
potentially large output capacitance of the load.
2ms/DIV
8033 F02
8033f