LTM8033
13
8033fb
For more information www.linear.com/LTM8033
APPLICATIONS INFORMATION
Frequency Selection
The LTM8033 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2.4MHz
by using a resistor tied from the RT pin to ground. Table 2
provides a list of R
T
resistor values and their resulting
frequencies.
Table 2. Switching Frequency vs R
T
Value
SWITCHING FREQUENCY (MHz) R
T
VALUE (kΩ)
0.2 215
0.3 137
0.4 100
0.5 76.8
0.6 63.4
0.7 52.3
0.8 44.2
0.9 38.3
1 34
1.2 25.5
1.4 21.5
1.6 17.8
1.8 14.7
2 12.1
2.2 9.76
2.4 8.25
Operating Frequency Trade-Offs
It is recommended that the user apply the optimal R
T
value given in Table 1 for the input and output operating
condition. System level or other considerations, however,
may necessitate another operating frequency. While the
LTM8033 is flexible enough to accommodate a wide range
of operating frequencies, a haphazardly chosen one may
result in undesirable operation under certain operating or
fault conditions. A frequency that is too high can reduce
efficiency, generate excessive heat or even damage the
LTM8033 if the output is overloaded or short-circuited.
A frequency that is too low can result in a final design
that has too much output ripple or too large of an output
capacitor.
BIAS Pin Considerations
The BIAS pin is used to provide drive power for the internal
power switching stage and operate other internal circuitry.
For proper operation, it must be powered by at least 2.8V. If
the output voltage is programmed to 2.8V or higher, BIAS
may be simply tied to V
OUT
. If V
OUT
is less than 2.8V, BIAS
can be tied to V
IN
or some other voltage source. If the BIAS
pin voltage is too high, the efficiency of the LTM8033 may
suffer. The optimum BIAS voltage is dependent upon many
factors, such as load current, input voltage, output voltage
and switching frequency, but 4V to 5V works well in many
applications. In all cases, ensure that the maximum voltage
at the BIAS pin is less than 25V and that the sum of V
IN
and BIAS is less 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 pin.
Load Sharing
Two or more LTM8033 may be paralleled to produce higher
currents. To do this, tie the V
IN
, ADJ, V
OUT
and SHARE
pins of all the paralleled LTM8033 together. To ensure that
paralleled modules start up together, the RUN/SS pins
may be tied together as well. If the RUN/SS pins are not
tied together, make sure that the same valued soft-start
capacitors are used for each module. Current sharing can
be improved by synchronizing the LTM8033s. An example
of two LTM8033 configured for load sharing is given in
the Typical Applications section.
Burst Mode Operation
To enhance efficiency at light loads, the LTM8033 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 L
TM8033 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 30μA and 75μA respectively during
the sleep time. As the load current decreases towards a
