ltm8023 Linear Technology Corporation, ltm8023 Datasheet - Page 11

• Manufacturer Part Number: ltm8023
• Description: 2a, 36v Dc/dc ?module
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
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. An example of two
LTM8023 modules confi gured for load sharing is given
in the Typical Applications section.
Burst Mode Operation
To enhance effi ciency at light loads, the LTM8023 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 LTM8023 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 LTM8023
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 LTM8023 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 2, it
takes only about 3.5V
output at light load. If RUN/SS is pulled up to V
5.5V
pin, the minimum voltage to start at light loads is lower,
about 4.5V. A similar curve for 5V
provided in Figure 2.
Soft-Start
The RUN/SS pin can be used to soft-start the LTM8023,
reducing the maximum input current during startup. The
IN
to start. If the LTM8023 is enabled via the RUN/SS
IN
IN
for the LTM8023 to run a 3.3V
and BIAS quiescent currents are
OUT
operation is also
IN
, it takes
RUN/SS pin is driven through an external RC fi lter to cre-
ate a voltage ramp at this pin. Figure 3 shows the startup
and shutdown waveforms with the soft-start circuit. By
choosing an appropriate RC time constant, the peak startup
0.22μF
Figure 2. The LTM8023 Needs More Voltage to Start Than to Run
RUN
15k
Figure 3. To Soft-Start the LTM8023, Add a
Resistor and Capacitor to the RUN/SS Pin
RUN/SS
6.0
5.5
5.0
4.5
4.0
3.5
3.0
7.5
7.0
6.5
6.0
5.5
5.0
GND
0
0
TO RUN
TO RUN
TO START
TO START
RUN/SS ENABLED
RUN/SS ENABLED
500
500
LOAD CURRENT (mA)
LOAD CURRENT (mA)
1000
1000
2ms/DIV
1500
1500
V
T
f = 650kHz
A
OUT
V
T
f = 650kHz
= 25°C
OUT
A
= 25°C
= 3.3V
LTM8023
= 5V
8023 F02
2000
2000
8023 F03
11
I
1A/DIV
V
2V/DIV
V
2V/DIV
L
RUN/SS
OUT
8023f