ltm8022 Linear Technology Corporation, ltm8022 Datasheet - Page 10

• Manufacturer Part Number: ltm8022
• Description: 1a, 36v Dc/dc ?module
• Manufacturer: Linear Technology Corporation

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
effi ciency, generate excessive heat or even damage the
LTM8022 if the output is overloaded or short-circuited. A
frequency that is too low can result in a fi nal design that has
too much output ripple or too large of an output cap.
The maximum frequency and corresponding R
which the LTM8022 should be allowed to switch is given in
Table 1 in the R
mended frequency and R
is given in the R
There are additional conditions that must be satisfi ed if
the synchronization function is used. Please refer to the
Synchronization section for details.
Burst Mode Operation
To enhance effi ciency at light loads, the LTM8022 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 LTM8022 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 LTM8022
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
source above 0.7V. Do not leave this pin fl oating.
BIAS Pin Considerations
The BIAS pin is used to provide drive power for the inter-
nal power switching stage and operate internal circuitry.
For proper operation, it must be powered by at least
2.4V. If the output voltage is programmed to be 2.4V or
higher, simply tie BIAS to AUX, which is internally tied to
V
some other voltage source. In all cases, ensure that the
LTM8022
APPLICATIONS INFORMATION
10
OUT
. If V
OUT
is less than 2.4V, BIAS can be tied to V
T(MIN)
T(OPTIMAL)
and f
IN
T
and BIAS quiescent currents are
value over the given input range
MAX
and f
columns, while the recom-
OPTIMAL
columns.
T
value at
IN
or
maximum voltage at the BIAS pin is less than 16V and
the sum of V
applied from a remote or noisy voltage source, it may
be necessary to apply a decoupling capacitor locally to
the LTM8022.
Minimum Input Voltage
The LTM8022 is a step-down converter, so a minimum
amount of headroom is required to keep the output in
regulation. For most applications at full load, the input
needs to be at least 1.5V above the desired output. In
addition, the input voltage required to turn on depends
upon how the RUN/SS pin is tied. As shown in Figure 1,
it takes more input voltage to turn on if RUN/SS is tied
to V
when V
in Figure 1.
IN
than if the turn-on is controlled by raising RUN/SS
IN
is in the required operating range. This is shown
Figure 1. The LTM8022 Needs More Voltage
to Start Than to Run
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
IN
0
0
TO RUN
+
TO START
TO START
BIAS is less than 56V. If BIAS power is
RUN/SS
ENABLE
200
200
TO RUN
LOAD CURRENT (mA)
LOAD CURRENT (mA)
400
400
RUN/SS ENABLED
600
600
V
T
f = 650kHz
A
OUT
V
T
f = 650kHz
800
800
OUT
A
= 25°C
= 25°C
= 3.3V
8022 F01a
= 5V
8022 F01b
1000
1000
8022f