Datasheet

LTM8023
9
8023fj
For more information www.linear.com/LTM8023
OPERATION
The LTM8023 is a standalone nonisolated step-down
switching DC/DC power supply. It can deliver up to 2A of
DC output current with only bulk external input and output
capacitors. This module provides a precisely regulated
output voltage programmable via one external resistor
from 0.8V
DC
to 10V
DC
. The input voltage range is 3.6V to
36V. Given that the LTM8023 is a step-down converter,
make sure that the input voltage is high enough to support
the desired output voltage and load current. A simplified
Block Diagram is given on the previous page.
The LTM8023 contains a current mode controller, power
switching element, power inductor, power Schottky diode
and a modest amount of input and output capacitance.
The LTM8023 is a fixed frequency PWM regulator. The
switching frequency is set by simply connecting the
appropriate resistor value from the R
T
pin to GND.
An internal regulator provides power to the control cir-
cuitry. The bias regulator normally draws power from the
V
IN
pin, but if the BIAS pin is connected to an external
voltage higher than 2.8V, bias power will be drawn from
the external source (typically the regulated output volt-
age). This improves efficiency. The RUN/SS pin is used
to place the LTM8023 in shutdown, disconnecting the
output and reducing the input current to less than 1µA.
To further optimize efficiency, the LTM8023 automatically
switches to Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down reducing the input supply
current to 50µ
A in a typical application. The oscillator
reduces the LTM8023’s operating frequency when the
voltage at the ADJ pin is low. This frequency foldback helps
to control the output current during start-up and overload.
The LTM8023 contains a power good comparator which
trips when the ADJ pin is at 92% of its regulated value.
The PGOOD output is an open-collector transistor that is
off when the output is in regulation, allowing an external
resistor to pull the PGOOD pin high. Power good is valid
when the LTM8023 is enabled and V
IN
is above 3.6V.
For most applications, the design process is straight
forward, summarized as follows:
1
. Look at Table 1 and find the row that has the desired
input range and output voltage.
2
. Apply the recommended C
IN
, C
OUT
, R
ADJ
and R
T
values.
3
. Connect BIAS as indicated.
While these component combinations have been tested
for proper operation, it is incumbent upon the user to
verify proper operation over the intended system’s line,
load and environmental conditions.
Capacitor Selection Considerations
The C
IN
and C
OUT
capacitor values in Table 1 are the
minimum recommended values for the associated oper-
ating conditions. Applying capacitor values below those
indicated in Table 1 is not recommended, and may result
in undesirable operation. Using larger values is generally
acceptable, and can yield improved dynamic response, if
it is necessary. Again, it is incumbent upon the user to
verify proper operation over the intended system’s line,
load and environmental conditions.
Ceramic capacitors are small, robust and have very low
ESR. However, not all ceramic capacitors are suitable.
X5R and X7R types are stable over temperature and ap-
plied voltage and give dependable service. Other types,
including Y5V and Z5U have very large temperature and
voltage coefficients of capacitance. In an application cir-
cuit they may have only a small fraction of their nominal
capacitance resulting in much higher output voltage ripple
than expected.
APPLICATIONS INFORMATION
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