ltm8025 Linear Technology Corporation, ltm8025 Datasheet - Page 9

• Manufacturer Part Number: ltm8025
• Description: 36v, 3a Step-down ?module Converter
• Manufacturer: Linear Technology Corporation

BLOCK DIAGRAM
OPERATION
The LTM8025 is a standalone nonisolated step-down
switching DC/DC power supply that can deliver up to 3A of
output current. This module provides a precisely regulated
output voltage programmable via one external resistor
from 0.8V to 25V. The input voltage range is 3.6V to 36V.
Given that the LTM8025 is a step-down converter, make
sure that the input voltage is high enough to support the
desired output voltage and load current.
As shown in the Block Diagram, the LTM8025 contains a
current mode controller, power switching element, power
inductor, power Schottky diode and a modest amount of
input and output capacitance. The LTM8025 is a fi xed
frequency PWM regulator. The switching frequency is set
by simply connecting the appropriate resistor value from
the RT pin to GND.
An internal regulator provides power to the control circuitry.
The bias regulator normally draws power from the V
pin, but if the BIAS pin is connected to an external volt-
age higher than 2.8V, bias power will be drawn from the
external source (typically the regulated output voltage).
This improves effi ciency. The RUN/SS pin is used to place
the LTM8025 in shutdown, disconnecting the output and
reducing the input current to less than 1μA.
V
RUN/SS
SHARE
SYNC
IN
0.2μF
CONTROLLER
CURRENT
MODE
GND
RT
IN
To further optimize effi ciency, the LTM8025 automatically
switches to Burst Mode
Between bursts, all circuitry associated with controlling
the output switch is shut down reducing the input supply
current to 50μA in a typical application.
The oscillator reduces the LTM8025’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 LTM8025 contains a power good comparator which
trips when the ADJ pin is at roughly 90% 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 LTM8025 is enabled and V
The LTM8025 is equipped with a thermal shutdown that
will inhibit power switching at high junction tempera-
tures. The activation threshold of this function, however,
is above 125°C to avoid interfering with normal operation.
Thus, prolonged or repetitive operation under a condition
in which the thermal shutdown activates may damage or
impair the reliability of the device.
Burst Mode is a registered trademark of Linear Technology Corporation.
PGOOD
8.2μH
15pF
ADJ
499k
4.4μF
®
operation in light load situations.
V
BIAS
AUX
OUT
8025 BD
IN
is above 3.6V.
LTM8025
9
8025f