Datasheet

LTM8020
7
8020fd
OPERATION
The LTM8020 is a standalone nonisolated step-down
switching DC/DC power supply. It can deliver up to
200mA 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 1.25VDC to 5VDC. The input voltage range
is 4V to 36V. Given that the LTM8020 is a step-down con-
verter, make sure that the input voltage is high enough to
support the desired output voltage and load current. See
Block Diagram.
The LTM8020 contains a current mode controller, power
switching element, power inductor, power Schottky diode
and a modest amount of input and output capacitance.
For some applications, as shown in Table 1, no output
capacitor is necessary.
With its high performance current mode controller and
internal feedback loop compensation, the LTM8020 module
has sufficient stability margin and good transient perfor-
mance under a wide range of operating conditions with a
wide range of output capacitors, even all ceramic ones (X5R
or X7R). Current mode control provides cycle-by-cycle fast
current limit, and automatic current limiting protects the
module in the event of a short circuit or overload fault.
The LTM8020 is built upon a variable frequency control-
ler. The on time, off time and switching frequency are
dependent upon the input voltage, output voltage and
load current.
The drive circuit for the internal power switching element
is powered through the BIAS pin. Power this pin with at
least 3V.
The LTM8020 is equipped with two operating modes,
dependant upon the load current. When the load current
is sufficiently high, the LTM8020 will switch continuously
(see Figure 1a). If the load is very light, or if the input
voltage is high relative to the output voltage, the part will
operate in Burst Mode
®
operation, alternating between its
micropower and switching states to keep the output in
regulation and hold the power dissipation to a minimum
(See Figure 1b).
If the SHDN pin is grounded, all internal circuits are turned
off and V
IN
current reduces to the device leakage current,
typically a few nanoamps.
Figure 1. Output Voltage and Internal Inductor Current
V
OUT
20mV/DIV
I
L
100mA/DIV
1ms/DIV
V
OUT
20mV/DIV
I
L
100mA/DIV
5μs/DIV
8020 F01b
NO LOAD
10mA LOAD
(1b) Burst Mode Operation
V
OUT
20mV/DIV
I
L
100mA/DIV
1μs/DIV
V
OUT
20mV/DIV
I
L
100mA/DIV
1μs/DIV
(1a) Continuous Operation
8020 F1a
200mA LOAD
150mA LOAD