ltc3025edc-trpbf Linear Technology Corporation, ltc3025edc-trpbf Datasheet - Page 7

• Manufacturer Part Number: ltc3025edc-trpbf
• Description: 300ma Micropower Vldo Linear Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Operation (Refer to Block Diagram)
The LTC3025 is a micropower, VLDO (very low dropout)
linear regulator which operates from input voltages as
low as 0.9V. The device provides a high accuracy output
that is capable of supplying 300mA of output current with
a typical dropout voltage of only 45mV. A single ceramic
capacitor as small as 1μF is all that is required for output
bypassing. A low reference voltage allows the LTC3025
output to be programmed to much lower voltages than
available in common LDOs (range of 0.4V to 3. 6V).
As shown in the Block Diagram, the BIAS input supplies
the internal reference and LDO circuitry while all output
current comes directly from the IN input for high effi ciency
regulation. The low quiescent supply currents I
I
shutdown making the LTC3025 an ideal choice for use in
battery-powered systems.
The device includes current limit and thermal overload
protection. The fast transient response of the follower
output stage overcomes the traditional tradeoff between
dropout voltage, quiescent current and load transient
response inherent in most LDO regulator architectures.
The LTC3025 also includes overshoot detection circuitry
which brings the output back into regulation when going
from heavy to light output loads (see Figure 1).
BIAS
= 50μA drop to I
I
OUT
20mV/DIV
300mA
V
0mA
Figure 1. LTC3025 Transient Response
OUT
AC
V
V
V
C
IN
OUT
BIAS
OUT
= 1.5V
= 1.2V
= 1 F
= 3.6V
IN
= 1μA, I
100 s/DIV
BIAS
= 0.01μA typical in
3025 F01
IN
= 4μA,
Adjustable Output Voltage
The output voltage is set by the ratio of two external resis-
tors as shown in Figure 2. The device servos the output
to maintain the ADJ pin voltage at 0.4V (referenced to
ground). Thus the current in R1 is equal to 0.4V/R1. For
good transient response, stability, and accuracy, the current
in R1 should be at least 8μA, thus the value of R1 should
be no greater than 50k. The current in R2 is the current in
R1 plus the ADJ pin bias current. Since the ADJ pin bias
current is typically <10nA, it can be ignored in the output
voltage calculation. The output voltage can be calculated
using the formula in Figure 2. Note that in shutdown the
output is turned off and the divider current will be zero
once C
The LTC3025 operates at a relatively high gain of –0.7μV/
mA referred to the ADJ input. Thus a load current change
of 1mA to 300mA produces a –0.2mV drop at the ADJ
input. To calculate the change referred to the output sim-
ply multiply by the gain of the feedback network (i. e. ,1
+ R2/R1). For example, to program the output for 1.2V
choose R2/R1 = 2. In this example, an output current
change of 1mA to 300mA produces –0.2mV • (1 + 2) =
0.6mV drop at the output.
Because the ADJ pin is relatively high impedance (depend-
ing on the resistor divider used) , stray capacitance at this
pin should be minimized (<10pF) to prevent phase shift
in the error amplifi er loop. Additionally, special attention
should be given to any stray capacitances that can couple
external signals onto the ADJ pin producing undesirable
output ripple. For optimum performance connect the ADJ
pin to R1 and R2 with a short PCB trace and minimize all
other stray capacitance to the ADJ pin.
OUT
is discharged.
Figure 2. Programming the LTC3025
GND
OUT
ADJ
R2
R1
3025 F02
C
OUT
V
OUT
= 0.4V 1 +
LTC3025
R2
R1
3025fa
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