LTC3025-1/LTC3025-2/
LTC3025-3/LTC3025-4
8
30251234ff
applicaTions inForMaTion
Operation (Refer to Block Diagram)
The LTC3025-X is a micropower, VLDO (very low dropout)
linear regulator which operates from input voltages as low
as 0.9V. The device provides a highly accurate output that
is capable of supplying 500mA of output current with a
typical dropout voltage of only 85mV. A single ceramic
capacitor as small as 1µF is all that is required for output
bypassing. A low reference voltage allows the LTC3025-1
output to be programmed to much lower voltages than
available in common LDOs (range of 0.4V to 3. 6V). The
LTC3025-2/LTC3025-3/LTC3025-4 have fixed outputs of
1.2V, 1.5V and 1.8V respectively, eliminating the need for
an external resistor divider.
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 efficiency
regulation. The low quiescent supply currents I
IN
= 4µA,
I
BIAS
= 50µA drop to I
IN
= 1µA, I
BIAS
= 0.01µA typical in
shutdown making the LTC3025-X 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-X also includes overshoot detection circuitry
which brings the output back into regulation when going
from heavy to light output loads (see Figure 1).
Figure 1. LTC3025-X Transient Response
300mA
0mA
I
OUT
V
OUT
AC
20mV/DIV
V
IN
= 1.5V
V
OUT
= 1.2V
V
BIAS
= 3.6V
C
OUT
= 1µF
100µs/DIV
30251234 F01
Adjustable Output Voltage (LTC3025-1)
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
OUT
is discharged.
The LTC3025-1 operates at a relatively high gain of –0.7µV/
mA referred to the ADJ input. Thus a load current change
of 1mA to 500mA produces a –0.35mV drop at the ADJ
input. To calculate the change referred to the output
simply 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 500mA produces –0.35mV • (1 + 2) =
1.05mV 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 amplifier 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.
Figure 2. Programming the LTC3025-1
( )
OUT
R1
R2
30251234 F02
C
OUT
R2
R1
V
OUT
= 0.4V 1 +
ADJ
GND
