ltc3025edc-1 Linear Technology Corporation, ltc3025edc-1 Datasheet - Page 7

• Manufacturer Part Number: ltc3025edc-1
• Description: 500ma Micropower Vldo Linear Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Operation (Refer to Block Diagram)
The LTC3025-1/LTC3025-2 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 75mV.
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 has a fi xed1.2V output
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 effi ciency
regulation. The low quiescent supply currents I
I
shutdown making the LTC3025-1/LTC3025-2 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 re-
sponse inherent in most LDO regulator architectures. The
LTC3025-1/LTC3025-2 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
Figure 1. LTC3025-1/LTC3025-2 Transient Response
I
OUT
20mV/DIV
300mA
V
0mA
OUT
AC
V
V
V
C
IN
OUT
BIAS
OUT
= 1.5V
= 1.2V
= 1μF
= 3.6V
IN
= 1μA, I
100μs/DIV
BIAS
= 0.01μA typical in
302512 F01
IN
= 4μA,
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
The LTC3025-1/LTC3025-2 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 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 LTC3025-1
LTC3025-1/LTC3025-2
GND
OUT
ADJ
R2
R1
302512 F02
C
OUT
V
OUT
= 0.4V 1 +
( )
R2
R1
302512fa
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