LTC3025-1 LINER [Linear Technology], LTC3025-1 Datasheet - Page 8

• Manufacturer Part Number: LTC3025-1
• Description: 500mA Micropower VLDO Linear Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3025-1/LTC3025-2/
LTC3025-3/LTC3025-4
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 operates at a relatively high gain of
–0.7μV/mA referred to the ADJ input. Thus a load cur-
rent 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.
8
Figure 3. Ceramic Capacitor DC Bias Characteristics
OUT
–100
–20
–40
–60
–80
is discharged.
20
0
0
2
DC BIAS VOLTAGE (V)
Y5V
BOTH CAPACITORS ARE 1μF,
10V, 0603 CASE SIZE
4
6
X5R
8
30251234 F03
10
Output Capacitance and Transient Response
The LTC3025-X is designed to be stable with a wide range
of ceramic output capacitors. The ESR of the output capaci-
tor affects stability, most notably with small capacitors. A
minimum output capacitor of 1μF with an ESR of 0.05Ω or
less is recommended to ensure stability. The LTC3025-X
is a micropower device and output transient response
will be a function of output capacitance. Larger values
of output capacitance decrease the peak deviations and
provide improved transient response for larger load current
changes. Note that bypass capacitors used to decouple
individual components powered by the LTC3025-X will
increase the effective output capacitor value. High ESR
tantalum and electrolytic capacitors may be used, but
a low ESR ceramic capacitor must be in parallel at the
output. There is no minimum ESR or maximum capacitor
size requirements.
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
temperature and applied voltage. The most common di-
electrics used are Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but exhibit large voltage and tem-
perature coeffi cients as shown in Figures 3 and 4. When
used with a 2V regulator, a 1μF Y5V capacitor can lose as
Figure 4. Ceramic Capacitor Temperature Characteristics
–100
–20
–40
–60
–80
20
0
–50
BOTH CAPACITORS ARE 1μF,
10V, 0603 CASE SIZE
–25
TEMPERATURE (°C)
0
25
Y5V
50
X5R
30251234 F04
75
30251234fc