LTC3026-1
9
30261f
OPERATION
The LTC3026-1 operates at a relatively high gain of
270μV/A referred to the ADJ input. Thus, a load current
change of 1mA to 1.5A produces a 400μV drop at the ADJ
input. To calculate the change in the output, simply mul-
tiply 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 1.5A produces –400μV • (1 + 2) = 1.2mV drop at
the output.
Power Good Operation
The LTC3026-1 includes an open-drain power good (PG)
output pin with hysteresis. If the chip is in shutdown or
under UVLO conditions (V
BIAS
< 4.25V typ.), PG is low
impedance to ground. PG becomes high impedance when
V
OUT
rises to 93% of its regulation voltage. PG stays high
impedance until V
OUT
falls back down to 91% of its regula-
tion value. A pull-up resistor can be inserted between PG
and a positive logic supply (such as IN, OUT, BIAS, etc.)
to signal a valid power good condition. V
IN
should be the
minimum operating voltage (1.14V) or greater for PG to
function correctly.
Output Capacitance and Transient Response
The LTC3026-1 is designed to be stable with a wide range
of ceramic output capacitors. The ESR of the output
capacitor affects stability, most notably with small ca-
pacitors. An output capacitor of 10μF or greater with an
ESR of 0.05Ω or less is recommended to ensure stability.
The LTC3026-1 is a micropower device and output tran-
sient 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
Figure 3. Programming the LTC3026-1
LTC3026-1 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 paral-
lel 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 strong voltage and tem-
perature coefficients as shown in Figures 4 and 5. When
used with a 2V regulator, a 10μF Y5V capacitor can exhibit
an effective value as low as 1μF to 2μF over the operating
temperature range. The X5R and X7R dielectrics result in
V
OUT
ADJ
GND
C
OUT
R2
R1
LTC3026-1
30261 F03
V
OUT
= 0.4V 1+
R2
R1
⎛
⎝
⎜
⎞
⎠
⎟
Figure 4. Ceramic Capacitor DC Bias Characteristics
Figure 5. Ceramic Capacitor Temperature Characteristics
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
30261 F04
20
0
–20
–40
–60
–80
–100
X5R
Y5V
BOTH CAPACITORS ARE 10μF,
6.3V, 0805 CASE SIZE
0123456
TEMPERATURE (°C)
–50
20
0
–20
–40
–60
–80
–100
25 75
30261 F05
–25 0
50
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 10μF,
6.3V, 0805 CASE SIZE
