Datasheet

LTC3035
7
3035f
Figure 2. Output Step Response
Adjustable Output Voltage
The output voltage is set by the ratio of two external
resistors as shown in Figure 4. 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 4. Note that in
shutdown the output is turned off and the divider current
will be zero once C
OUT
is discharged.
The LTC3035 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 refered to
the output simply multiply by the gain of the feedback
network (i.e., 1 + R2/R1). For example, to program the
output for 3.3V choose R2/R1 = 7.25. In this example an
output current change of 1mA to 300mA produces
–0.2mV • (1 + 7.25) = 1.65mV drop at the output.
Output Capacitance and Transient Response
The LTC3035 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. An
output capacitor of 1µF or greater with an ESR of 0.05Ω or
less is recommended to ensure stability. The LTC3035 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 LTC3035 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 strong voltage and tem-
perature coefficients as shown in Figures 5 and 6. When
used with a 3.3V regulator, a 1µF Y5V capacitor can lose
as much as 80% of its rated capacitance over the operating
V
OUT
20mV/DIV
AC
I
OUT
300mA
200µs/DIVV
IN
= 3.6V
V
OUT
= 3.3V
C
OUT
= 1µF
3035 F02
0mA
ON
OFF
V
BIAS
2V/DIV
V
OUT
2V/DIV
0V
SHDN
500µs/DIVV
IN
= 3.6V
V
OUT
= 3.3V
C
OUT
= 1µF
C
BIAS
= 1µF
3035 F03
0V
Figure 4. Programming the LTC3035
Figure 3. Bias and Output Start-Up Waveforms
V
OUT
C
OUT
R2
V
OUT
= 0.4V
3035 F04
R1
ADJ
GND
R2
R1
1 +
()
APPLICATIO S I FOR ATIO
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