13
LT1579
A small capacitor placed in parallel with the top resistor
(R2) of the output divider is necessary for stability and
transient performance of the adjustable LT1579. The
impedance of C
FB
at 10kHz should be less than the value
of R1.
The adjustable LT1579 is tested and specified with the
output pin tied to the adjust pin and a 3µA load (unless
otherwise noted) for an output voltage of 1.5V. Specifica-
tions for output voltages greater than 1.5V are propor-
tional to the ratio of the desired output voltage to 1.5V;
(V
OUT
/1.5V). For example, load regulation for an output
current change of 1mA to 300mA is –2mV typical at
V
OUT
= 1.5V. At V
OUT
= 12V, load regulation is:
12
15
216
V
V
mV mV
.
––
()
=
Output Capacitance and Transient Response
The LT1579 is designed to be stable with a wide range of
output capacitors. The ESR of the output capacitor affects
stability, most notably with small capacitors. A minimum
output capacitor of 4.7µF with an ESR of 3Ω or less is
recommended to prevent oscillations. Smaller value ca-
pacitors may be used, but capacitors which have a low
ESR (i.e. ceramics) may need a small series resistor added
to bring the ESR into the range suggested in Table 1. The
LT1579 is a micropower device and output transient
response is a function of output capacitance. Larger
values of output capacitance decrease the peak deviations
and provide improved output transient response for larger
load current changes. Bypass capacitors, used to decouple
individual components powered by the LT1579, will
increase the effective output capacitor value.
Table 1. Suggested ESR Range
OUTPUT CAPACITANCE SUGGESTED ESR RANGE
1.5µF1Ω to 3Ω
2.2µF 0.5Ω to 3Ω
3.3µF 0.2Ω to 3Ω
≥4.7µF0Ω to 3Ω
APPLICATIONS INFORMATION
WUU
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BIASCOMP Pin Compensation
The BIASCOMP pin is a connection to a compensation
point for the internal bias circuitry. It must be bypassed
with a 0.01µF capacitor for stability during the switch from
V
IN1
to V
IN2
.
“Hot” Plugging and Unplugging of Inputs
The LT1579 is designed to maintain regulation even if one
of the outputs is instantaneously removed. If the primary
input is supplying load current, removal and insertion of
the secondary input creates no noticeable transient at the
output. In this case, the LT1579 continues to supply
current from the primary; no switching is required. How-
ever, when load current is being supplied from the primary
input and it is removed, load current must be switched
from the primary to the secondary input. In this case, the
LT1579 sees the input capacitor as a rapidly discharging
battery. If it discharges too quickly, the LT1579 does not
have ample time to switch over without a large transient
occurring at the output. The input capacitor must be large
enough to supply load current during the transition from
primary to secondary input. Replacement of the primary
creates a smaller transient on the output because both
inputs are present during the transition. For a 100mA load,
input and output capacitors of 10µF will limit peak output
deviations to less than 50mV. See the “Hot” Plugging and
Unplugging Transient Response in the Typical Perfor-
mance Characteristics. Proportionally larger values for
input and output capacitors are needed to limit peak
deviations on the output when delivering larger load
currents.
Standby Mode
“Standby” mode is where one input draws a minimum
quiescent current when the other input is delivering all
bias and load currents . In this mode, the standby current
is the quiescent current drawn from the standby input. The
secondary input will be in standby mode, when the pri-
mary input is delivering all load and bias currents. When
the secondary input is in standby mode the current drawn
from the secondary input will be 3µA if V
IN1
> V
IN2
and 5µA
