LT1579 datasheet(13/20 Pages) LINER

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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 CFB 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 3

µA 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;

(VOUT/1.5V). For example, load regulation for an output

current change of 1mA to 300mA is – 2mV typical at

VOUT = 1.5V. At VOUT = 12V, load regulation is:

216

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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 4.7

µF 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

1.5

µF1Ω to 3Ω

2.2

µF

0.5

Ω to 3Ω

3.3

µF

0.2

Ω to 3Ω

≥4.7µF0Ω to 3Ω

APPLICATIONS INFORMATION

BIASCOMP Pin Compensation

The BIASCOMP pin is a connection to a compensation

point for the internal bias circuitry. It must be bypassed

with a 0.01

µF capacitor for stability during the switch from

VIN1 to VIN2.

“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 10

µF 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 3

µA if VIN1 > VIN2 and 5µA

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