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LT1185

1185ff

APPLICATIO S I FOR ATIO

Figure 3. Shutdown Techniques

RLIM

†

–

VIN

30k

2.4k

†

VOUT

2N3906

LT1185 • F3a

*CMOS LOGIC

†FOR HIGHER VALUES OF RLIM, MAKE R7 = (RLIM)(0.6)

300k

REF

GND

VOUT

VIN

LT1185

5V Logic, Positive Regulated Output

5V Logic, Negative Regulated Output

RLIM

–

LT1185 • F03b

REF

GND

VOUT

LT1185

VIN

33k

“HI” = OUTPUT “OFF”

3 EA 1N4148

VIN

2N3906

for remote sense applications, they may need to be con-

sidered. Ground lead resistance of 0.4

Ω would cause an

output voltage error of up to (3A/40)(0.4

Ω) = 30mV, or

0.6% at VOUT = 5V. Note that if the sense leads are

connected as shown in Figure 2, with ra ≈ 0Ω, this error is

a fixed number of millivolts, and does not increase as a

function of DC output voltage.

Shutdown Techniques

The LT1185 can be shut down by open-circuiting the REF

pin. The current flowing into this pin must be less than

0.4

µA to guarantee shutdown. Figure 3 details several

ways to create the “open” condition, with various logic

levels. For variations on these schemes, simply remember

that the voltage on the REF pin is 2.4V negative with

respect to the ground pin.

Output Overshoot

Very high input voltage slew rate during start-up may

cause the LT1185 output to overshoot. Up to 20% over-

shoot could occur with input voltage ramp-up rate exceed-

ing 1V/

µs. This condition cannot occur with normal 50Hz

to 400Hz rectified AC inputs because parasitic resistance

and inductance will limit rate of rise even if the power

switch is closed at the peak of the AC line voltage. This

assumes that the switch is in the AC portion of the circuit.

If instead, a switch is placed directly in the regulator input

so that a large filter capacitor is precharged, fast input slew

rates will occur on switch closure. The output of the

regulator will slew at a rate set by current limit and output

capacitor size; dVdt = ILIM/COUT. With ILIM = 3.6A and COUT

= 2.2

µF, the output will slew at 1.6V/µs and overshoot can

occur. This overshoot can be reduced to a few hundred

millivolts or less by increasing the output capacitor to

µF and/or reducing current limit so that output slew rate

is held below 0.5V/

µs.

A second possibility for creating output overshoot is

recovery from an output short. Again, the output slews at

a rate set by current limit and output capacitance. To avoid

overshoot, the ratio ILIM/COUT should be less than

0.5

× 106. Remember that load capacitance can be added

to COUT for this calculation. Many loads will have multiple

supply bypass capacitors that total more than COUT.

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