LT1074CK datasheet(11/16 Pages) LINER

11 / 16 page

LT1074/LT1076

Figure 8. ILIM Pin Circuit

Figure 9. Foldback Current Limit

DESCRIPTIO S

ILIM PIN

The ILIM pin is used to reduce current limit below the

preset value of 6.5A. The equivalent circuit for this pin is

shown in Figure 8.

TO LIMIT

CIRCUIT

VIN

320 A

ILIM

4.3V

LT1047•PD12

When ILIM is left open, the voltage at Q1 base clamps at 5V

through D2. Internal current limit is determined by the

current through Q1. If an external resistor is connected

between ILIM and ground, the voltage at Q1 base can be

reduced for lower current limit. The resistor will have a

voltage across it equal to (320

µA)(R), limited to ≈5V when

clamped by D2. Resistance required for a given current

limit is:

RLIM = ILIM(2kΩ) + 1kΩ (LT1074)

RLIM = ILIM(5.5kΩ) + 1kΩ (LT1076)

As an example, a 3A current limit would require

3A(2k) + 1k = 7k

Ω for the LT1074. The accuracy of these

formulas is

±25% for 2A ≤ ILIM ≤ 5A (LT1074) and

≤ ILIM ≤ 1.8A (LT1076), so ILIM should be set at least

25% above the

peak switch current required.

Foldback current limiting can be easily implemented by

adding a resistor from the output to the ILIM pin as shown

in Figure 9. This allows full desired current limit (with or

without RLIM) when the output is regulating, but reduces

current limit under short-circuit conditions. A typical value

for RFB is 5kΩ, but this may be adjusted up or down to set

the amount of foldback. D2 prevents the output voltage

from forcing current back into the ILIM pin. To calculate a

value for RFB, first calculate RLIM, the RFB:

Rin k

LSC

−

()( )

−Ω

()−

Ω

()

044

*Change 0.44 to 0.16, and 0.5 to 0.18 for LT1076.

Example: ILIM = 4A, ISC = 1.5A, RLIM = (4)(2k) + 1k = 9k

FB =

−

() Ω

()

−

()−

Ω

()

15 044 9

05 9

Error Amplifier

The error amplifier in Figure 10 is a single stage design

with added inverters to allow the output to swing above

and below the common mode input voltage. One side of

the amplifier is tied to a trimmed internal reference voltage

of 2.21V. The other input is brought out as the FB (feed-

back) pin. This amplifier has a GM (voltage “in” to current

“out”) transfer function of

≈5000µmho. Voltage gain is

determined by multiplying GM times the total equivalent

output loading, consisting of the output resistance of Q4

and Q6 in parallel with the series RC external frequency

compensation network. At DC, the external RC is ignored,

and with a parallel output impedance for Q4 and Q6 of

400k

Ω, voltage gain is ≈2000. At frequencies above a few

hertz, voltage gain is determined by the external compen-

sation, RC and CC.

VOUT

ILIM

RLIM

RFB

1N4148

LT1074•PD13

LT1074

LT1074CK Datasheet(PDF) 11 Page - Linear Technology

LT1074CK Datasheet(HTML) 11 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL