LT1074 Linear Technology, LT1074 Datasheet - Page 11

• Manufacturer Part Number: LT1074
• Description: Step-Down Switching Regulator
• Manufacturer: Linear Technology

PI
I
The I
preset value of 6.5A. The equivalent circuit for this pin is
shown in Figure 8.
When I
through D2. Internal current limit is determined by the
current through Q1. If an external resistor is connected
between I
reduced for lower current limit. The resistor will have a
voltage across it equal to (320 A)(R), limited to 5V when
clamped by D2. Resistance required for a given current
limit is:
As an example, a 3A current limit would require
3A(2k) + 1k = 7k for the LT1074. The accuracy of these
formulas is 25% for 2A
7A I
25% above the peak switch current required.
Foldback current limiting can be easily implemented by
adding a resistor from the output to the I
in Figure 9. This allows full desired current limit (with or
without R
current limit under short-circuit conditions. A typical value
for R
the amount of foldback. D2 prevents the output voltage
LIM
R
R
U
LIM
LIM
PIN
FB
LIM
LIM
DESCRIPTIO S
LIM
is 5k , but this may be adjusted up or down to set
= I
= I
pin is used to reduce current limit below the
LIM
LIM
is left open, the voltage at Q1 base clamps at 5V
LIM
LIM
1.8A (LT1076), so I
) when the output is regulating, but reduces
and ground, the voltage at Q1 base can be
(2k ) + 1k (LT1074)
(5.5k ) + 1k (LT1076)
TO LIMIT
CIRCUIT
R1
8K
Q1
Figure 8. I
I
V
LIM
IN
D1
LIM
U
320 A
D2
Pin Circuit
I
LIM
LIM
D3
6V
LT1047•PD12
should be set at least
4.3V
5A (LT1074) and
LIM
pin as shown
from forcing current back into the I
value for R
Example: I
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 G
“out”) transfer function of 5000 mho. Voltage gain is
determined by multiplying G
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 , voltage gain is 2000. At frequencies above a few
hertz, voltage gain is determined by the external compen-
sation, R
*Change 0.44 to 0.16, and 0.5 to 0.18 for LT1076.
R
R
FB
FB
C
0 5
0 5 9
1 5 0 44 9
and C
FB
LIM
. *
.
.
I
, first calculate R
SC
R
Figure 9. Foldback Current Limit
= 4A, ISC = 1.5A, R
LIM
k
R
C
L
I
.
.
0 44
LIM
1
LT1074
.
k
1
R
k
FB
*
k
1 5
FB
R
.
1N4148
L
LT1074/LT1076
D2
I
M
SC
3 8
LIM
.
times the total equivalent
M
R in k
k
, the R
(voltage “in” to current
LIM
L
LIM
LT1074•PD13
= (4)(2k) + 1k = 9k
pin. To calculate a
FB
V
OUT
:
11