LT1074I Datasheet, PDF (11/16 Pages) Linear Technology – Step-Down Switching Regulator

LT1074/LT1076
PI DESCRIPTIO S
I_LIM PIN
The I_LIM 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
V_IN
320µ A
D2
Q1
D1
4.3V
R1
8K
D3
6V
I _LIM
LT1047•PD12
Figure 8. I_LIM Pin Circuit
When I_LIM 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 I_LIM and ground, the voltage at Q1 base can be
reduced for lower current limit. The resistor will have a
voltage across it equal to (320µA)(R), limited to ≈5V when
clamped by D2. Resistance required for a given current
limit is:
R_LIM = I_LIM(2kΩ) + 1kΩ (LT1074)
R_LIM = I_LIM(5.5kΩ) + 1kΩ (LT1076)
As an example, a 3A current limit would require
3A(2k) + 1k = 7kΩ for the LT1074. The accuracy of these
formulas is ±25% for 2A ≤ I_LIM ≤ 5A (LT1074) and
7A ≤ I_LIM ≤ 1.8A (LT1076), so I_LIM 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 I_LIM pin as shown
in Figure 9. This allows full desired current limit (with or
without R_LIM) when the output is regulating, but reduces
current limit under short-circuit conditions. A typical value
for R_FB 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 I_LIM pin. To calculate a
value for R_FB, first calculate R_LIM, the R_FB:
( )( ) ^ISC ^{− 0.44 * R}L
( ) ^RFB ⁼
R_L in kΩ
( ) ^{0.5 * R}L ^{− 1kΩ − I}SC
*Change 0.44 to 0.16, and 0.5 to 0.18 for LT1076.
Example: I_LIM = 4A, ISC = 1.5A, R_LIM = (4)(2k) + 1k = 9k
( )( ) ^{1.5 − 0.44 9kΩ}
( ) ^RFB ⁼
3.8kΩ
( ) ^{0.5 9k − 1k − 1.5}
LT1074
FB
I _LIM
V_OUT
R_LIM
R_FB
D2
1N4148
LT1074•PD13
Figure 9. Foldback Current Limit
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_M (voltage “in” to current
“out”) transfer function of ≈5000µmho. Voltage gain is
determined by multiplying G_M 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
400kΩ, voltage gain is ≈2000. At frequencies above a few
hertz, voltage gain is determined by the external compen-
sation, R_C and C_C.
sn1074 1074fds
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