LT1374C Linear Technology, LT1374C Datasheet - Page 23

• Manufacturer Part Number: LT1374C
• Description: 4.5A/ 500kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
If everything looks OK, I use a heat gun and cold spray on
the circuit (especially the output capacitor) to bring out
any temperature-dependent characteristics.
Keep in mind that this procedure does not take initial
component tolerance into account. You should see fairly
clean response under all load and line conditions to ensure
that component variations will not cause problems. One
note here: according to Murphy, the component most
likely to be changed in production is the output capacitor,
because that is the component most likely to have manu-
facturer variations (in ESR) large enough to cause prob-
lems. It would be a wise move to lock down the sources of
the output capacitor in production.
A possible exception to the “clean response” rule is at very
light loads, as evidenced in Figure 14 with I
Switching regulators tend to have dramatic shifts in loop
response at very light loads, mostly because the inductor
current becomes discontinuous. One common result is very
slow but stable characteristics. A second possibility is low
phase margin, as evidenced by ringing at the output with
transients. The good news is that the low phase margin at
light loads is not particularly sensitive to component varia-
tion, so if it looks reasonable under a transient test, it will
probably not be a problem in production. Note that fre-
quency of the light load ringing may vary with component
tolerance but phase margin generally hangs in there.
POSITIVE-TO-NEGATIVE CONVERTER
The circuit in Figure 15 is a classic positive-to-negative
topology using a grounded inductor. It differs from the
standard approach in the way the IC chip derives its
feedback signal, however, because the LT1374 accepts
only positive feedback signals, the ground pin must be tied
to the regulated negative output. A resistor divider to
ground or, in this case, the sense pin, then provides the
proper feedback voltage for the chip.
Inverting regulators differ from buck regulators in the
basic switching network. Current is delivered to the output
as square waves with a peak-to-peak amplitude much
greater than load current . This means that maximum load
current will be significantly less than the LT1374’s 4.5A
maximum switch current, even with large inductor values .
U
INFORMATION
U
W
LOAD
U
= 50mA.
The buck converter in comparison, delivers current to the
output as a triangular wave superimposed on a DC level
equal to load current, and load current can approach 4.5A
with large inductors. Output ripple voltage for the positive-
to-negative converter will be much higher than a buck
converter. Ripple current in the output capacitor will also
be much higher. The following equations can be used to
calculate operating conditions for the positive-to-negative
converter.
Maximum load current:
I
V
V
V
0.35 = Switch voltage drop at 4.5A
Example: with V
V
not take into account that maximum rated switch current
(I
above 50%. If duty cycle is expected to exceed 50% (input
voltage less than output voltage), use the actual I
from the Electrical Characteristics table.
5.5V TO
I
** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE
P
MAX
INPUT
* INCREASE L1 TO 10 H OR 20 H FOR HIGHER CURRENT APPLICATIONS.
IN
OUT
F
F
10 F TO
P
SEE APPLICATIONS INFORMATION
AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION
= Maximum rated switch current
= 0.5V, I
) on the LT1374 is reduced slightly for duty cycles
20V
= Catch diode forward voltage
50 F
= Minimum input voltage
C3
= Output voltage
+
I
P
Figure 15. Positive-to-Negative Converter
P
= 4.5A: I
V
2
IN
V
GND
V
IN(MIN)
OUT
LT1374-5
OUT
BOOST
V
IN
MAX
V
C
SENSE
V
V
V
= 2A. Note that this equation does
R
= 5.5V, V
OUT
V
IN
IN
C
C
SW
C
f L
0 35
.
D2
MBRS330T3
C2
0.27 F
OUT
V
V
OUT
OUT
1N4148
= 5V, L = 10 H,
5 H
L1*
D1
V
+
V
IN
LT1374
F
C1
100 F
10V TANT
2
0 35
23
P
OUTPUT**
–5V, 1.8A
.
value
1374 F15