LT1576/LT1576-5
APPLICATIONS INFORMATION
Table 1
OUTPUT
VOLTAGE
(V)
3
3.3
5
6
8
10
12
15
R2
(k
Ω
)
4.99
4.99
4.99
4.99
4.99
4.99
4.99
4.99
R1
(NEAREST 1%)
(k
Ω
)
7.32
8.66
15.8
19.6
28.0
36.5
44.2
56.2
% ERROR AT OUTPUT
DUE TO DISCREET 1%
RESISTOR STEPS
– 0.50
+ 0.30
+ 0.83
– 0.62
– 0.01
+ 0.61
– 0.60
– 1.08
More Than Just Voltage Feedback
The feedback pin is used for more than just output voltage
sensing. It also reduces switching frequency and current
limit when output voltage is very low (see the Frequency
Foldback graph in Typical Performance Characteristics).
This is done to control power dissipation in both the IC and
the external diode and inductor during short-circuit con-
ditions. A shorted output requires the switching regulator
to operate at very low duty cycles, and the average current
through the diode and inductor is equal to the short-circuit
current limit of the switch (typically 2A for the LT1576,
folding back to less than 0.77A). Minimum switch on time
limitations would prevent the switcher from attaining a
LT1576
TO FREQUENCY
SHIFTING
1.4V
ERROR
AMPLIFIER
Q1
+
–
R5
5k
Q2
TO SYNC CIRCUIT
V
C
GND
Figure 2. Frequency and Current Limit Foldback
U
W
U
U
sufficiently low duty cycle if switching frequency were
maintained at 200kHz, so frequency is reduced by about
5:1 when the feedback pin voltage drops below 0.7V (see
Frequency Foldback graph). This does not affect operation
with normal load conditions; one simply sees a gear shift
in switching frequency during start-up as the output
voltage rises.
In addition to lower switching frequency, the LT1576 also
operates at lower switch current limit when the feedback
pin voltage drops below 0.7V. Q2 in Figure 2 performs this
function by clamping the V
C
pin to a voltage less than its
normal 2.1V upper clamp level. This
foldback current limit
greatly reduces power dissipation in the IC, diode and
inductor during short-circuit conditions. External synchro-
nization is also disabled to prevent interference with
foldback operation. Again, it is nearly transparent to the
user under normal load conditions. The only loads that may
be affected are current source loads which maintain full
load current with output voltage less than 50% of final value.
In these rare situations the feedback pin can be clamped
above 0.7V to defeat foldback current limit.
Caution:
clamp-
ing the feedback pin means that frequency shifting will also
be defeated, so a combination of high input voltage and
dead shorted output may cause the LT1576 to lose control
of current limit.
V
SW
OUTPUT
5V
1.21V
R3
1k
R4
1k
FB
R1
+
R2
5k
1576 F02
9
LINER [ LINEAR TECHNOLOGY ]
