LT1977
W U U
U
APPLICATIO S I FOR ATIO
and Soft-Start Current graphs in Typical Performance
Characteristics).
V
OUT
LT1977
SW
2
9
C1
Frequencyfoldbackisusedtocontrolpowerdissipationin
both the IC and in the external diode and inductor during
short-circuit conditions. A shorted output requires the
switching regulator to operate at very low duty cycles. As
a result the average current through the diode and induc-
tor is equal to the short-circuit current limit of the switch
(typically 2A for the LT1977). Minimum switch on time
limitations would prevent the switcher from operating at a
sufficiently low duty cycle if switching frequency were
maintained at 500kHz, so frequency is reduced by about
4:1 when the FB pin voltage drops below 0.4V (see
Frequency Foldback graph). In addition, if the current in
the switch exceeds 1.5 times the current limitations speci-
fied by the VC pin, due to minimum switch on time, the
LT1977 will skip the next switch cycle. As the feedback
voltagerises,theswitchingfrequencyincreasesto500kHz
with 0.95V on the FB pin. During frequency foldback,
external synchronization is disabled to prevent interfer-
ence with foldback operation. Frequency foldback does
not affect operation during normal load conditions.
C
SS
SOFT-START
500kHz
OSCILLATOR
FOLDBACK
DETECT
R1
FB
+
–
12
11
ERROR
AMP
R2
1.25V
V
C
1977 F02
Figure 2. Feedback Network
R1
Table 2
OUTPUT
VOLTAGE
(V)
OUTPUT
ERROR
(%)
R2
NEAREST (1%)
(kΩ, 1%)
(kΩ)
2.5
3
100
100
100
100
100
100
100
100
100
140
165
300
383
536
698
866
0
0
In addition to lowering switching frequency the soft-start
ramp rate is also affected by the feedback voltage. Large
capacitive loads or high input voltages can cause a high
input current surge during start-up. The soft-start func-
tion reduces input current surge by regulating switch
current via the VC pin to maintain a constant voltage ramp
rate(dV/dt)attheoutput. Acapacitor(C1inFigure2)from
the CSS pin to the output determines the maximum output
dV/dt. Whenthefeedbackvoltageisbelow0.4V, theVC pin
will rise, resulting in an increase in switch current and
outputvoltage.IfthedV/dtoftheoutputcausesthecurrent
through the CSS capacitor to exceed ICSS the VC voltage is
reduced resulting in a constant dV/dt at the output. As the
feedback voltage increases ICSS increases, resulting in an
increased dV/dt until the soft-start function is defeated
with 0.9V present at the FB pin. The soft-start function
does not affect operation during normal load conditions.
However, if a momentary short (brown out condition) is
present at the output which causes the FB voltage to drop
below 0.9V, the soft-start circuitry will become active.
3.3
5
0.38
0
6
0.63
–0.63
–0.25
0.63
8
10
12
50nA FB pin input current. The suggested value for the
output divider resistor (see Figure 2) from FB to ground
(R2) is 100k or less. A formula for R1 is shown below. A
table of standard 1% values is shown in Table 2 for
common output voltages.
VOUT – 1.25
R1= R2 •
1.25 +R2 • 50nA
More Than Just Voltage Feedback
The FB pin is used for more than just output voltage
sensing. It also reduces switching frequency and con-
trolsthesoft-startvoltagerampratewhenoutputvoltage
is below the regulated level (see the Frequency Foldback
1977f
10
Linear [ Linear ]
