lt1976 Linear Technology Corporation, lt1976 Datasheet - Page 12

• Manufacturer Part Number: lt1976
• Description: High Voltage 1.5a, 200khz Step-down Switching Regulator With 100?a Quiescent Current
• Manufacturer: Linear Technology Corporation

LT1976/LT1976B
APPLICATIO S I FOR ATIO
FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT1976 is used to set output
voltage and provide several overload protection features.
The first part of this section deals with selecting resistors
to set output voltage and the remaining part talks about
frequency foldback and soft-start features. Please read
both parts before committing to a final design.
Referring to Figure 2, the output voltage is determined by
a voltage divider from V
1.25V at the FB pin. Since the output divider is a load on the
output care must be taken when choosing the resistor
divider values. For light load applications the resistor
values should be as large as possible to achieve peak
efficiency in Burst Mode operation. Extremely large values
for resistor R1 will cause an output voltage error due to the
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.
For LT1976B aplications, the suggested value for R2 is 10k
or less, eliminating output voltage errors due to feedback
pin current and reducing noise susceptibility.
12
LT1976
R
OSCILLATOR
1
200kHz
=
R
2
•
1 25
.
V
Figure 2. Feedback Network
OUT
+
U
R
– .
2 50
1 25
•
SOFT-START
OUT
FOLDBACK
U
DETECT
nA
ERROR
to ground which generates
AMP
–
+
W
1.25V
C
SW
SS
FB
V
C
12
11
2
9
1976 F02
U
C1
R1
R2
V
OUT
More Than Just Voltage Feedback
The FB pin is used for more than just output voltage
sensing. It also reduces switching frequency and con-
trols the soft-start voltage ramp rate when output voltage
is below the regulated level (see the Frequency Foldback
and Soft-Start Current graphs in Typical Performance
Characteristics).
Frequency foldback is done to control power dissipation in
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 2.4A for the LT1976). Minimum switch on time
limitations would prevent the switcher from attaining a
sufficiently low duty cycle if switching frequency were
maintained at 200kHz, 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 V
LT1976 will skip the next switch cycle. As the feedback
voltage rises, the switching frequency increases to 200kHz
with 0.95V on the FB pin. During frequency foldback,
external syncronization is disabled to prevent interference
with foldback operation. Frequency foldback does not
affect operation during normal load conditions.
In addition to lowering switching frequency the soft-start
ramp rate is also affected by the feedback voltage. Large
Table 2
VOLTAGE
OUTPUT
(V)
2.5
3.3
10
12
3
5
6
8
C
pin, due to minimum switch on time, the
(kΩ, 1%)
100
100
100
100
100
100
100
100
R2
NEAREST (1%)
(kΩ)
100
140
165
300
383
536
698
866
R1
OUTPUT
ERROR
– 0.63
– 0.25
0.38
0.63
0.63
(%)
0
0
0
1976bfg