Datasheet

LT3434
9
3434fb
BLOCK DIAGRA
W
power from the V
IN
pin, but if the BIAS pin is connected to
an external voltage higher than 3V bias power will be
drawn from the external source (typically the regulated
output voltage). This improves efficiency.
High switch efficiency is attained by using the BOOST pin
to provide a voltage to the switch driver which is higher
than the input voltage, allowing switch to be saturated.
This boosted voltage is generated with an external capaci-
tor and diode.
To further optimize efficiency, the LT3434 automatically
switches to Burst Mode operation in light load situations.
In Burst Mode operation, all circuitry associated with
controlling the output switch is shut down reducing the
input supply current to 45µA.
The LT3434 contains a power good flag with a program-
mable threshold and delay time. A logic-level low on the
SHDN pin disables the IC and reduces input suppy current
to less than 1µA.
APPLICATIO S I FOR ATIO
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FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT3434 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
OUT
to ground which generates
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 1 for
common output voltages.
RR
V
RnA
OUT
12
125
125 2 50
=
+
•
–.
.•
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
Table 1
OUTPUT R1 OUTPUT
VOLTAGE R2 NEAREST (1%) ERROR
(V) (kΩ, 1%) (kΩ)(%)
2.5 100 100 0
3 100 140 0
3.3 100 165 0.38
5 100 300 0
6 100 383 0.63
8 100 536 –0.63
10 100 698 – 0.25
12 100 866 0.63
SOFT-START
FOLDBACK
DETECT
200kHz
OSCILLATOR
–
+
ERROR
AMP
1.25V
V
C
11
FB
12
C
SS
V
OUT
9
SW
LT3434
C1
R1
R2
3434 F02
2
Figure 2. Feedback Network