LTC1435A Просмотр технического описания (PDF) - Linear Technology
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LTC1435A Datasheet PDF : 20 Pages
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LTC1435A
W
FUNCTIONAL DIAGRA
1 COSC
COSC
4 SFB
13 VIN
SGND 5
1µA
–
1.19V
REF
OSC
+
OV
1.28V –
1.19V +
SHUTDOWN
S
Q
R
0.6V +
DROP
OUT
DET
SWITCH
LOGIC
VOSENSE
6
VFB –
R2
EA
1.19V +
gm = 1m
R1 3µA
6V
SHUTDOWN
RUN
SOFT
START
I1
180k
+
–
30k
–
4k
–
I2
+
VIN
INTVCC
5V
LDO
REG
4.8V +
8k
–
2 RUN/SS
CSS
RC
3 ITH
CC
DFB*
SENSE+ 8
7 SENSE–
9 EXTVCC
* FOLDBACK CURRENT LIMITING OPTION
VIN
+
CIN
INTVCC
DB
BOOST
15
CB
TG
16
SW
14
D1
INTVCC
12
+
VSEC
+
CSEC
BG
11
PGND
10
VOUT
+
COUT
RSENSE
1435A • FD
U
OPERATION (Refer to Functional Diagram)
Main Control Loop
The LTC1435A uses a constant frequency, current mode
step-down architecture. During normal operation, the top
MOSFET is turned on each cycle when the oscillator sets
the RS latch, and turned off when the main current com-
parator I1 resets the RS latch. The peak inductor current at
which I1 resets the RS latch is controlled by the voltage on
the ITH pin , which is the output of error amplifier EA. The
VOSENSE pin, described in the Pin Functions section, allows
EA to receive an output feedback voltage VFB from an ex-
ternal resistive divider. When the load current increases,
it causes a slight decrease in VFB relative to the 1.19V ref-
erence, which in turn causes the ITH voltage to increase until
the average inductor current matches the new load current.
While the top MOSFET is off, the bottom MOSFET is turned
on until either the inductor current starts to reverse, as
indicated by current comparator I2, or the beginning of the
next cycle.
The top MOSFET driver is biased from floating bootstrap
capacitor CB, which normally is recharged during each off
cycle. However, when VIN decreases to a voltage close to
VOUT, the loop may enter dropout and attempt to turn on
the top MOSFET continuously. The dropout detector counts
the number of oscillator cycles that the top MOSFET remains
6
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