Datasheet

6
LTC1435A
FUNCTIONAL DIAGRA
UU

W
–
+
+
–
+
–
+
–
SWITCH
LOGIC
DROP
OUT
DET
0.6V
1.19V
Q
S
R
I
1
INTV
CC
4k
–
+
8k
180k
30k
+
–
–
+
EA
SHUTDOWN
6V
3µA
1.19V
1.28V
–
+
4.8V
V
IN
SHUTDOWN
1.19V
REF
C
SS
C
C
R
C
RUN/SS
2
I
TH
3
D
FB
*
SENSE
+
78
SENSE
–
EXTV
CC
9
+
R2
V
OSENSE
OV
1µA

OSC
PGND
10
BG
INTV
CC
12
SW
14
TG
16
BOOST
SGND
V
IN
13
SFB
4
C
OSC
1
R
SENSE
+
+
V
SEC
C
SEC
V
OUT
C
OUT
D1
C
B
D
B
INTV
CC
+
C
IN
V
IN
C
OSC
5V
LDO
REG
RUN
SOFT
START
1435A • FD
V
FB
R1
I
2
g
m
= 1m
Ω
* FOLDBACK CURRENT LIMITING OPTION
5
6
11
15
(Refer to Functional Diagram)
OPERATION
U
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 I
1
resets the RS latch. The peak inductor current at
which I
1
resets the RS latch is controlled by the voltage on
the I
TH
pin , which is the output of error amplifier EA. The
V
OSENSE
pin, described in the Pin Functions section, allows
EA to receive an output feedback voltage V
FB
from an ex-
ternal resistive divider. When the load current increases,
it causes a slight decrease in V
FB
relative to the 1.19V ref-
erence, which in turn causes the I
TH
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 I
2
, or the beginning of the
next cycle.
The top MOSFET driver is biased from floating bootstrap
capacitor C
B
, which normally is recharged during each off
cycle. However, when V
IN
decreases to a voltage close to
V
OUT
, 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