LTC1735I-1 Linear Technology, LTC1735I-1 Datasheet - Page 8

• Manufacturer Part Number: LTC1735I-1
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

FU CTIO AL DIAGRA
OPERATIO
LTC1735-1
Main Control Loop:
The LTC1735-1 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
comparator I
current at which I
voltage on Pin I
EA. Pin V
to receive an output feedback voltage V
resistive divider. When the load current increases, it
causes a slight decrease in V
ence, which in turn causes the I
8
PGOOD 4
U
R1
INTV
V
OSENSE
–
+
OSENSE
CC
100k
7
R2
0.74V
6V
RUN/SS
1.2 A
C
1
U
SS
0.8V
V
0.86V
FB
resets the RS latch. The peak inductor
, described in the Pin Functions, allows EA
TH
2
1
U
, which is the output of error amplifier
+
–
resets the RS latch is controlled by the
EA
LATCHOFF
CURRENT
+
–
C
g
START
OVER-
m
SOFT-
OSC
RUN
C
SD
C
=1.3m
OV
+
(Refer to Functional Diagram)
1
R
SGND
C
FB
C
3
TH
OSC
OSC
8
relative to the 0.8V refer-
4(V
I
TH
0.86V
voltage to increase until
SLOPE COMP
FB
)
W
SYNC
FB
ICMP
BURST
DISABLE
FC
from the external
A
C
I
1
2.4V
–
+
–
+
S
R
BUFFERED
FORCE BOT
–
1.2V
SENSE
I
TH
2k
Q
0.17 A
+
+
DROP
OUT
DET
6
TOP ON
+
45k
30k
3mV
INTV
0.55V
0.8V
–
BOT
CC
5
45k
30k
SENSE
–
+
+
–
–
+
the average inductor current matches the new load cur-
rent. 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
the next cycle.
The top MOSFET driver is powered from a floating boot-
strap capacitor C
from INTV
top MOSFET is turned off. As V
the converter will attempt to turn on the top MOSFET con-
tinuously (“dropout’’). A dropout counter detects this con-
dition and forces the top MOSFET to turn off for about 500ns
every tenth cycle to recharge the bootstrap capacitor.
I
2
F
B
IREV
–
SD
INTV
FC
4.8V
SWITCH
CC
LOGIC
0.8V
REF
13
+
–
V
CC
IN
V
UVL
TOP
BOT
IN
through an external Schottky diode when the
5.2V
LDO
REG
EXTV
B
CC
. This capacitor is normally recharged
BOOST
SW
15
TG
16
14
9
INTV
CC
C
D
C
D
INTV
INTVCC
PGND
B
B
1
12
BG
11
10
IN
CC
decreases towards V
+
V
IN
2
, or the beginning of
+
L
C
IN
R
1735-1 FD
SENSE
+
OUT
C
OUT
V
OUT
,