Datasheet
9
LTC1735
1735fc
FU CTIO AL DIAGRA
UU
W
SW
–
+
+
–
0.86V
–
+
0.55V
2.4V
0.8V
0.86V
I
1
+
–
I
2
+
–
EA
A
BURST
DISABLE
FC
OV
gm =1.3m
B
–
+
4.7V
IREV
+
–
+
–
F
FC
S
R
Q
DROP
OUT
DET
0.8V
REF
SWITCH
LOGIC
SD
6V
R1
RUN/SS
C
SS
R
C
V
OSENSE
V
FB
1.2µA
RUN
SOFT-
START
+
OVER-
CURRENT
LATCHOFF
SD
I
TH
C
C
0.17µA
OSC
4(V
FB
)
BUFFERED
I
TH
SLOPE COMP
+– –+
3mV
ICMP
R2
2k
45k
BOT
TOP ON
FORCE BOT
45k
30k 30k
SENSE
+
SENSE
–
SYNC
1.2V 0.8V
C
TOP
UVL
BOT
INTV
CC
5.2V
LDO
REG
V
IN
+
C
INTVCC
V
OUT
V
SEC
INTV
CC
BG
PGND
V
IN
V
IN
BOOST
TG
INTV
CC
C
B
D
B
D
1
C
OSC
+
C
IN
+
C
SEC
+
C
OUT
EXTV
CC
FCBSGNDC
OSC
R
SENSE
1735 FD
PGOOD
LTC1735F
ONLY
Ω
+
–
0.74V
OPERATIO
U
(Refer to Functional Diagram)
Main Control Loop
The LTC1735 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
1
resets the RS latch. The peak inductor
current at which I
1
resets the RS latch is controlled by the
voltage on Pin 3 (I
TH
), which is the output of error
amplifier EA. Pin␣ 6 (V
OSENSE
), described in the pin func-
tions, allows EA to receive an output feedback voltage V
FB
from an external resistive divider. When the load current
increases, it causes a slight decrease in V
FB
relative to the
0.8V refer
ence, 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 powered from a floating boot-
strap capacitor C
B
. This capacitor is normally recharged
from INTV
CC
through an external diode when the top
MOSFET is turned off. As V
IN
decreases towards V
OUT
, the
converter will attempt to turn on the top MOSFET continu-
ously (“dropout’’). A dropout counter detects this condi-
tion and forces the top MOSFET to turn off for about 500ns
every tenth cycle to recharge the bootstrap capacitor.