LTC1735
FU CTIO AL DIAGRA
PGOOD
C
OSC
C
OSC
SGND
C
0.17µA
SYNC
–
+
1.2V
FORCE BOT
DROP
OUT
DET
+
0.86V
–
OV
S
Q
R
TOP ON
0.55V
+
–
2.4V
2k
–
+
EA
0.86V
SD
RUN
SOFT-
START
+
OVER-
CURRENT
LATCHOFF
R
C
RUN/SS
C
SS
C
C
I
TH
SENSE
gm =1.3m
Ω
SD
45k
45k
–
+
I
2
IREV
B
0.8V
FCB
FC
–
+
F
OSC
–
0.74V
LTC1735F
ONLY
+
V
OSENSE
V
FB
0.8V
R1
R2
1.2µA
4(V
FB
)
6V
SLOPE COMP
OPERATIO
(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 reference, which in turn causes the I
TH
voltage to
W
V
IN
V
IN
0.8V
REF
UVL
INTV
CC
BOOST
D
B
V
SEC
TOP
TG
C
B
U
U
U
+
C
IN
SW
BOT
SWITCH
LOGIC
+
C
SEC
D
1
V
OUT
BOT
INTV
CC
V
IN
5.2V
LDO
REG
+
–
PGND
C
INTVCC
ICMP
I
1
–
+
–
+
+
3mV
–
+
C
OUT
BURST
DISABLE
FC
A
+
INTV
CC
BUFFERED
I
TH
30k
30k
4.7V
BG
+
SENSE
–
EXTV
CC
R
SENSE
1735 FD
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.
1735fc
9
LINER [ LINEAR TECHNOLOGY ]
