8
LTC1736
FU CTIO AL DIAGRA
U
U
W
OPERATIO
U
(Refer to Functional Diagram)
Main Control Loop
The LTC1736 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 I1 resets the RS latch. The peak inductor
current at which I1 resets the RS latch is controlled by the
voltage on Pin I
TH
, which is the output of the error
amplifier EA. Pin V
OSENSE
, described in the Pin Functions,
allows EA to receive an output feedback voltage V
FB
from
the internal 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
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 com-
parator I2, or the beginning of the next cycle.
The top MOSFET driver is powered from a floating
bootstrap capacitor C
B
. This capacitor is normally re-
charged from INTV
CC
through an external Schottky 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 continuously (‘’dropout’’). A dropout counter
detects this condition and forces the top MOSFET to turn
off for about 500ns every tenth cycle to recharge the
bootstrap capacitor.
SW
–
+
+
–
0.86V
0.74V
–
+
0.55V
2.4V
0.8V
47pF
0.86V
I1
+
–
I2
+
–
EA
A
BURST
DISABLE
FC
OV
B
–
+
4.8V
IREV
+
–
+
–
F
FC
S
R
Q
DROP
OUT
DET
0.8V
REF
SWITCH
LOGIC
SD
6V
RUN/SS
C
SS
R
C
V
FB
40k
1.2
µ
A
RUN
SOFT
START
+
OVER-
CURRENT
LATCH-OFF
SD
I
TH
C
C
0.17
µ
A
OSC
4(V
FB
)
BUFFERED
I
TH
SLOPE COMP
+
–
–
+
3mV
ICMP
R2
10k
R1
SGND
V
FB
V
OSENSE
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
FCB
R4
R3
C
OSC
R
SENSE
1736 FD
1
PGOOD
6
9
10
5
VIDV
CC
VID4
INTV
CC
15
VID3 14
VID2 13
VID1 12
VID0 11
4
21
23
24
22
20
19
18
17
7
8
3
2
–
+
VID
DECODER
16
g
m
=1.3m
Ω
