Datasheet
9
LTC3727/LTC3727-1
3727fc
Figure 2
FU CTIO AL DIAGRA
U
U
W
(Refer to Functional Diagram)
OPERATIO
U
Main Control Loop
The LTC3727/LTC3727-1 use a constant frequency, cur-
rent mode step-down architecture with the two controller
channels operating 180 degrees out of phase. During
normal operation, each top MOSFET is turned on when the
clock for that channel 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 the I
TH
pin, which is the
output of each error amplifier EA. The V
OSENSE
pin receives
SWITCH
LOGIC
–
+
0.8V
7.3V
7.5V
V
IN
V
IN
7V
BINH
CLK2
CLK1
0.18μA
R6
R5
+
–
FCB
+
–
–
+
–
+
–
+
V
REF
INTERNAL
SUPPLY
3.3V
OUT
V
SEC
R
LP
C
LP
1.5V
FCB
EXTV
CC
INTV
CC
SGND
+
7.5V
LDO
REG
SW
SHDN
0.55V
TOP
BOOST
TG
C
B
C
IN
D
1
D
B
PGND
BOT
BG
INTV
CC
INTV
CC
V
IN
+
C
SEC
C
OUT
V
OUT
3727 F02
D
SEC
R
SENSE
R2
+
V
OSENSE
DROP
OUT
DET
RUN
SOFT
START
BOT
TOP ON
S
R
Q
Q
OSCILLATOR
PHASE DET
PLLFLTR
PLLIN
FCB
EA
0.86V
0.80V
OV
V
FB
1.2μA
6V
R1
–
+
R
C
4(V
FB
)
RST
SHDN
RUN/SS
I
TH
C
C
C
C2
C
SS
+
4(V
FB
)
0.86V
SLOPE
COMP
3mV
+
–
–
+
SENSE
–
SENSE
+
INTV
CC
50k
25k
2.4V
25k
50k
I1 I2
B
DUPLICATE FOR SECOND
CONTROLLER CHANNEL
+– –+
50k
F
IN
+
–
+
–
+
–
+
–
PGOOD
V
OSENSE1
V
OSENSE2
0.86V
0.74V
0.86V
0.74V
the voltage feedback signal, which is compared to the
internal reference voltage by the EA. When the load current
increases, it causes a slight decrease in V
OSENSE
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. After the top MOSFET has turned off, the
bottom MOSFET is turned on until either the inductor
current starts to reverse, as indicated by current compara-
tor I
2
, or the beginning of the next cycle.