LTC1735 데이터 시트보기 (PDF) - Linear Technology
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LTC1735 Datasheet PDF : 32 Pages
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W
FU CTIO AL DIAGRA
LTC1735
VIN
VIN
PGOOD
COSC
COSC
–
0.74V +
LTC1735F
ONLY
0.86V
OV
+
–
VOSENSE VFB
0.8V
R1
R2
gm =1.3m
–
EA
+
SD
1.2µA
6V
RUN/SS
CSS
RUN
SOFT-
START
+
OVER-
CURRENT
LATCHOFF
RC
CC
SGND
0.17µA
C
SYNC
OSC
1.2V
FORCE BOT
FCB
–
0.8V +
0.8V UVL
REF
FC
F
TOP
INTVCC
BOOST
DB
CB
TG
DROP
OUT
SW
DET
BOT
SWITCH
S
Q
TOP ON
LOGIC
R
0.55V + B
0.86V
2.4V
2k
ICMP
I1 – +
BURST
DISABLE
FC
4(VFB)
A
BUFFERED
ITH
SLOPE COMP
–
SD
45k
45k
– IREV
+ – I2
BOT
+
3mV
VIN
5.2V
INTVCC
LDO
REG
4.7V +
30k
30k
–
ITH
SENSE+
SENSE–
EXTVCC
D1
INTVCC
+
CINTVCC
BG
PGND
+
CIN
VSEC
+
CSEC
VOUT
+
COUT
RSENSE
1735 FD
U
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 I1 resets the RS latch. The peak inductor
current at which I1 resets the RS latch is controlled by the
voltage on Pin 3 (ITH), which is the output of error
amplifier EA. Pin␣ 6 (VOSENSE), described in the pin func-
tions, allows EA to receive an output feedback voltage VFB
from an external resistive divider. When the load current
increases, it causes a slight decrease in VFB relative to the
0.8V reference, which in turn causes the ITH 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 I2, or
the beginning of the next cycle.
The top MOSFET driver is powered from a floating boot-
strap capacitor CB. This capacitor is normally recharged
from INTVCC through an external diode when the top
MOSFET is turned off. As VIN decreases towards VOUT, 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
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