LTC1735_15 Datasheet, PDF (9/32 Pages) Linear Technology – High Efficiency Synchronous Step-Down Switching Regulator

W
FU CTIO AL DIAGRA
LTC1735
V_IN
V_IN
PGOOD
C_OSC
C_OSC
–
0.74V +
LTC1735F
ONLY
0.86V
OV
+
–
V_{OSENSE VFB}
0.8V
R1
R2
gm =1.3m
–
EA
+
SD
1.2µA
6V
RUN/SS
C_SS
RUN
SOFT-
START
+
OVER-
CURRENT
LATCHOFF
R_C
C_C
SGND
0.17µA
C
SYNC
OSC
1.2V
FORCE BOT
FCB
–
0.8V +
0.8V ^UVL
REF
FC
F
TOP
INTV_CC
BOOST
D_B
C_B
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(V_FB)
A
BUFFERED
I_TH
SLOPE COMP
–
SD
45k
45k
– ^IREV
+ – I2
BOT
+
3mV
V_IN
5.2V
INTV_CC
LDO
REG
4.7V +
30k
30k
–
I_TH
SENSE⁺
SENSE^–
EXTV_CC
D₁
INTV_CC
+
C_INTVCC
BG
PGND
+
C_IN
V_SEC
+
C_SEC
V_OUT
+
C_OUT
R_SENSE
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 (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
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 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
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