5
DEMO MANUAL DC257
NO-DESIGN SWITCHER
OPERATIO
U
V
FB
V
IN
3
V
REF
2
1
6
5
4
Q
Q
R
S
0.12V
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
0.6V
I
TH
RUN/SS
0.8V
0.4V
0.86V
SYNC/MODE
Y = “0” ONLY WHEN X IS A CONSTANT “1”
SHUTDOWN
SLEEP
6
Ω
1.5
µ
A
2.25
µ
A
V
IN
V
IN
V
IN
V
IN
SW
GND
DC257 F03
V
IN
BURST
EN
7
OSC
FREQ
SHIFT
1.19V
REF
UVLO
TRIP = 2.7V
Y
X
BURST
DEFEAT
SLOPE
COMP
–
+
OVDET
–
+
–
+
–
+
–
+
–
+
EA
I
COMP
–
+
I
RCMP
RUN/SOFT
START
ANTI-
SHOOT-THRU
8
Figure 3. Functional Block Diagram
Main Control Loop (Refer to Functional Diagram)
The LTC1707 uses a constant-frequency, current mode
step-down architecture. Both the main and synchronous
switches, consisting of top P-channel and bottom
N-channel power MOSFETs, are internal. During normal
operation, the internal top power MOSFET is turned on
during each cycle when the oscillator sets the RS latch,
and turned off when the current comparator, I
COMP
, resets
the RS latch. The peak inductor current at which I
COMP
resets the RS latch is controlled by the voltage on the I
TH
pin, which is the output of error amplifier EA. The V
FB
pin
allows EA to receive an output feedback voltage from an
external resistive divider. When the load current increases,
it causes a slight decrease in the feedback voltage 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 the
current reversal comparator I
RCMP
, or the next cycle
begins.
The main control loop is shut down by pulling the RUN/
SS pin low. Releasing RUN/SS allows an internal 2.25
µ
A
current source to charge soft-start capacitor C
SS
. When
C
SS
reaches 0.7V, the main control loop is enabled with
the I
TH
voltage clamped at approximately 5% of its