LTC3407-2_15 Datasheet, PDF (6/16 Pages) Linear Technology – Dual Synchronous, 800mA, 2.25MHz Step-Down DC/DC Regulator

LTC3407-2
BLOCK DIAGRAM
MODE/SYNC 6
REGULATOR 1
V_FB1 1
0.6V
+
EA
I_TH
–
SLOPE
COMP
0.35V
–
EN
+
BURST
SLEEP
0.55V
–
UV
UVDET
+
S
Q
RS
LATCH
R
Q
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
BURST
CLAMP
–+
I_COMP
ANTI
SHOOT-
THRU
0.65V
SHUTDOWN
+
OV
OVDET
–
RUN1 2
0.6V REF
OSC
RUN2 9
OSC
V_FB2 10
REGULATOR 2 (IDENTICAL TO REGULATOR 1)
I_RCMP
PGOOD1
POR
COUNTER
PGOOD2
V_IN
57
4 SW1
11 GND
V_IN
3 V_IN
8 POR
5 GND
7 SW2
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OPERATION
The LTC3407-2 uses a constant-frequency, current mode
architecture. The operating frequency is set at 2.25MHz
and can be synchronized to an external oscillator. Both
channels share the same clock and run in-phase. To suit a
variety of applications, the selectable Mode pin allows the
user to choose between low noise and high efficiency.
The output voltage is set by an external divider returned
to the V_FB pins. An error amplifier compares the divided
output voltage with a reference voltage of 0.6V and adjusts
the peak inductor current accordingly. Overvoltage and
undervoltage comparators will pull the POR output low if
the output voltage is not within ±8.5%. The POR output
will go high after 262,144 clock cycles (about 117ms) of
achieving regulation.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle
6
when the V_FB voltage is below the the reference voltage.
The current into the inductor and the load increases until
the current limit is reached. The switch turns off and
energy stored in the inductor flows through the bottom
switch (N-channel MOSFET) into the load until the next
clock cycle.
The peak inductor current is controlled by the internally
compensated I_TH voltage, which is the output of the er-
ror amplifier.This amplifier compares the V_FB pin to the
0.6V reference. When the load current increases, the
V_FB voltage decreases slightly below the reference. This
decrease causes the error amplifier to increase the I_TH
voltage until the average inductor current matches the
new load current.
The main control loop is shut down by pulling the RUN
pin to ground.
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