ltc3406bes5-1.8 Linear Technology Corporation, ltc3406bes5-1.8 Datasheet - Page 6

• Manufacturer Part Number: ltc3406bes5-1.8
• Description: 1.5mhz, 600ma Synchronous Step-down Regulator In Thinsot
• Manufacturer: Linear Technology Corporation

FU CTIO AL DIAGRA
OPERATIO
LTC3406B
Main Control Loop
The LTC3406B uses a constant frequency, current mode
step-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET) switches
are internal. During normal operation, the internal top
power MOSFET is turned on each cycle when the oscillator
sets the RS latch, and turned off when the current com-
parator, I
current at which I
the output of error amplifier EA. When the load current
increases, it causes a slight decrease in the feedback
voltage, FB, relative to the 0.6V reference, which in turn,
causes the EA amplifier’s output voltage to increase until
the average inductor current matches the new load cur-
rent. 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
the beginning of the next clock cycle. The comparator
OVDET guards against transient overshoots >7.8% by
turning the main switch off and keeping it off until the fault
is removed.
6
V
LTC3406B-1.5
R1 + R2 = 550k
LTC3406B-1.8
R1 + R2 = 540k
FB
RUN
U
/V
5
1
OUT
COMP
0.6V REF
U
V
IN
, resets the RS latch. The peak inductor
R1
R2
SHUTDOWN
COMP
U
0.6V + ∆V
(Refer to Functional Diagram)
FB
SHIFT
resets the RS latch, is controlled by
FREQ
OSC
0.6V
OVL
OVDET
+
+
–
–
EA
SLOPE
COMP
W
–
+
OSC
RCMP
OV
, or
Pulse Skipping Mode Operation
At light loads, the inductor current may reach zero or re-
verse on each pulse. The bottom MOSFET is turned off by
the current reversal comparator, I
voltage will ring. This is discontinuous mode operation,
and is normal behavior for the switching regulator. At very
light loads, the LTC3406B will automatically skip pulses in
pulse skipping mode operation to maintain output regula-
tion. Refer to LTC3406 data sheet if Burst Mode operation
is preferred.
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator is reduced to about 210kHz, 1/7 the nominal
frequency. This frequency foldback ensures that the in-
ductor current has more time to decay, thereby preventing
runaway. The oscillator’s frequency will progressively
increase to 1.5MHz when V
RS LATCH
R
S
Q
Q
SWITCHING
BLANKING
CIRCUIT
LOGIC
AND
–
I
SHOOT-
COMP
THRU
ANTI-
I
RCMP
+
+
–
FB
or V
OUT
RCMP
rises above 0V.
, and the switch
5Ω
3406B BD
4
3
2
3406bfa
V
SW
GND
IN