LTC3406B-2 Linear Technology, LTC3406B-2 Datasheet - Page 6

• Manufacturer Part Number: LTC3406B-2
• Description: (LTC3406B-1/-2) 600mA Synchronous Step-Down Regulator in ThinSOT
• Manufacturer: Linear Technology

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FU CTIO AL DIAGRA
OPERATIO
LTC3406B-1.2
Main Control Loop
The LTC3406B-1.2 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 0.8V 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,
6
4
U
TO 5.5V
2.7V
V
IN
COMP
V
RUN
OUT
1
5
U
C
4.7µF
CER
, resets the RS latch. The peak inductor
IN
Figure 1. Typical Application
**
**
COMP
*
0.8V REF
U
†
MURATA LQH3C2R2M24
TAIYO YUDEN JMK212BJ475MG
TAIYO YUDEN JMK316BJ106ML
V
IN
4
1
60k
120k
SHUTDOWN
(Refer to Functional Diagram)
V
LTC3406B-1.2
RUN
IN
resets the RS latch, is controlled by
GND
0.8V + ∆V
FB
2
SHIFT
FREQ
OSC
V
OUT
SW
0.8V
OVL
3
5
2.2µH*
OVDET
+
+
–
–
W
EA
SLOPE
COMP
3406B12 F01
–
+
OSC
C
10µF
CER
OUT
†
V
1.2V
600mA
OUT
OV
RS LATCH
as indicated by the current reversal comparator I
the beginning of the next clock cycle. The comparator
OVDET guards against transient overshoots >6.25% by
turning the main switch off and keeping it off until the fault
is removed.
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-1.2 will automatically skip pulses
in pulse skipping mode operation to maintain output regu-
lation. Refer to LTC3406-1.2 data sheet if Burst Mode op-
eration 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
S
R
Q
Q
SWITCHING
BLANKING
CIRCUIT
LOGIC
AND
–
I
SHOOT-
COMP
ANTI-
THRU
I
RCMP
+
+
–
OUT
rises above 0V.
5Ω
RCMP
3406B12 BD
, and the switch
4
3
2
V
SW
GND
IN
sn3406b12 3406b12fs
RCMP
, or