LTC3405A_15 Datasheet, PDF (6/16 Pages) Linear Technology – 1.5MHz, 300mA Synchronous Step-Down Regulator in ThinSOT

LTC3405A
W
FU CTIO AL DIAGRA
MODE
6
SLOPE
COMP
OSC
OSC
0.65V
FREQ
SHIFT
V_FB
5
+
0.8V
– ^EA
V_IN
RUN
1
0.8V REF
–
OV
OVDET
0.85V +
SHUTDOWN
– ^EN _SLEEP
0.4V
+
BURST
I_COMP
SQ
RQ
RS LATCH
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTI-
SHOOT-
THRU
I_RCMP
4 V_IN
5Ω
3 SW
2 GND
3405A BD
U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
The LTC3405A 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_COMP, resets the RS latch. The peak inductor
current at which I_COMP resets the RS latch, is controlled by
the output of error amplifier EA. The V_FB pin, described in
the Pin Functions section, 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 EA amplifier’s output voltage to in-
crease 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
6
comparator I_RCMP, or the beginning of the next clock
cycle.
Comparator OVDET guards against transient overshoots
> 6.25% by turning the main switch off and keeping it off
until the fault is removed.
Burst Mode Operation
The LTC3405A is capable of Burst Mode operation in
which the internal power MOSFETs operate intermittently
based on load demand. To enable Burst Mode operation,
simply connect the MODE pin to GND. To disable Burst
Mode operation and enable PWM pulse skipping mode,
connect the MODE pin to V_IN or drive it with a logic high
(V_MODE > 1.5V). In this mode, the efficiency is lower at
light loads, but becomes comparable to Burst Mode
operation when the output load exceeds 25mA. The ad-
vantage of pulse skipping mode is lower output ripple and
less interference to audio circuitry.
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