LTC3405A-1.375_15 Datasheet, PDF (6/12 Pages) Linear Technology – 1.375V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT

LTC3405A-1.375
W
FU CTIO AL DIAGRA
MODE
6
SLOPE
COMP
OSC
OSC
V_OUT
5
FREQ
SHIFT
R1
+
95k
0.9V
– ^EA
R2
180k
V_FB
V_IN
RUN
1
0.9V REF
–
OV
OVDET
0.95V +
SHUTDOWN
0.65V
– ^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
3405A1375 BD
U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
The LTC3405A-1.375 uses a constant frequency, current
mode step-down architecture. 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. When the load current
increases, the output voltage decreases which causes a
slight decrease in V_FB relative to the 0.9V 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
comparator I_RCMP, or the beginning of the next clock cycle.
6
Comparator OVDET guards against transient overshoots
> 5.6% by turning the main switch off and keeping it off
until the fault is removed.
Burst Mode Operation
The LTC3405A-1.375 is capable of Burst Mode operation
in which the internal power MOSFETs operate intermit-
tently 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 advantage of pulse skipping mode is lower
output ripple and less interference to audio circuitry.
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