LTC3405A-1.8_15 Datasheet, PDF (6/16 Pages) Linear Technology – 1.5V, 1.8V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT

LTC3405A-1.5/LTC3405A-1.8
W
FU CTIO AL DIAGRA
MODE
6
SLOPE
COMP
OSC
OSC
0.65V
V_OUT
5
LTC3405A-1.5
R1
R1 = 110k
R2 = 440k
LTC3405A-1.8
R2
R1 = 180k
R2 = 360k
V_IN
FREQ
SHIFT
+
1.2V
– ^EA
V_FB
RUN
1
1.2V REF
–
OV
OVDET
1.294V +
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
3405A1518 BD
U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
The LTC3405A Fixed output voltage series parts use a
constant frequency, current mode step-down architec-
ture. Their 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 comparator, 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 de-
creases which causes a slight decrease in V_FB relative to
the 1.2V reference, which in turn, causes the EA amplifier’s
output voltage to increase 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
> 7.8% by turning the main switch off and keeping it off
until the fault is removed.
Burst Mode Operation
The LTC3405A series parts are 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 advantage of pulse skipping mode is lower
output ripple and less interference to audio circuitry.
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