LTC3405A-1.5 LINER [Linear Technology], LTC3405A-1.5 Datasheet - Page 6

• Manufacturer Part Number: LTC3405A-1.5
• Description: 1.5V, 1.8V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT
• Manufacturer: LINER [Linear Technology]

FU CTIO AL DIAGRA
LTC3405A-1.5/LTC3405A-1.8
OPERATIO
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
latch. The peak inductor current at which I
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
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
the next clock cycle.
6
MODE
V
LTC3405A-1.5
R1 = 110k
R2 = 440k
LTC3405A-1.8
R1 = 180k
R2 = 360k
RUN
OUT
6
5
1
U
1.2V REF
V
IN
U
R1
R2
SHUTDOWN
U
1.294V
SHIFT
FREQ
OSC
(Refer to Functional Diagram)
1.2V
V
FB
–
OVDET
+
+
–
EA
SLOPE
RCMP
COMP
W
COMP
, or the beginning of
–
+
OV
OSC
, resets the RS
COMP
FB
relative to
resets the
0.4V
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
with a logic high (V
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.
RS LATCH
+
–
S
R
BURST
EN
Q
Q
SLEEP
SWITCHING
BLANKING
CIRCUIT
LOGIC
AND
0.65V
–
I
SHOOT-
COMP
ANTI-
THRU
MODE
I
RCMP
+
–
+
> 1.5V). In this mode, the
5
sn3405a1518 3405a1518fs
IN
3405A1518 BD
or drive it
4
3
2
V
SW
GND
IN