ltc3405a-1.375 Linear Technology Corporation, ltc3405a-1.375 Datasheet - Page 6

• Manufacturer Part Number: ltc3405a-1.375
• Description: 1.375v, 1.5mhz, 300ma Synchronous Step-down Regulators In Thinsot
• Manufacturer: Linear Technology Corporation

FU CTIO AL DIAGRA
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
current at which I
the output of error amplifier EA. When the load current
increases, the output voltage decreases which causes a
slight decrease in V
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
LTC3405A-1.375
OPERATIO
6
MODE
V
RUN
OUT
U
1
6
5
COMP
0.9V REF
V
IN
RCMP
R1
95k
R2
180k
U
SHUTDOWN
, resets the RS latch. The peak inductor
COMP
U
, or the beginning of the next clock cycle.
0.95V
FB
SHIFT
FREQ
OSC
(Refer to Functional Diagram)
relative to the 0.9V reference, which
resets the RS latch, is controlled by
0.9V
V
FB
+
–
OVDET
+
–
EA
SLOPE
COMP
W
–
+
OV
OSC
0.4V
RS LATCH
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
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.
+
–
S
R
BURST
EN
Q
Q
SLEEP
SWITCHING
BLANKING
CIRCUIT
LOGIC
AND
0.65V
–
I
SHOOT-
COMP
ANTI-
THRU
I
MODE
RCMP
+
–
+
> 1.5V). In this mode, the
5Ω
IN
3405A1375 BD
or drive it
4
3
2
3405a1375f
V
SW
GND
IN