ltc3447edd-trpbf Linear Technology Corporation, ltc3447edd-trpbf Datasheet - Page 7

• Manufacturer Part Number: ltc3447edd-trpbf
• Description: I2c Controllable Buck Regulator In 3mm ? 3mm Dfn
• Manufacturer: Linear Technology Corporation

OPERATIO
Main Control Loop
The LTC3447 uses current mode step-down architecture
with both the main (P-channel MOSFET) and synchronous
(N-channel MOSFET) switches 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
the RS latch. The peak inductor current at which I
resets the RS latch, is controlled by the output of error
amplifi er EA. When the load current increases, it causes
a slight decrease in the feedback voltage, FB, relative to
an internal reference voltage, which in turn, causes the
EA amplifi er’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
of the next clock cycle.
Burst Mode Operation
The LTC3447 is capable of Burst Mode operation in which
the internal power MOSFETs operate intermittently based
on load demand. Burst Mode operation can be disabled via
the I
the regulator is in pulse skipping mode operation.
During Burst Mode operation, the LTC3447’s internal
circuits sense when the inductor peak current falls below
BUCK OPERATION
BUCK REGULATOR
V
2
V
REF
C interface. When Burst Mode operation is disabled,
FB
Figure 3. LTC3447 Buck Regulator Diagram
OSC
EA
U
S
R
LATCH
RS
BURST
QB
Q
PEAK CURRENT LEVEL REF
LOGIC
RCMP
I
I
RCMP
COMP
, or the beginning
NFET
PFET
COMP
V
IN
RS
, resets
SW
3447 AI01
COMP
50mA. When below this level, the power MOSFETs and any
unneeded circuitry are turned off, reducing the quiescent
current to 33µA, and the peak current level reference level
is held at 150mA. The LTC3447 remains in this sleep state
until the output voltage falls below the output voltage set-
ting. Once this occurs, the regulator wakes up and allows
the inductor to develop 150mA current pulses. For light
loads, this will cause the output voltage to increase and the
internal peak current reference to decrease. When the peak
current reference falls to below 50mA, the part re-enters
sleep mode and the cycle is repeated. This process repeats
at a rate that is dependent on the load demand.
Pulse Skipping Mode Operation
At light loads, the inductor current may reach zero or
reverse on each pulse. The bottom MOSFET is turned off
by the current reversal comparator, I
voltage will ring. This is discontinuous mode operation,
and is normal behavior for a switching regulator. At very
light loads, the LTC3447 will automatically skip pulses
in pulse skipping mode operation to maintain output
regulation. This feature is enabled when the Burst Mode
operation is disabled.
Short-Circuit Protection
When the output is shorted to ground, the frequency of
the oscillator is reduced to about 160kHz. This frequency
foldback ensures that the inductor current has more
time to decay, thereby preventing thermal runaway. The
oscillator’s frequency will progressively increase to 1MHz
when V
Dropout Operation
When using the optional external feedback resistors, it is
possible for V
the input supply voltage decreases to a value approaching
the output voltage, the duty cycle increases toward the
maximum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one cycle
until it reaches 100% duty cycle. The output voltage will
then be determined by the input voltage minus the voltage
drop across the P-channel MOSFET and the inductor.
An important detail to remember is that at low input supply
voltages, the R
(see Typical Performance Characteristics). Therefore,
OUT
rises above 0V.
IN
DS(ON)
to approach the output voltage level. As
of the P-channel switch increases
RCMP
LTC3447
, and the switch
7
3447f