LTC3446
9
3446ff
The LTC3446 combines a constant frequency, current mode
synchronous buck converter with two very low dropout
(VLDO) linear DC regulators to provide up to three high
efficiency, low voltage outputs from a single higher voltage
input source. Each output can be independently enabled
and disabled. A power good circuit monitors all three sup-
plies. The LTC3446 incorporates an undervoltage lockout
circuit that shuts down the IC when the input voltage drops
below about 2.4V to prevent unstable operation.
SYNCHRONOUS BUCK OPERATION
A buck converter takes power from a high input voltage,
V
IN
, and delivers it at a lower output voltage, V
OUT
. The buck
converter inside the LTC3446 achieves over 80% efficient
power conversion under a wide range of V
IN
, V
OUT
and load
conditions, whereas a linear regulator is limited by physics
to a maximum efficiency of (V
OUT
/V
IN
) ?100%.
Main Control Loop
During normal operation, the internal oscillator produces a
constant frequency 2.25MHz clock. The top power switch
(P-channel MOSFET) turns on at the beginning of a clock
cycle. Inductor current increases to a peak value which is
set by the voltage on the I
TH
pin. Then the top switch turns
off and the energy stored in the inductor flows through
the bottom switch (N-channel MOSFET) into the load until
the next clock cycle.
The peak inductor current is controlled by the voltage on
the I
TH
pin, which is the output of the error amplifier. This
amplifier compares the BUCKFB pin to the 0.8V reference.
When the load current increases, the BUCKFB voltage de-
creases slightly below the reference. This decrease causes
the error amplifier to increase the I
TH
voltage until the
average inductor current matches the new load current.
The main control loop is shut down by pulling the ENBUCK
pin to ground.
Overcurrent Protection
To help avert inductor current runaway in case the buck
output is accidentally shorted to ground, the LTC3446
features a bottom switch NMOS overcurrent limit, which
works as follows.
When the buck output is shorted to ground, inductor
current will rise to its maximum peak level, I
MAXP
, such
that on every oscillator cycle the PMOS top switch will
turn on for only its minimum duty cycle, and the bottom
switch NMOS turns on for the remainder of the cycle.
Temporarily ignoring inductor, switch and parasitic resis-
tance drops, which in most applications are designed to
be small in order to maximize buck converter efficiency,
it is to first order true that when the PMOS is on, the V
IN
supply voltage is placed across the inductor, increasing
the inductor current, but when the NMOS is on, there is no
output voltage to be placed across the inductor to reduce
its current. Inductor current ratchets up each cycle and
could lead to the destruction of the buck IC.
The NMOS overcurrent limit helps prevent this by sensing
the current through the NMOS bottom switch, and for as
long as this current exceeds the overcurrent limit level,
I
MAXN
, it:
1. Keeps the NMOS on, allowing the tiny voltage drops from
parasitic resistances to reduce the inductor current.
2. Refuses to allow the PMOS to turn on, preventing any
additional energy from being fed into the system.
Low Current Operation
The MODESEL pin controls the buck converters behavior at
light load currents to help optimize efficiency, output ripple
and noise. When the load is relatively light and MODESEL
is grounded, the buck converter automatically switches
into Burst Mode operation, which operates the PMOS
operaTion
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