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LTC3446 Datasheet, PDF (10/20 Pages) Linear Technology – Monolithic Buck Regulator with Dual VLDO Regulators
LTC3446
OPERATION
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, and has its own independent soft-start cir-
cuit to help reduce inrush current. A power good circuit
monitors all three supplies. 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,
VIN, and delivers it at a lower output voltage, VOUT. The buck
converter inside the LTC3446 achieves over 80% efficient
power conversion under a wide range of VIN, VOUT and load
conditions, whereas a linear regulator is limited by physics
to a maximum efficiency of (VOUT/VIN) × 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 ITH 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 ITH 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 ITH 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. A soft-start is enabled whenever ENBUCK
is brought high. Soft-start limits the peak inductor current
from reaching maximum for the first millisecond after
ENBUCK is brought high.
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, IMAXP, 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 VIN
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,
IMAXN, 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 converter’s 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
switch intermittently based on load demand rather than
at a constant frequency. Every switch cycle during Burst
Mode operation delivers more energy than would occur
in constant frequency operation, minimizing the switch-
ing loss per unit of energy delivered. Since the dominant
power loss at light loads is gate charge switching loss in
the power MOSFETs, operating in Burst Mode operation
3446fd
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