NCP5424 Datasheet, PDF (10/20 Pages) ON Semiconductor – Dual Synchronous Buck Controller with Input Current Sharing

NCP5424
Out-of-Phase Synchronization
In out-of-phase synchronization, the turn-on of the
second channel is delayed by half the switching cycle. This
delay is supervised by the oscillator, which supplies a clock
signal to the second channel which is 180° out of phase with
the clock signal of the first channel.
The advantages of out-of-phase synchronization are
many. Since the input current pulses are interleaved with one
another, the overlap time is reduced. The effect of this
overlap reduction is to reduce the input filter requirement,
allowing the use of smaller components. In addition, since
peak current occurs during a shorter time period, emitted
EMI is also reduced, thereby reducing shielding
requirements.
Overvoltage Protection
Overvoltage Protection (OVP) is provided as a result of
the normal operation of the V² control method and requires
no additional external components. The control loop
responds to an overvoltage condition within 150 ns, turning
off the upper MOSFET and disconnecting the regulator
from its input voltage. This results in a crowbar action to
clamp the output voltage preventing damage to the load. The
regulator remains in this state until the overvoltage
condition ceases.
Input Current Sharing
In contemporary high-end applications, part of a system
may require more power than is available from one supply.
The NCP5424 dual controller can address this requirement
in two ways.
In many cases, it is sufficient to be able to set the input
power sharing as a ratio so that one source always supplies
a certain percentage of the total. This is achieved by having
the Error Amplifier inputs from Slave side, Controller Two,
brought to external pins so its’ reference is available.
Current information from the Master, Controller One,
provides a reference for the Slave. Current information from
the Slave is fed back to the error amplifier’s inverting input.
The Slave will try to adjust its current to match the current
information fed to its reference input from the Master. If this
information is 1/2 the voltage developed across the Master’s
output inductor, the Slave will run at half current and supply
a percentage, nominally 33% in this case, of the total current.
In other applications however, the user may not only wish
to draw a percentage of power from one source, but also may
need to limit the power drawn from that source. The Slave
has a Cycle-By-Cycle current limit. In this case, the Slave
can be programmed to budget the maximum input power.
For example, a designer may wish to draw equal amounts of
power from two 5-volt sources, but only 2 amps are
available from one of the supplies. In this case, the dual
controller will draw equally from the two sources up to a
total of 4 amps. At this point, the Slave controller goes into
current limit and draws no more than its preset budget. The
Master continues to supply the remaining output current up
to the maximum that the application requires.
Current Limiting
The NCP5424 employs two types of current limits.
Controller One has a Hiccup Mode Current Limit and
Controller Two has Cycle-By-Cycle current limit. Any
overcurrent condition on Controller One results in the
immediate shutdown of both output phases. In a dual output
application, independent current limits are not supported.
The NCP5424 has two current limiting amplifiers that
have a built in 70 mV offset. These differential amplifiers
have a common mode range from zero to 5.5 volts and low
input current. They share a common negative input that in
single output voltage application is not a limitation.
However in dual output applications independent current
limits are not supported.
Once a voltage greater than 70 mV is applied to the current
limiting amplifier of Controller 2; it produces an output that,
as shown in the block diagram, resets the output RS flip flop.
This ends the PWM pulse for the particular cycle and in so
doing, limits the energy delivered to the load on a
cycle-by-cycle basis. One advantage of this current limiting
scheme is that the NCP5424 will limit transient currents and
will resume normal operation the cycle after the transient
goes away.
A second benefit is that this action of limiting the PWM
pulse width means that in an input power sharing
application, one controller can be current limiting while the
other supplies the remaining current needs.
The fault latch immediately turns off the error amplifier
and discharges both COMP capacitors. The capacitor
connected to COMP1 is discharged through a 5.0 mA current
sink in order to provide timing for the reset cycle. When
COMP1 has fallen below 0.25 V, a comparator resets the
fault latch and error amplifier 1 begins to charge COMP1
with a 30 mA source current. When COMP1 exceeds the
feedback voltage plus the PWM Comparator offset voltage,
the normal switching cycle will resume. If the short circuit
condition persists through the restart cycle, the overcurrent
reset cycle will repeat itself until the short circuit is removed,
resulting in small “hiccup” output pulses while the COMP
capacitor charges and discharges. Please see the section
titled “Current Sharing Compensation Capacitor Selection”
for proper Comp capacitor selection.
Cycle-By-Cycle current limit controls the amount of
current available from Controller 2. Controller 2 has a
current limiting comparator that, by truncating the
respective controller’s PWM pulse width, limits the
available current on a pulse-by-pulse basis. This
comparator has a built in 70 mV offset that provides a
reference for setting current limit.
Output Enable
On/Off control of the regulator outputs can be
implemented by pulling the COMP pins low. The COMP
pins must be driven below the 0.40 V PWM comparator
offset voltage in order to disable the switching of the GATE
drivers.
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