IRFR1018EPBF Datasheet, PDF (12/15 Pages) International Rectifier – HEXFET TM Power MOSFET

NCP5383
APPLICATIONS INFORMATION
General
The NCP5383 dual edge modulated multiphase PWM
controller is specifically designed with the necessary
features for a high current power system. The IC consists
of the following blocks: High Performance Voltage Error
Amplifier, Precision Oscillator and Triangle Wave
Generators, and PWM Comparators. Protection features
include Undervoltage Lockout, Soft-Start, Overcurrent
Protection, Thermal Shutdown and Power Good Monitor.
High Performance Voltage Error Amplifier
The error amplifier is designed to provide high slew rate
and bandwidth. A capacitor from COMP to V_FB is required
for stable unity gain test configurations.
Oscillator and Triangle Wave Generator
A programmable precision oscillator is provided. The
oscillator 's frequency is programmed by the resistance
connected from the R_OSC pin to ground. The user will
usually form this resistance from two resistors in order to
create a voltage divider that uses the R_OSC output voltage
as the reference for creating the current limit setpoint
voltage. The oscillator frequency range is 100 kHz/phase
to 400 kHz/phase. The oscillator generates 2 triangle
waveforms (symmetrical rising and falling slopes)
between 1.3 V and 2.3 V. The triangle waves have a phase
delay between them such that for 2 phase operation the
PWM outputs are separated by 180 degrees.
PWM Comparators with Hysteresis
Two PWM comparators receive the error amplifier
output signal at their non inverting input. Each comparator
receives one of the triangle waves offset by 1.3 V at it's
inverting input. During steady state operation, the duty
cycle will center on the valley of the triangle waveform,
with steady state duty cycle calculated by V_out/V_in. During
a transient event, both high and low comparator output
transitions shift phase to the points where the error
amplifier output intersects the down and up ramp of the
triangle wave.
PROTECTION FEATURES
Undervoltage Lockout
An undervoltage lockout (UVLO) senses the V_CC input.
During powerup, the input voltage to the controller is
monitored, and the PWM outputs and the soft-start circuit
are disabled until the input voltage exceeds the threshold
voltage of the UVLO comparator. The UVLO comparator
incorporates hysteresis to avoid chattering, since V_CC is
likely to decrease as soon as the converter initiates
soft-start. There is a separate undervoltage lockout
(UVLO) for the drivers that sense the V_CCP inputs.
Overcurrent Shutdown
A programmable overcurrent function is incorporated
within the IC. A comparator and latch makeup this
function. The inverting input of the comparator is
connected to the ILIM pin. The voltage at this pin sets the
maximum output current the converter can produce. The
ROSC pin provides a convenient and accurate reference
voltage from which a resistor divider can create the
overcurrent setpoint voltage. Although not actually
disabled, tying the ILIM pin directly to the ROSC pin sets
the limit above useful levels – effectively disabling
overcurrent shutdown. The comparator non inverting input
is the summed current information from the current sense
amplifiers. The overcurrent latch is set when the current
information exceeds the voltage at the ILIM pin. The
outputs are immediately, and the soft-start is pulled low.
The outputs will remain disabled until the V_CC voltage is
removed and re-applied, or the ENABLE input is brought
low and then high.
Power Good Monitor
NCP5383 has a power good monitor set at 125% of Vfb
or 75% of Vfb for upper and lower thresholds respectively.
It is an open drain type output.
Soft-Start
The NCP5383 incorporates an externally programmable
soft-start. The soft-start circuit works by controlling the
ramp-up of the Vref voltage during powerup. The initial
soft-start pin voltage is 0 V. The soft-start sequence ends
when V_SS = 0.8 V. The soft-start pin is pulled to 0 V if there
is an overcurrent shutdown, if V_CC is below the UVLO
threshold, or if V_CCP is below the UVLO threshold.
Programming the Current Limit and Oscillator Frequency
The OSC pin provides a 2.0 V reference voltage which
is divided down with a resistor divider and fed into the
current limit pin ILIM. Calculate the total series resistance
to set the frequency and then calculate the individual values
for current limit divider. The series resistors RLIM1 and
RLIM2 sink current to ground. This current is internally
mirrored into a capacitor to create an oscillator. The period
is proportional to the resistance and frequency is inversely
proportional to the resistance.
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