MP4558 – 1A, 2MHz, 55V STEP-DOWN CONVERTER
Current Comparator and Current Limit
Startup and Shutdown
A current-sense MOSFET accurately senses the
current in the power MOSFET. This signal is then
fed to the high speed current comparator for
current-mode–control purposes, which uses it as
one of its inputs with the COMP voltage. When
the power MOSFET turns on, the comparator is
first blanked until the end of the turn-on transition
to avoid noise issues. When the sensed current
exceeds the COMP voltage, the comparator
output is low and the power MOSFET turns off.
The cycle-by-cycle maximum current of the
internal power MOSFET is internally limited.
If both VIN and VEN exceed their appropriate
thresholds, the chip starts. The reference block
starts first, generating stable reference voltage
and currents, and then the internal regulator is
enabled. The regulator provides stable supply for
the remaining circuitries.
While the internal supply rail is up, an internal
timer blanks the power MOSFET OFF for about
50µs to avoid start-up glitches. When the internal
soft-start block is enabled, it first holds its SS
output low to ensure the other circuits are ready
and then slowly ramps up.
Short-Circuit Protection
Three events can shut down the chip: EN low, VIN
low and thermal shutdown. In shutdown, the
power MOSFET turns off first to avoid any fault
triggering. The COMP voltage and the internal
supply rail are then pulled down.
When the output is shorted to the ground, the
switching frequency folds back and the current
limit falls to reduce the short circuit current. When
the FB voltage equals 0V, the current limit falls to
about 50% of its full current limit. The FB voltage
reaches its 100% of its current limit when it
exceeds 0.4V
Programmable Oscillator
An external resistor—RFREQ connected from the
FREQ pin to GND—sets the MP4558 oscillating
frequency. Calculate the value of RFREQ from:
When the short-circuit FB voltage is low, the SS
drops by VFB and SS ≈ VFB + 100mV. If the short
circuit is removed, the output voltage recovers at
the SS rate. When FB is high enough, the
frequency and current limit return to normal
values.
100000
RFREQ(kΩ) =
-5
fS(kHz)
For fSW=500kHz, RFREQ=195kΩ.
MP4558 Rev. 1.01
10/28/2013
www.MonolithicPower.com
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