MP4560 – 2A, 2MHz, 55V STEP-DOWN CONVERTER
Current Comparator and Current Limit
Startup and Shutdown
The power MOSFET current is accurately sensed
via a current sense MOSFET. It is then fed to the
high speed current comparator for the current
mode control purpose. The current comparator
takes this sensed current as one of its inputs.
When the power MOSFET is turned on, the
comparator is first blanked till the end of the turn-
on transition to avoid noise issues. The
comparator then compares the power switch
current with the COMP voltage. When the
sensed current is higher than the COMP voltage,
the comparator output is low, turning off the
power MOSFET. The cycle-by-cycle maximum
current of the internal power MOSFET is
internally limited.
If both VIN and EN are higher than 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 holds the power MOSFET OFF for about
50µs to blank the startup glitches. When the
internal soft-start block is enabled, it first holds its
SS output low to ensure the remaining circuitries
are ready and then slowly ramps up.
Three events can shut down the chip: EN low,
VIN low and thermal shutdown. In the shutdown
procedure, power MOSFET is turned off first to
avoid any fault triggering. The COMP voltage and
the internal supply rail are then pulled down.
Short Circuit Protection
When the output is shorted to the ground, the
switching frequency is folded back and the
current limit is reduced to lower the short circuit
current. When the voltage of FB is at zero, the
current limit is reduced to about 50% of its full
current limit. When FB voltage is higher than
0.4V, current limit reaches 100%.
Programmable Oscillator
The MP4560 oscillating frequency is set by an
external resistor, RFREQ from the FREQ pin to
ground. The value of RFREQ can be calculated
from:
In short circuit FB voltage is low, the SS is pulled
down by FB and SS is about 100mV above FB.
In case the short circuit is removed, the output
voltage will recover at the SS pace. When FB is
high enough, the frequency and current limit
return to normal values.
100000
RFREQ(kꢀ) =
-5
fS(kHz)
To get fSW=500kHz, RFREQ=195kꢀ.
MP4560 Rev. 1.0
11/5/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
9
MPS [ MONOLITHIC POWER SYSTEMS ]
