MPQ4559 – 1.5A, 2MHz, 55V STEP-DOWN CONVERTER AEC-Q100 QUALIFIED
Floating Driver and Bootstrap Charging
Current Comparator and Current Limit
The floating power MOSFET driver is powered by
an external bootstrap capacitor. This floating
driver has its own UVLO protection. This UVLO’s
rising threshold is 2.2V with a hysteresis of
150mV. The driver’s UVLO is soft-start related. In
case the bootstrap voltage hits its UVLO, the
soft-start circuit is reset. To prevent noise, there
is 20µs delay before the reset action. When
bootstrap UVLO is gone, the reset is off and then
soft-start process resumes.
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.
The bootstrap capacitor is charged and regulated
to about 5V by the dedicated internal bootstrap
regulator. When the voltage between the BST
and SW nodes is lower than its regulation, a
PMOS pass transistor connected from VIN to
BST is turned on. The charging current path is
from VIN, BST and then to SW. External circuit
should provide enough voltage headroom to
facilitate the charging.
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%.
As long as VIN is sufficiently higher than SW, the
bootstrap capacitor can be charged. When the
power MOSFET is ON, VIN is about equal to SW
so the bootstrap capacitor cannot be charged.
When the external diode is on, the difference
between VIN and SW is largest, thus making it
the best period to charge. When there is no
current in the inductor, SW equals the output
voltage VOUT so the difference between VIN and
VOUT can be used to charge the bootstrap
capacitor.
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.
Startup and Shutdown
At higher duty cycle operation condition, the time
period available to the bootstrap charging is less
so the bootstrap capacitor may not be sufficiently
charged.
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.
In case the internal circuit does not have
sufficient voltage and the bootstrap capacitor is
not charged, extra external circuitry can be used
to ensure the bootstrap voltage is in the normal
operational region. Refer to External Bootstrap
Diode in Application section.
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.
The DC quiescent current of the floating driver is
about 20µA. Make sure the bleeding current at
the SW node is higher than this value, such that:
VO
IO
20A
(R1 R2)
MPQ4559 Rev. 1.01
5/24/2016
www.MonolithicPower.com
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