MPQ4560 Datasheet, PDF (11/21 Pages) Monolithic Power Systems – Industrial-Grade, 2A, 2MHz, 55V Step-Down Converter Available in AEC-Q100

MPQ4560 – 2A, 2MHz, 55V STEP-DOWN CONVERTER
~5V. When the voltage between the BST and SW
nodes is less than its regulation, a PMOS pass
transistor from VIN to BST turns ON. The
charging current path is from VIN, BST and then
to SW. An external circuit must provide enough
voltage headroom to facilitate charging.
As long as V_IN is sufficiently higher than V_SW, the
bootstrap capacitor can charge. When the power
MOSFET is ON, V_IN≈V_SW so the bootstrap
capacitor cannot charge. When the external
diode is ON, the difference between V_IN and V_SW
is at its largest, thus making it the best period to
charge. When there is no current in the inductor,
V_SW=V_OUT so the difference between V_IN and V_OUT
can charge the bootstrap capacitor.
At higher duty cycles, the time period available
for bootstrap charging is shorter so the bootstrap
capacitor may not sufficiently charge. If the
internal circuit does not have sufficient voltage
and the bootstrap capacitor is not charged, extra
external circuitry can ensure the bootstrap
voltage is within the normal operational region.
The DC quiescent current of the floating driver is
about 20µA. Make sure the bleeding current at
the SW node exceeds this value, such that:
I_O

V_O
(R1 R2)

20A
Current Comparator and Current Limit
A current-sense MOSFET accurately senses the
power MOSFET’s current. The result goes to the
high-speed current comparator for current-mode
control.: When the power MOSFET turns 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 to V_COMP. When the sensed current
exceeds V_COMP, the comparator output is LOW,
turning OFF the power MOSFET. The
cycle-by-cycle maximum current of the internal
power MOSFET is internally limited.
Short Circuit Protection
When the output is shorted to the ground, the
switching frequency folds back and the current
limit falls to lower the short-circuit current. When
V_FB is zero, the current limit drops to about 50%
of its full current limit. When V_FB exceeds 0.4V,
current limit reaches 100%.
During a short circuit, the V_FB voltage is low and
pulls down V_SS to ~100mV above V_FB. Removing
the short circuit causes the output voltage to
recover with V_SS. When V_FB is high enough, the
frequency and current limit return to normal
values.
Startup and Shutdown
If both V_IN and V_EN exceed their respective
thresholds, the chip starts. The reference block
initiates to generate a stable reference voltage
and currents, and then the internal regulator is
enabled. The regulator provides a 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 noise. 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: V_EN LOW,
V_IN LOW and thermal shutdown. During
shutdown, the power MOSFET turns OFF first to
avoid any fault triggering. Then V_COMP and the
internal supply rail drop.
Programmable Oscillator
An external resistor (R_FREQ) from the FREQ pin to
ground sets the MPQ4560 oscillating frequency.
The value of R_FREQ can be calculated from:
R_FREQ (kΩ)
=
100000
f_S (kHz)
-
5
For example, for f_SW=500kHz, R_FREQ=195kΩ.
MPQ4560 Rev. 1.1
www.MonolithicPower.com
11
3/29/2013
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