MP4560 – 2A, 2MHz, 55V STEP-DOWN CONVERTER
Internal Regulator
Floating Driver and Bootstrap Charging
Most of the internal circuitries are powered from
the 2.6V internal regulator. This regulator takes
the VIN input and operates in the full VIN range.
When VIN is greater than 3.0V, the output of the
regulator is in full regulation. When VIN is lower
than 3.0V, the output decreases.
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.
Enable Control
The MP4560 has a dedicated enable control pin
(EN). With high enough input voltage, the chip
can be enabled and disabled by EN which has
positive logic. Its falling threshold is a precision
1.2V, and its rising threshold is 1.5V (300mV
higher).
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.
When floating, EN is pulled up to about 3.0V by
an internal 1µA current source so it is enabled.
To pull it down, 1µA current capability is needed.
When EN is pulled down below 1.2V, the chip is
put into the lowest shutdown current mode.
When EN is higher than zero but lower than its
rising threshold, the chip is still in shutdown
mode but the shutdown current increases slightly.
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.
Under-Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) is implemented to
protect the chip from operating at insufficient
supply voltage. The UVLO rising threshold is
about 3.0V while its falling threshold is a
consistent 2.6V.
Internal Soft-Start
The soft-start is implemented to prevent the
converter output voltage from overshooting
during startup and short circuit recovery. When
the chip starts, the internal circuitry generates a
soft-start voltage (SS) ramping up from 0V to
2.6V. When it is lower than the internal reference
(REF), SS overrides REF so the error amplifier
uses SS as the reference. When SS is higher
than REF, REF regains control.
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.
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.
Thermal Shutdown
Thermal shutdown is implemented to prevent the
chip from operating at exceedingly high
temperatures. When the silicon die temperature
is higher than its upper threshold, it shuts down
the whole chip. When the temperature is lower
than its lower threshold, the chip is enabled again.
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
+
> 20μA
(R1+ R2)
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.
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