MPQ4459 – INDUSTRIAL GRADE, 1.5A, 4MHz, 36V STEP-DOWN CONVERTER
MPQ4459 Rev. 1.01 www.MonolithicPower.com 8
1/24/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
OPERATION
The MPQ4459 is a variable frequency,
non-synchronous, step-down switching regulator
with an integrated high-side high voltage power
MOSFET. It provides a single highly efficient
solution with current mode control for fast loop
response and easy compensation. It features a
wide input voltage range, internal soft-start
control and precision current limiting. Its very low
operational quiescent current makes it suitable
for battery powered applications.
PWM Control
At moderate to high output current, the MPQ4459
operates in a fixed frequency, peak current
control mode to regulate the output voltage. A
PWM cycle is initiated by the internal clock. The
power MOSFET is turned on and remains on
until its current reaches the value set by the
COMP voltage. When the power switch is off, it
remains off for at least 100ns before the next
cycle starts. If, in one PWM period, the current in
the power MOSFET does not reach the COMP
set current value, the power MOSFET remains
on, saving a turn-off operation. Error Amplifier
The error amplifier compares the FB pin voltage
with the internal reference (REF) and outputs a
current proportional to the difference between the
two. This output current is then used to charge
the external compensation network to form the
COMP voltage, which is used to control the
power MOSFET current.
During operation, the minimum COMP voltage is
clamped to 0.9V and its maximum is clamped to
2.0V. COMP is internally pulled down to GND in
shutdown mode. COMP should not be pulled up
beyond 2.6V.
Internal Regulator
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.
Enable Control
The MPQ4459 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).
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.
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. 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.
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.
Floating Driver and Bootstrap Charging
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 threshold of
150mV.
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
