MP4688 – 80V INPUT, 1A HIGH POWER LED DRIVER
OPERATION
Hysteresis Current Control with Adaptive
Threshold Adjustment
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.
MP4688 operates in a hysteresis current control
mode to regulate the LED current accurately.
FB pin is the sensed voltage of LED current
across the sensing resistor. The power
MOSFET is turned on and remains on until FB
pin rises to 215mV. The power MOSFET is
turned off and remains off until FB pin falls to
185mV. The two thresholds of 215mV and
185mV are adaptive adjusted to compensate all
the circuit delays so that the LED current is
regulated very accurately with 200mV average
value at FB pin.
LED Short Protection
The LED current is well regulated with FB pin
voltage at 200mV. As long as the LED sensing
resistor is not failed, the output current is within
limitation. If the LED sensing resistor is failed or
the output is shorted to GND directly, the FB pin
voltage is low even the power MOSFET is
turned on. The power MOSFET will be shut off
if such failure time is longer than 10us. MP4688
will retry the operation after about 300us delay.
Enable Control
The MP4688 has a dedicated enable control pin
(EN) with the positive logic. Its falling threshold
is a precision 1.2V, and its rising threshold is
1.5V (300mV higher).
The power MOSFET current is also accurately
sensed via a current sense MOSFET. If the
current is over 3.2A, the IC is shut down. This
offers extra protection under output short
conditions.
When floating, EN is pulled up to about 3.0V by
an internal 1µA current source so it is enabled.
To pull it down, over 1µA current capability is
needed.
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.
In case the internal circuit does not have
sufficient voltage and the bootstrap capacitor is
not well 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.
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 4.0V while its falling threshold is a
consistent 3.65V.
MP4688 Rev. 1.12
3/28/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
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