MP4688 – 80V INPUT, 1A HIGH POWER LED DRIVER

MP4688 Rev. 1.12

www.MonolithicPower.com

7

3/28/2012

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OPERATION

Hysteresis Current Control with Adaptive

Threshold Adjustment

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.

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).

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.

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.

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.

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.