MP2487 – 55V, 1A, HIGH-POWER LEDS DRIVER

MP2487 Rev. 1.0

www.MonolithicPower.com

7

9/22/2011

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© 2011 MPS. All Rights Reserved.

OPERATION

PWM Control Mode

MP2487 operates in a fixed frequency, peak

current control mode to regulate the LED current.

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. 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 MP2487 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 LED

current 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.

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 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.

The bootstrap capacitor is charged and regulated

to about 4V 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.