MP2487 MPS, MP2487 Datasheet - Page 8

• Manufacturer Part Number: MP2487
• Description: 55v, 1a High Power Led Driver
• Manufacturer: MPS

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 V
V
capacitor.
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.
Current Comparator and Current Limit
The power MOSFET current is accurately sensed
via a current sense MOSFET. It is then fed to the
high speed current comparator for the current
mode control purpose. The current comparator
takes this sensed current as one of its inputs.
When the power MOSFET is turned on, the
comparator is first blanked till the end of the turn-
on
comparator then compares the power switch
current with the COMP voltage. When the
sensed current is higher than the COMP voltage,
the comparator output is low, turning off the
power MOSFET. The cycle-by-cycle maximum
current of the internal power MOSFET is
internally limited.
Startup and Shutdown
If both VIN and EN are higher than their
appropriate thresholds, the chip starts. The
reference block starts first, generating stable
reference voltage and currents, and then the
internal regulator is enabled. The regulator
provides
circuitries.
While the internal supply rail is up, an internal
timer holds the power MOSFET OFF for about
MP2487 Rev. 0.9
1/26/2010
OUT
transition
can be used to charge the bootstrap
OUT
stable
so the difference between V
to
supply
avoid
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noise
for
the
issues.
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remaining
www.MonolithicPower.com
IN
The
and
MP2487 – 55V, 1A, HIGH-POWER LEDS DRIVER
50µs to blank the startup glitches. When the
internal soft-start block is enabled, it first holds its
SS output low to ensure the remaining circuitries
are ready and then slowly ramps up.
Three events can shut down the chip: EN low,
VIN low and thermal shutdown. In the shutdown
procedure, power MOSFET is turned off first to
avoid any fault triggering. The COMP voltage and
the internal supply rail are then pulled down.
Programmable Oscillator
The MP2487 oscillating frequency is set by an
external resistor, R
ground. The value of R
from:
To get f
A ceramic capacitor C
R
most applications.
FREQ
to decouple the noise, 1nF is enough for
S
=200kHz, R
R
FREQ
(kΩ) =
FREQ
FREQ
FREQ
=495kΩ.
FREQ
100000
from the FREQ pin to
f (kHz)
S
should be Parallel to
can be calculated
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