MP2487 MPS, MP2487 Datasheet - Page 10

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

Output Capacitor C
The output capacitor (C
the LED current ripple. Ceramic, tantalum, or low
ESR electrolytic capacitors are recommended.
Low ESR capacitors are preferred to keep the
output voltage ripple low so that the AC ripple
current through the LEDs is small. The output
voltage ripple can be estimated by:
For most application, a 2.2uF~4.7uF ceramic
capacitor is recommended.
Compensation Components
MP2487 employs current mode control for easy
compensation and fast transient response. The
system stability and transient response are
controlled through the COMP pin. COMP pin is
the output of the internal error amplifier. A series
capacitor-resistor combination (R
sets a pole-zero combination to control the
characteristics of the control system. The DC
gain of the current feedback loop is given by:
Where A
400V/V;
transconductance, 2.6A/V; R
sensing resistor value.
The system has two poles of importance. One is
due to the compensation capacitor (C
the
(R
transconductance, 500µA/V. The other is due to
the output capacitor and the LEDs’ AC resistor
(R
The system has one zero of importance, due to
the compensation capacitor (C
compensation resistor (R
located at:
MP2487 Rev. 0.9
1/26/2010
EA
LED
=A
=∆V
∆V
output
VEA
OUT
VEA
OUT
/G
G
EA
/∆I
=
A
is the error amplifier voltage gain,
CS
f
).
f
P2
8 f
VDC
P1
LED
×
resistor
=
=
). These poles are located at:
G
S
=
2
2
2
is
EA
R
π ×
π ×
OUT
V
×
FB
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
OUT
L1 C
C
C
OUT
is
×
×
COM1
OUT
the
G
1
1
) is required to reduce
of
CS
OUT
COM
the
×
×
×
R
FB
R
). This zero is
×
A
LED
current
error
EA
⎛
⎜
⎝
VEA
COM
error
COM1
1
is the current
−
V
and C
) and the
V
OUT
© 2010 MPS. All Rights Reserved.
IN
COM1
amplifier
amplifier
www.MonolithicPower.com
⎞
⎟
⎠
sense
) and
COM1
)
MP2487 – 55V, 1A, HIGH-POWER LEDS DRIVER
The
importance, if the output capacitor has a large
capacitance and/or a high ESR value. The zero,
due to the ESR and capacitance of the output
capacitor, is located at:
In this case, a third pole set by the compensation
capacitor (C
(R
ESR zero on the loop gain. This pole is located
at:
The goal of compensation design is to shape the
converter transfer function to get a desired loop
gain and phase margin. The system crossover
frequency where the feedback loop has the unity
gain is important. Lower crossover frequencies
result in slower line and load transient responses,
while higher crossover frequencies could cause
system unstable. A good rule of thumb is to set
the crossover frequency to approximately one-
tenth of the switching frequency. To optimize the
compensation components for conditions, the
following procedure can be used.
1. Choose the compensation resistor (R
set the desired crossover frequency. Determine
the R
Where f
2. Choose the compensation capacitor (C
achieve
applications with typical inductor values, setting
the compensation zero, f
crossover frequency provides sufficient phase
margin. Determine the C
following equation:
COM
COM
) is used to compensate the effect of the
system
C
value by the following equation:
is the desired crossover frequency.
the
R
COM
COM2
f
f
C
P3
f
ESR
Z1
COM1
may
=
=
) and the compensation resistor
desired
=
=
2
2
2
2
π ×
π ×
π ×
>
R
π ×
2
FB
C
C
have
C
π ×
C
COM2
COM1
OUT
×
Z1
OUT
G
R
1
, below one forth of the
1
phase
1
EA
COM
4
×
COM1
×
×
×
R
R
another
×
R
R
LED
COM
×
G
COM
ESR
f
CS
C
value by the
×
margin.
f
C
zero
COM1
COM
) to
) to
For
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of